Pseudopolydora (Annelida: Spionidae) from the Arabian Gulf, Kuwait

Seven species of Pseudopolydora are described and illustrated from the Arabian Gulf, Kuwait: P. achaeta Radashevsky & Hsieh, 2000, P. antennata (Claparède, 1868), P. arabica Radashevsky & Al-Kandari, 2020, P. auha sp. nov., P. kuwaiti sp. nov., P. melanopalpa sp. nov., and P. multispinosa sp. nov. The morphology of the developed planktonic larvae is described for P. antennata and P. kuwaiti sp. nov. Adults of all species live in tubes in soft sediments, while adults of P. kuwaiti sp. nov. also bore in shells of gastropods and dead corals encrusted by coralline algae. Pseudopolydora antennata and P. arabica form dense settlements up to 50 000 individuals per 1 m2, while other species are comparatively rare. The phylogenetic relationships between the examined species and other Pseudopolydora (18 species in total) were assessed in an analysis of sequence data of four gene fragments: mitochondrial 16S rDNA, nuclear 18S rDNA and 28S rDNA, and Histone 3 (2473 bp in total). Three species complexes are distinguished involving P. antennata, P. diopatra Hsieh, 1992 and P. paucibranchiata (Okuda, 1937). Sets of adult morphological features shared by species of each complex and an identification key to Pseudopolydora species from the Arabian Gulf are provided.


Description
Two specimens were found in Sulaibikhat Bay, Kuwait, comprising 20-chaetiger anterior fragment of a small juvenile and a 70-chaetiger complete female about 16 mm long and 0.8 mm wide (MIMB 40934; Fig. 3A-B). Transverse bands of diffused black pigment present on dorsal side of up to 15 anterior chaetigers; small middorsal melanophores present from chaetigers 4−6 to chaetigers 10−12. Prostomium anteriorly weakly incised, notched or almost blunt, posteriorly extending to end of chaetiger 2 as a low caruncle. Occipital antenna present. Chaetiger 1 reduced, weakly separated from peristomium, with small notopodial and well developed neuropodial lamellae; notochaetae absent; neurochaetae comprising 1−5 very fine, hair-like capillaries. Chaetiger 5 same in size as chaetigers 4 or 6, with dorsal superior and ventral capillaries same in shape and number as those chaetae on chaetigers 4 or 6; two kinds of heavy spines arranged in a vertical slightly curved double row; noto-and neuropodial postchaetal lamellae present (Fig. 3C−E). Anterior-row spines pennoned, with curved pointed tip, without subdistal constriction (Fig. 3F), up to 22 in a series; posterior-row spines simple falcate (Fig. 3G), up to 19 in a series. Bidentate hooded hooks in neuropodia from chaetiger 8, up to 15 in a series. Branchiae from chaetiger 7 to chaetiger 15. Pygidium flaring disc with wide dorsal gap and dorso-lateral processes ( Fig. 3H-I). Glandular pouches in neuropodia from chaetiger 1, largest and paired in each neuropodium in chaetigers 6 and 7, single in other neuropodia.

Remarks
Pseudopolydora achaeta was originally described from the South China Sea, Taiwan, as a common polychaete inhabiting tubes in soft sediments in brackish-water environments (Radashevsky & Hsieh 2000). Since then, the species was reported from Paraná and São Paulo (Brazil) (Lana et al. 2006;Radashevsky & Migotto 2006), Sea of Japan (Russia) (Zvyagintsev et al. 2011), Pacific side of Honshu Island (Japan) (Abe et al. 2014(Abe et al. , 2016Abe & Sato-Okoshi 2021), and from Florida (USA) (Bogantes et al. 2021). Here, for the first time, we report it for the Arabian Gulf (Kuwait). The only female had small oocytes up to 65 µm in diameter developing from chaetiger 15 onwards (Fig. 3J). The morphology of the specimens fits the diagnostic characters of P. achaeta. The same pattern of methylene green staining was observed in the type specimens of P. achaeta from Taiwan (MIMB 3401). The wide distribution of this species outside of its native area in the Northwest Pacific can be explained by unintentional human-mediated transportations of larvae with ballast water of ships, followed by successful invasions.

Description
Adults up to 16 mm long, 1 mm wide with 86 chaetigers. No pigmentation on body and palps; fine black pigment scattered on dorsal side of anterior chaetigers without particular pattern in some individuals. Prostomium anteriorly bifurcated, with two long, distally narrowing lobes ( Fig. 4A-D), posteriorly extending to end of chaetiger 4 as a low caruncle, shorter in small individuals. Long cirriform occipital antenna present on caruncle between palps (Fig. 4C). Two pairs of black eyes arranged trapezoidally, comprising one pair of median eyes, and one pair of lateral eyes situated anteriorly and set wider apart. Palps as long as 15-25 chaetigers, with frontal longitudinal groove lined with fine cilia, and short compound non-motile cilia arising directly from palp surface sparsely arranged in line on sides of groove and sparsely scattered on lateral and abfrontal palp surfaces.
Chaetiger 5 same in size as chaetiger 4 or 6, with dorsal superior capillaries, two kinds of notopodial spines arranged in a double U-shaped row, and ventral capillaries; notopodial postchaetal lamellae reduced, but neuropodial lamellae same as on chaetigers 4 and 6 ( Fig. 6B). Dorsal superior capillaries shorter and fewer than those capillaries on chaetigers 4 and 6. Ventral capillaries same in size, number and arrangement (in three groups) as those on chaetiger 4 (Fig. 6B). Upper posterior part of double U-shaped row of spines equal to or slightly lower than upper anterior part. Newly developed spines in posterior upper part of U-shaped row slightly larger than older spines in anterior upper part of row. Outer (anterior-row) notopodial spines up to 11 in a series, with distal part of stem enlarged, with concavity on top and large triangular tooth on its side directed upwards and facing towards the inside of the U-shaped row of spines; fine bristles arising from concavity forming long flag-like pointed transparent tip which usually broken in worn old spines in anterior upper part of row ( Fig. 6B-C). Inner (posterior-row) notopodial spines up to eight in a series, falcate, with short rounded distal part geniculate, with subdistal bulbous swelling bearing very short fine bristles and facing towards the inside of the U-shaped row of spines ( Fig. 6B-C).  Nototrochs usually from chaetiger 7 onwards, occasionally from chaetiger 6 or 8, composed of single rows of cilia in both sexes. On branchiate chaetigers, nototroch cilia long, arranged in transverse lines and extending onto branchiae; on posterior abranchiate chaetigers, cilia arranged in U-shaped bands, with arms directed posteriorly. Additional ciliation on chaetigers absent.
Pygidium bilobed, with two semi-oval lateral lobes (Fig. 4E), white due to great number of spindleshaped glandular cells with striated content.   (Claparède, 1868). D. P. arabica Radashevsky & Al-Kandari, 2020. E. P. kuwaiti sp. nov. A. Relationships between caruncle length (in chaetiger numbers) and total number of chaetigers, and between distribution of branchiae (referring to number of the last branchiate chaetiger) and total number of chaetigers. B. Relationships between distribution of gametes (referring to number of the first and the last chaetigers with gametes) and total number of chaetigers. C-E. Relationships between distribution of branchiae (referring to number of the last branchiate chaetiger LBC divided by the total number of chaetigers TNC) and total number of chaetigers. Digestive tract without ventral buccal bulb and gizzard-like structure, not pigmented.
Main dorsal blood vessel transformed into gut sinus in anterior part of midgut. Heart body absent. Blood red, without globules or other elements.
Nephridia from chaetiger 4 onwards, very narrow in chaetigers 4-6, well-developed from chaetiger 7. In female fertile chaetigers, paired nephridia on each chaetiger opening to exterior via common middorsal nephridiopore situated anterior to nototroch; walls of distal parts of nephridia containing glandular cells marking nephridia in live individuals ( Fig. 7D-F) and also in fixed specimens stained with MG (Fig. 8C).

MG staining
Intensely stained dorsal sides of the prostomium and peristomium; most intensely stained blotches on the dorsal and lateral sides from chaetigers 7-9 onwards; wide transverse bands on the ventral side from chaetigers 7-8 to chaetigers 10-11 ( Fig. 8A-F).

Habitat
Adult P. antennata inhabit silty tubes in muddy sand intertidally and in shallow waters. The population density in some local places can reach a maximum of 50 000 individuals per 1 m 2 .

Reproduction
Pseudopolydora antennata is gonochoristic. Both females and males become mature after growing to about 10 mm long with 70 chaetigers. Of 21 examined mature individuals, 14 were females and seven were males. The gametes developed from chaetiger 12 to chaetigers 40−45 (Fig. 5B). Paired testes or ovaries were attached to segmental blood vessels in fertile chaetigers.
Oogenesis was partly intraovarian. Developed coelomic oocytes were spherical, about 130 µm in diameter, with smooth envelope less than 1 µm thick ( Fig. 7E-F). Spermatogonia proliferated in testes; spermatogenesis occurred in the coelomic cavity. Spermatids, each about 4 µm in diameter, were joined in tetrads. Spermatozoa were introsperm with pointed acrosome about 1 µm long, straight elongated nucleus about 1.5 µm in diameter and 6 µm long, midpiece 33 µm (head + midpiece about 40 µm long), and a flagellum about 40 µm long (the precision of measurement of spermatids and parts of head of spermatozoa were at maximum to the nearest 0.5 µm and flagellum to the nearest 2 µm).

Larval development in the plankton
The spermatophores and egg capsules, typically produced by Pseudopolydora, were not observed in P. antennata from the Arabian Gulf. The 13−15-chaetiger larvae of the species were collected in plankton in March 2016. The larvae were identified by the morphology of the heavy spines in the notopodia of chaetiger 5. Although similar spines are also present in worms of two other species described below, those species were quite rare and not found in the area where numerous larvae were collected; P. antennata worms were common in that area and the larvae are therefore referred to this species. The larvae were maintained in Petri dishes in the laboratory but did not settle during the study.
Chromatophores absent in lateral peristomial lips and on ventral side of pygidium. Both melanophores and chromatophores changing their shape and size according to light intensity, thus appearing small and compact ( Fig. 9E-F) or greatly expanded and ramified (Fig. 9A, C).
Prostomium wide and rounded anteriorly, posteriorly narrow, extending to end of chaetiger 1 as a low caruncle. Occipital antenna absent. Three pairs of black eyes arranged almost in a transverse line, comprising one pair of small rounded median eyes and two pairs of lateral eyes; lateral eyes on each side situated close to each other and appearing as one comma-shaped eye. Black pigment spot situated between lateral and median eyes on each side. One pair of spherical unpigmented ocelli, each about 20 µm in diameter, situated in front of pigmented eyes. Palps arising from posterior dorso-lateral edges of peristomium, posterior to prototroch; each palps as long as about three chaetigers. Ciliation on palps not yet developed.
Prototroch and telotroch well developed. Nototrochs and grasping cilia from chaetiger 3 onwards. Triangular neurotroch extending from ventral peristomial lip to end of chaetiger 1. Two small ciliated cells situated on each side of neurotroch. Ventral ciliary pit absent. Gastrotroch on chaetiger 3 composed of two small cells with short cilia; those on chaetigers 5 and 7 each composed of 8-10 large cells with long numerous cilia; additional gastrotroch usually present on chaetiger 9, and in some larvae also on chaetiger 10 and/or chaetiger 11, each composed of one to two pairs of small cells with short cilia.
Chaetiger 5 with two kinds of heavy spines in notopodia in addition to short adult capillaries and long provisional serrated bristles. Spines comprising 2-4 anterior-row spines with enlarged distal end of stem and pointed tip, and 1-2 posterior-row simple falcate spines (Fig. 9J).
Hooks in neuropodia from chaetiger 8 onwards, up to seven in a series, not accompanied by capillaries. Hooks bidentate, with upper tooth closely applied to main fang, with outer hood; lower part of shaft bent almost at right angle (Fig. 9K).
Large lateral lips and small ventral lip of peristomium forming voluminous vestibulum. Narrow oesophagus extending through 2-3 chaetigers. Gizzard-like structure absent in digestive tract. Wall of voluminous midgut with numerous lipid globules, each up to 15 µm in diameter.
Circulatory system developed and functional; blood transparent, without pigment.
Two pairs of protonephridia in chaetigers 1 and 2. Metanephridia not yet developed.

Remarks
Adult Pseudopolydora from the Arabian Gulf described above appear identical to P. antennata from Italy recently re-described by Radashevsky (2021), and are consequently referred to this species. They have the prostomia anteriorly bifurcated, with two long, distally narrowing lobes, caruncles extending to the end of chaetiger 4, comparatively short occipital antennae, chaetiger 5 spines of the same morphology, bilobed pygidia with two fleshy semi-oval lateral lobes, and gametes developing from chaetiger 12.
The exact dimensions of the gametes of P. antennata from the Mediterranean remain unknown but worms from the Arabian Gulf are unusual among Pseudopolydora in having spermatids joined in tetrads instead of octads, and the spermatozoa with an extremely long midpiece, about 33 µm instead of 3.2−4.6 µm long as in other examined species (see review by Blake & Arnofsky 1999: table 2).
The morphology of genuine P. antennata larvae from the Mediterranean has never been described or illustrated (see review by Radashevsky 2021). Hannerz (1956) described and illustrated pelagic, ready to metamorphose 11- and 13−14-chaetiger Pseudopolydora larvae, and a 19-chaetiger juvenile from Gullmar Fjord, Sweden. He referred them to P. antennata and reported that the larvae occurred in the Fjord from July through November but were relatively scarce. The Arabic larvae herein referred to P. antennata are similar to those described by Hannerz (1956) only in having dorso-lateral ramified melanophores from chaetiger 4 onwards, but differ significantly by having 1−2 pairs of subspherical yellow chromatophores in front of lateral eyes, large yellow melanophores on the ventral side of chaetigers, main gastrotrochs on chaetigers 3, 5, 7, and small additional gastrotrochs on chaetigers 9, 10 and/or chaetiger 11, instead of gastrotrochs on chaetigers 5 and 7 only. Moreover, the Arabic larvae are unique among Pseudopolydora larvae in the absence of large ramified middorsal melanophore on chaetiger 1. Rasmussen (1973: 113) described and perfectly illustrated egg capsules laid by Pseudopolydora in Isefjord, Denmark, and noted that "The hatched larvae from the Isefjord agreed closely with the description [of P. antennata] given by Hannerz (1956, pp 126−130)". Following Hannerz (1956), Rasmussen (1973) referred Danish worms to P. antennata. Obviously, larvae from the Arabian Gulf referred to P. antennata in the present study are not conspecific with those from Gullmar Fjord described by Hannerz (1956). Which of them belong to P. antennata remains to be clarified.
The larvae from the Arabian Gulf referred to P. antennata are characterised by the following features: one pair of large ramified dorsal melanophores on chaetiger 3 (one or both of them can be absent), large ramified dorso-lateral melanophores from chaetiger 4 onwards, middorsal melanophore on the pygidium, paired yellow chromatophores in the anterior part of prostomium, and large median, usually unpaired, yellow chromatophores on the ventral side from chaetiger 2 onwards. Melanophores in lateral peristomial lips, middorsal and lateral melanophores on chaetigers, and chromatophores on the ventral side of pygidium, characteristic for larvae of other species of Pseudopolydora, are absent.

MG staining
Intensely stained dorsal side of the prostomium in front of eyes, dorsal side of the peristomium, and ventral side of chaetigers: complete transverse paired bands moderate intense on chaetigers 1-4, very intense complete transverse paired bands on chaetigers 5-7, and paired transverse bands split into three parts from chaetiger 8 onwards ( Fig. 10D-G). Remarkably, the three-parts banding was associated with the start of the hooded hooks in neuropodia and was invariable in all examined specimens.

Remarks
Pseudopolydora arabica Radashevsky & Al-Kandari, 2020 was originally described from the Arabian Gulf as one of the common polychaetes inhabiting soft sediments. The population density of the species in some places reaches 50 000 individuals per 1 m 2 .

Diagnosis
Caruncle to end of chaetiger 6. Branchiae present beyond the middle of the body. Pygidium bilobed.

Etymology
The name refers to the type locality of the species in the intertidal of Auha Island, Arabian Gulf.

Description
Two complete individuals in good condition with palps were found together in a shell limestone encrusted by coralline alga. The 22-chaetiger individual is designated as the holotype. The 42-chaetiger paratype was examined and then used entirely for molecular analysis.
Holotype about 2 mm long, 0.37 mm wide with 22 chaetigers (Fig. 11A). Paratype about 5 mm long, with 42 chaetigers. Pigmentation absent on body and palps. Prostomium anteriorly bifid, with two short triangular lobes (Fig. 11A), posteriorly extending to end of chaetiger 6 as a low caruncle in both types. Occipital antenna present on caruncle between palps. Two pairs of black eyes arranged trapezoidally, comprising one pair of median eyes, and one pair of lateral eyes situated anteriorly and set wider apart. Palps as long as 15-20 chaetigers, with frontal longitudinal groove lined with fine cilia, and short compound non-motile cilia arising directly from palp surface sparsely arranged in line on sides of groove and sparsely scattered on lateral and abfrontal palp surfaces.
Chaetiger 1 with short capillaries in neuropodia, and small lamellae in both rami; notochaetae absent. Chaetiger 2 notochaetae all slender capillaries with narrow limbation. Anterior-row notopodial capillaries on chaetiger 3 with slightly enlarged wing; capillaries on chaetiger 4 with wing slightly larger than on chaetigers 3. Anterior-row capillaries on chaetigers 3, 4, 6 and 7 arranged in J-shaped series; posteriorrow capillaries on these chaetigers arranged in vertical series. Posterior notopodia with a few long alimbate capillary chaetae.
Chaetiger 5 slightly larger than chaetigers 4 or 6, with dorsal superior capillaries, two kinds of notopodial spines arranged in a double U-shaped row, and ventral capillaries; noto-and neuropodial postchaetal lamellae same as on chaetiger 4 (Fig. 11D). Dorsal superior capillaries slightly shorter and fewer than those capillaries on chaetiger 4. Ventral capillaries same in size, number and arrangement (in three groups) as those on chaetiger 4. Newly developed spines in posterior upper part of U-shaped row slightly larger than older spines in anterior upper part of row. Outer (anterior-row) notopodial spines 4-6 in a series, with distal part of stem enlarged, with concavity on top and large triangular tooth on its side directed upwards and facing towards the inside of the U-shaped row of spines; fine bristles arising from tooth and concavity forming a long flag-like pointed transparent tip which is usually broken in worn old spines in the anterior upper part of row (Fig. 11E). Inner (posterior-row) notopodial spines 4-5 in a series, falcate, with short rounded distal part geniculate, with subdistal bulbous swelling bearing very short fine bristles and facing towards the inside of the U-shaped row of spines (Fig. 11E).
Hooks in neuropodia from chaetiger 8, up to 20 in a series, not accompanied by capillaries, in posterior chaetigers situated on top of prominent fleshy neuropodial lobes. Hooks bidentate, with upper tooth closely applied to main fang; upper part of shaft with constriction; lower part of shaft bent at right angle (Fig. 11F).
Branchiae from chaetiger 7 to chaetiger 15 in holotype, and to chaetiger 24 in paratype, on chaetiger 7 slightly shorter than those on chaetiger 8, full-sized from chaetigers 9-10, free from notopodial postchaetal lamellae, flattened, with surfaces oriented perpendicular to body axis, with longitudinal ciliation (extension of nototroch) running on inner edge. Pygidium bilobed, with two semi-oval lateral lobes, white due to many spindle-shaped glandular cells with striated content. Glandular pouches in neuropodia from chaetiger 1, largest and paired in each neuropodium in chaetigers 6 and 7, single in other neuropodia.
Digestive tract without ventral buccal bulb and gizzard-like structure, without pigmentation.

MG staining
Intensely stained anterior part of the prostomium in front of eyes, and dorso-lateral sides of the peristomium. Weakly stained lateral sides of chaetigers and scattered glandular cells on the ventral side of chaetigers.

Habitat
The two types of P. auha sp. nov. were found together in a shell limestone encrusted by a coralline alga. Whether the worms made the burrows themselves or occupied empty holes made by other organisms remains uncertain.

Reproduction
Both examined individuals of P. auha sp. nov. were juveniles.

Remarks
In the Arabian Gulf, adults of P. auha sp. nov. appear very similar to those of P. antennata and P. melanopalpa sp. nov. (see below). They all have heavy spines in the notopodia of chaetiger 5 of the same morphology, branchiae beyond the middle of the body, and the bilobed pygidia with dorsal and ventral clefts (characters shared by all members of the P. antennata-complex, see below in the Discussion). However, they can be distinguished by the prostomium shape, length of the caruncle, and palp pigmentation. In P. antennata, the prostomium is anteriorly bifurcated, with two long pointed antero-lateral processes (to which the name of the species was referred), whereas in P. auha sp. nov. and P. melanopalpa sp. nov. the prostomia are incised, with two short lobes. In P. antennata, the caruncle extends posteriorly maximum to the end of chaetiger 4, whereas in P. melanopalpa sp. nov. the caruncle extends to the end of chaetiger 5, and in P. auha sp. nov. it extends to the end of chaetiger 6.
Adults of P. auha sp. nov. appear very similar to those of P. uphondo Simon, Sato-Okoshi & Abe, 2017 and P. eriyali Simon, Sato-Okoshi & Abe, 2017 both of which were described from South Africa by Simon et al. (2019). They share the characters of the P. antennata-complex (see below in the Discussion); the morphological differences between the three species are very subtle (see Simon et al. 2019: table 1) although genetically they are well separated (Fig. 2). Adult P. eriyali and P. auha sp. nov. have caruncles extending to the end of chaetiger 6, while in P. uphondo the caruncle extends to the end of chaetiger 5. Additional specimens of P. auha sp. nov. should be examined to understand the morphological variability of this species and the differences between this species and P. eriyali and P. uphondo.

Diagnosis
Prostomium anteriorly notched to weakly incised, with two rounded lobes. Palps with up to 11 yellow bands. Occipital antenna absent. Anterior-row capillaries in notopodia from chaetiger 7 to chaetigers 9−14 with wide subtriangular, pennoned limbation. Chaetiger 5 larger than chaetigers 4 and 6, dorsally overlapping anterior part of chaetiger 6; lower (anterior-row) notopodial spines, with distal part of stem enlarged, cup-shaped, with a concavity on top and long pennoned, pointed tip. Pygidium small and fleshy, disc-like to cup-shaped, with dorsal gap to incision.

Etymology
The name refers to the type locality of the species in Kuwaiti waters.

Description
Adults up to 6 mm long, 0.4 mm wide with 35 chaetigers (Fig. 12A). Fine black pigment usually scattered on dorsal and ventral sides of head and 10−13 anterior chaetigers (Fig. 13A-C). Up to 11 yellow bands (branched chromatophores) present on each palp when alive (Figs 12A-C, E, 13B, 14A), not visible after fixation. Prostomium anteriorly usually notched or weakly incised, with two rounded lobes (Figs 12B, 13D), occasionally blunt (Fig. 13A), posteriorly extending to end of chaetiger 2 as a low caruncle (Fig. 14C). Usually two pairs of black eyes present. Occipital antenna absent (Fig. 12D). Palps as long as 15−20 chaetigers, with longitudinal frontal groove lined with fine cilia, short motile compound cilia regularly arranged in one row along ciliated groove, and non-motile cilia arising from palp surface and scattered on lateral and abfrontal surfaces.
Chaetiger 5 larger than chaetigers 4 and 6, dorsally overlapping anterior part of chaetiger 6, with up to four dorsal superior capillaries, two kinds of notopodial spines arranged in a curved diagonal or almost horizontal double row, and ten ventral capillaries; notopodial lamellae lacking but neuropodial postchaetal lamellae present, same as on chaetigers 4 and 6 (Figs 12B, F, 13A). Dorsal superior capillaries shorter and fewer than those capillaries on chaetigers 4 and 6. Ventral capillaries same in size, number and arrangement (in three groups) as those on chaetiger 4. Lower (anterior-row) notopodial spines, with distal part of stem enlarged, cup-shaped, with concavity on top and long pennoned, pointed distal tip (Fig. 15C), up to ten in a series. Upper (posterior-row) notopodial spines simple falcate (Fig. 15C), up to seven in a series. Newly developed spines in posterior part of curved series slightly larger than older spines in anterior part of series (Fig. 12F).
Hooks in neuropodia from chaetiger 8, up to 11 in a series, not accompanied by capillaries. Hooks bidentate, with upper tooth closely applied to main fang; upper part of shaft with constriction; lower part of shaft bent at right angle in hooks in posterior neuropodia ( Fig. 15D-E).
In immature individuals and females, short nototroch present on chaetiger 3 and longer nototrochs with longer cilia from chaetiger 7 onwards, each composed of single row of ciliated cells extending onto branchiae on branchiate chaetigers. In males, nototrochs present from chaetiger 2 onwards, each composed of two rows of ciliated cells (anterior row extending onto branchiae; posterior row running only across chaetiger), and additional intersegmental transverse ciliary rows present from chaetiger 3, situated on anterior edge of chaetigers; cilia of nototrochs longer than those of intersegmental rows. In both sexes, nototrochs on branchiate chaetigers as transverse ciliary bands, on postbranchiate chaetigers gradually becoming U-shaped, with arms directed posteriorly. One or two pairs of short compound motile cilia usually present on dorsal side of each of chaetigers 4 and 5.
Pygidium small and fleshy, disc-like to cup-shaped, with dorsal gap to incision (Fig. 13E-F), white due to numerous fusiform glandular cells with striated content. Glandular pouches in neuropodia from chaetiger 1, largest in chaetigers 6 and 7, single pouch in each neuropodium (Fig. 15D).
Main dorsal blood vessel transformed into gut sinus in anterior part of midgut. Heart body absent. Blood red, without globules or other elements. Nephridia from chaetiger 4 onwards. In female fertile chaetigers, paired nephridia on each chaetiger opening to exterior via common middorsal nephridiopore anteriorly to nototroch.

Habitat
Adult P. kuwaiti sp. nov. inhabit silty tubes in muddy sand intertidally and in shallow waters and also bore in live mollusc shells, dead corals and shell limestone encrusted by coralline algae. While the species is comparatively rare, the population density in some local places can reach a maximum of one thousand individuals per 1 m 2 . In hard substrata, worms occasionally form aggregations of 1−3 individuals per 1 cm 2 .

Reproduction
Pseudopolydora kuwaiti sp. nov. is gonochoristic. Both in females and males, gametes developed from chaetiger 12 to chaetigers 16−29 (Fig. 14D). Paired testes or ovaries were attached to segmental blood vessels in fertile chaetigers. Oogenesis was partly intraovarian (Fig. 15F). Developed coelomic oocytes were spherical, about 100 µm in diameter. Oocyte envelope was smooth, less than 1 µm thick. Spermatogonia proliferated in testes; spermatogenesis occurred in the coelomic cavity. Spermatids, each 3−4 µm in diameter, were joined in octads. Spermatozoa were introsperm with pointed acrosome about 1.5 µm long, straight elongated nucleus about 1.5 µm in diameter and 6 µm long, midpiece 3.5 µm (head + midpiece about 11 µm long), and a flagellum about 32 µm long (the precision of measurement of spermatids and parts of head of spermatozoa were at maximum to the nearest 0.5 µm and flagellum to the nearest 2 µm).

Fig. 14.
Adult characteristics of Pseudopolydora kuwaiti sp. nov. A. Relationships between number of yellow bands on palp and total number of chaetigers. B. Relationships between distribution of branchiae (referring to number of the last branchiate chaetiger) and total number of chaetigers. C. Relationships between caruncle length (in chaetiger numbers) and total number of chaetigers, and distribution of pennoned capillaries in notopodia (referring to number of the last chaetiger with pennoned capillaries) and total number of chaetigers. D. Relationships between distribution of gametes (referring to number of the first and the last chaetigers with gametes) and total number of chaetigers.

Larval development in the plankton
The spermatophores and egg capsules, typically produced by Pseudopolydora, were not observed in Pseudopolydora kuwaiti sp. nov. The 10−14-chaetiger larvae of this species were collected in plankton in March 2016. The 14-chaetiger larvae were easily identified by the specific heavy spines in the notopodia of chaetiger 5. Small larvae, with heavy spines not yet developed, were identified by the specific pigmentation, revealed in larger larvae.
Branchiae not yet developed. Hooks in neuropodia from chaetiger 8. Glandular pouches in neuropodia from chaetiger 3 onwards; pouches in chaetigers 3−5 each composed of one small glandular cell; from chaetiger 6 onwards, each pouch composed of two cells. One pair of protonephridia in chaetiger 1.

14-chaetiger larvae
Larvae 700−750 µm long and up to 200 µm wide in middle part (Fig. 16B−E). Intense pigmentation comprising small melanophores and fine yellowish pigment scattered all along body, in anterior part of prostomium, in pharynx and in pygidium; distinct chromatophores absent. A small black spot present between median and lateral eyes on each side of prostomium. Small middorsal melanophores in anterior part of chaetiger 1 and on pygidium at level of telotroch; chaetiger 1 melanophore occasionally heavily ramified and hardly discernible. Small paired dorsal melanophores from chaetiger 3 onwards. Small dorso-lateral melanophores from chaetigers 3−4 onwards, usually indistinct, difficult to recognize. Single small lateral melanophores on either side on anterior edge of chaetiger 2; in some larvae, lateral melanophores appearing as indistinct spots of diffused black pigment.
Prostomium anteriorly wide and rounded, posteriorly extending to middle of chaetiger 1 as a low narrow caruncle. Nuchal organs large round ciliary patches on sides of caruncle on anterior half of chaetiger 1. Occipital antenna absent. Three pairs of black eyes arranged almost in a transverse line, comprising one pair of small rounded median eyes and two pairs of lateral eyes; lateral eyes on each side situated close to each other and appearing as one comma-shaped eye. One pair of spherical unpigmented ocelli, each about 17 µm in diameter, present in front of pigmented eyes. One pair of compound motile cilia, each about 20 µm long, situated near median eyes, and one pair of compound cells, each about 75 µm long, situated near lateral eyes. One pair of palps arising from posterior dorso-lateral edges of peristomium, posterior to prototroch; each palp as long as about four chaetigers. Shallow frontal longitudinal groove on palps densely ciliated.
Prototroch and telotroch well developed. Nototrochs and grasping cilia from chaetiger 3 onwards. Triangular neurotroch extending over ventral peristomial lip from mouth to end of chaetiger 1. Two small ciliated cells present on each side of neurotroch. Ventral ciliated pit absent. Gastrotrochs on chaetigers 5, 7 and 11, each composed of six large cells with numerous long cilia.
Provisional serrated bristles in all notopodia including chaetiger 5; those in anterior chaetigers up to 175 µm long, gradually becoming shorted in succeeding chaetiger. Short adult capillaries in notopodia from chaetiger 2, and in neuropodia in chaetigers 1−7. Single capillary chaetae with flag-like limbation in notopodia of chaetigers 7 and 8.
Chaetiger 5 with two kinds of heavy spines in notopodia in addition to short adult capillaries and long provisional bristles. Spines comprising 3−4 anterior-row spines with wide cup-shaped distal end of stem bearing long transparent pointed tip, and 2−3 posterior-row simple falcate spines.
Hooks in neuropodia from chaetiger 8 onwards, up to five in a series, not accompanied by capillaries. Hooks bidentate, with upper tooth closely applied to main fang, with outer hood; lower part of shaft bent almost at right angle.
Branchiae short, on chaetigers 7 and 8. Pygidium semispherical, divided into two rounded lobes by vertical median furrow and bearing four short papillae.
Numerous oval to spherical glandular cells with striated content present in anterior part of prostomium, on dorsal side of chaetigers, in pygidium and palps (Fig. 16E).
Glandular pouches in neuropodia from chaetigers 1−2, very small in first chaetigers, large in chaetigers 6 and 7, gradually diminishing in size in succeeding chaetigers.
Large lateral lips and small ventral lip of peristomium lined with numerous short cilia and forming voluminous vestibulum. Compound motile cilia up to 65 µm long present on lateral and ventral sides of lateral peristomial lips. Narrow oesophagus extending through three chaetigers. Gizzard-like structure absent in digestive tract. Numerous lipid globules up to 10 µm in diameter present in wall of voluminous midgut.
Circulatory system developed and functional; blood transparent, without pigment.

14-chaetiger larvae
Larvae about 700 µm long easily settled and underwent gradual metamorphosis in Petri dishes. They started crawling and in the absence of sediment built transparent mucous tubes. The metamorphosis comprised loss of larval features, such as provisional bristles in notopodia, proto-, telo-, neuro-, gastrotrochs and grasping cilia, nototrochs on chaetigers 3−5, and also the transformation of the head and rapid elongation of the palps. The prostomium became distinctly separated from the peristomium. Nuchal organs transformed from oval ciliary patches to longitudinal narrow ciliary bands on the posterior sides of the prostomium. Lateral peristomial lips became reduced and transformed into dorso-lateral ciliary folds of the foregut. The ventral peristomial lip enlarged and formed the adult peristomium and basement for the palps. Unpigmented ocelli in the prostomium and larval protonephridia in chaetigers 1 and 2 became reduced and disintegrated.

Remarks
Adult P. kuwaiti sp. nov. appear similar to the Asian Pacific Pseudopolydora diopatra Hsieh, 1992. Both have yellow ramified chromatophores on palps, caruncles without occipital antenna extending to the end of chaetiger 2, and small cup-shaped pygidia (Hsieh 1992). They differ from other Pseudopolydora by having greatly enlarged chaetiger 5 (instead of chaetiger 5 being similar in size to chaetigers 4 and 6) with the notopodial heavy spines arranged in a diagonal instead of a vertical double row, and also by the unique morphology of the pennoned spines, which have a stem with a cup-shaped distal end and a long pointed tip which is easily broken at the base. The two species differ from each other genetically but morphologically are very similar and can only be distinguished by small paired melanophores on the ventral side of the anterior chaetigers which are present in P. diopatra from the Asian Pacific and absent in P. kuwaiti sp. nov. from the Arabian Gulf.
Larvae of P. kuwaiti sp. nov. appear similar to those of P. arabica (see . Both occur in the plankton at the same time, have gastrotrochs on chaetigers 3, 5, 7 and 11, and are intensely pigmented with yellow and black pigments. They differ in that the larvae of P. arabica may postpone metamorphosis in the absence of substratum and attain 1250 µm long with 23 chaetigers, whereas the largest larvae of P. kuwaiti sp. nov. caught from the plankton were 750 µm long with 14 chaetigers. The developed 15−17-chaetiger larvae of P. arabica have one to three yellow ramified chromatophores on each palp, paired dorsal melanophores from chaetigers 6−7, dorso-lateral melanophores from chaetiger 4 to chaetigers 6−10, and one pair of small melanophores in lateral peristomial lips. In P. kuwaiti sp. nov., chromatophores on palps appear in juveniles; the developed larvae have paired dorsal melanophores from chaetiger 3 onwards, and dorso-lateral melanophores from chaetigers 3−4 onwards; melanophores in lateral peristomial lips are absent. Pseudopolydora kuwaiti sp. nov. is the only species of the genus inhabiting soft sediments as well as boring into mollusc shells and dead corrals encrusted by coralline algae. Our genetic analysis confirmed the conspecificity of the tube-dwelling and shell-boring individuals (Fig. 2).

Diagnosis
Prostomium anteriorly incised, with two short rounded lobes. Caruncle to end of chaetiger 5. Fine black pigment scattered on distal part of palps.

Etymology
The species name refers to fine black pigment scattered on the distal part of palps in adults.

Description
All types: anterior fragments of females in good condition with palps; one 25-chaetiger fragment with a few small newly regenerated posterior achaetous segments with a small bilobed pygidium. Holotype: 39-chaetiger anterior fragment about 10 mm long, broken in the middle (Fig. 17A). Specimens up to 10 mm long, 1 mm wide with 45 chaetigers; when complete, worms apparently up to 15 mm long with at least 60 chaetigers. No pigmentation on body; fine black pigment scattered on distal part of palps (Figs 17A, C, E, 18C). Prostomium anteriorly incised, with two short rounded lobes (Figs 17C-E, 18A-B), posteriorly extending to end of chaetiger 5 (end of chaetiger 4 in holotype) as a low caruncle. Peristomium with two lateral lips as thick folds in front of large ventral lip (Fig. 17A). Short occipital antenna present on caruncle between palps (Fig. 18C). Two pairs of black eyes arranged trapezoidally, comprising one pair of median eyes, and one pair of slightly larger lateral eyes situated anteriorly and set wider apart. Palps as long as 10-15 chaetigers, with frontal longitudinal groove lined with fine cilia, and short compound non-motile cilia arising directly from palp surface sparsely arranged in line on sides of groove and sparsely scattered on lateral and abfrontal palp surfaces.
Chaetiger 5 same in size as chaetigers 4 or 6, with dorsal superior capillaries, two kinds of notopodial spines arranged in a double U-shaped row, and ventral capillaries; notopodial postchaetal lamellae absent; neuropodial lamellae same as on chaetigers 4 and 6 (Figs 17A-B, 18C). Dorsal superior capillaries shorter and fewer than those capillaries on chaetigers 4 and 6. Ventral capillaries same in size, number and arrangement (in three groups) as those on chaetigers 4 and 6. Upper posterior part of double U-shaped row of spines slightly lower than upper anterior part. Newly developed spines in posterior upper part of U-shaped row slightly larger than older spines in anterior upper part of row. Outer (anterior-row) notopodial spines up to 12 in a series, with distal part of stem enlarged, with concavity on top and large triangular tooth on its side directed upwards and facing towards the inside of the U-shaped row of spines; fine bristles arising from concavity forming long flag-like pointed transparent tip which usually broken in worn old spines in anterior upper part of row ( Fig. 19A-B, D). Inner (posterior-row) notopodial spines up to nine in a series, falcate, with short rounded distal part geniculate, with subdistal bulbous swelling bearing very short fine bristles and facing towards the inside of the U-shaped row of spines (Fig. 19A, C-D).
Hooks in neuropodia from chaetiger 8, up to 30 in a series, not accompanied by capillaries. Hooks bidentate, with upper tooth closely applied to main fang; upper part of shaft with constriction; lower part of shaft bent at right angle ( Fig. 19E-F).
Branchiae from chaetiger 7 to chaetiger 42, up to 36 pairs, on chaetiger 7 1.5-2 times as short as those on chaetiger 8, full-sized from chaetigers 10-11, fewer in small individuals, free from notopodial Pygidium bilobed, with two semi-oval lateral lobes, white due to great number of spindle-shaped glandular cells with striated content. Subspherical to irregular shape glandular cells with striated content present on dorsal side of chaetigers. Cells few on anterior and posterior chaetigers, forming distinct paired gatherings from chaetiger 12 to chaetigers 30-35 (Figs 17D, 18A). Glandular pouches in neuropodia from chaetiger 1, largest and paired in each neuropodium in chaetigers 6 and 7, single in other neuropodia (Fig. 19E).
Digestive tract without ventral buccal bulb and gizzard-like structure, without pigmentation. Nephridia from chaetiger 4 onwards. Distal parts of nephridia on female fertile chaetigers enlarged, containing glandular cells which absorbing MG and therefore well seen in fixed specimens from chaetiger 13 (Fig. 18A, E-F). Nephridia of chaetiger 13 used to release gametes from chaetiger 12. In female fertile chaetigers, paired nephridia on each chaetiger opening to exterior via common middorsal nephridiopore anteriorly to nototroch (Fig. 18F).

MG staining
Intensely stained anterior part of prostomium in front of eyes, dorso-lateral sides of peristomium, scattered glandular cells on dorsal side of chaetigers 7-11, and paired gatherings of glandular cells on dorsal side from chaetiger 12 to chaetigers 30-35. Weakly stained lateral sides of chaetigers; scattered glandular cells on ventral side of chaetigers (Figs 17-18).

Habitat
Adult P. melanopalpa sp. nov. inhabit silty tubes in the muddy intertidal.

Reproduction
All six type specimens of P. melanopalpa sp. nov. are females with oocytes from chaetiger 12 to chaetigers 32-40. Paired ovaries are attached to the segmental blood vessels in fertile chaetigers (Fig. 19G). The developed coelomic oocytes are about 120 µm in diameter, with smooth thin envelope less than 1 µm thick.

Remarks
Adult P. melanopalpa sp. nov. share a series of characters with members of the P. antennata-complex (see below in the Discussion). They differ, however, from other species of this complex by having black pigment on the distal part of palps, and the prostomium with two rounded lobed instead of pointed fronto-lateral extensions (see Simon et al. 2019: table 1).

Etymology
The species name refers to the large number of heavy spines in the notopodia of chaetiger 5.

Holotype
About 10 mm long, 1 mm wide for about 40 chaetigers with small regenerating posterior chaetigers and pygidium. Pigmentation absent on palps and body. Prostomium anteriorly bifurcated, with two long, pointed fronto-lateral processes (left process broken) (Fig. 20D-E), posteriorly extending to end of chaetiger 6 as a low caruncle (Fig. 20B-C). Long cirriform occipital antenna present on caruncle between palps. Two pairs of black eyes arranged trapezoidally. In life, palps longer than body, flexible, with frontal longitudinal groove lined with fine cilia, and short compound non-motile cilia arising directly from palp surface arranged on sides of groove and sparsely scattered on lateral and abfrontal palp surfaces.
Chaetiger 1 well separated from peristomium, with short winged capillaries in neuropodia and well developed cirriform noto-and neuropodial postchaetal lamellae; notochaetae absent. Chaetiger 2 notochaetae all slender capillaries with narrow limbation. Anterior-row notopodial capillaries on chaetiger 3 with slightly enlarged wing; capillaries on chaetiger 4 with wing slightly wider than on chaetigers 3, intermediate between those on chaetigers 3 and 5. Anterior-row capillaries on chaetigers 3, 4, 6 and 7 arranged in J-shaped series; posterior-row capillaries on these chaetigers arranged in vertical series (Fig. 20A). Posterior notopodia with a few long alimbate capillaries arising from elongated fleshy notopodial lobes. Posterior neuropodial lobes also elongated and fleshy, each with a series of hooks on top.
Chaetiger 5 as same in size as chaetigers 4 and 6, with dorsal superior capillaries, two kinds of dorsal spines arranged in a double U-shaped row, and ventral capillaries; postchaetal lamellae well developed in both rami, same as on chaetigers 4 and 6 (Figs 20A-B, 21A). Dorsal superior and ventral capillaries similar in size, number and arrangement to those on chaetigers 4 and 6 (Fig. 21A). Outer (anteriorrow) notopodial spines with distal part of stem curved, gradually narrowing, with wide, distally pointed limbation ( Fig. 21B-C  Hooks in neuropodia from chaetiger 8, up to 30 in a series, not accompanied by capillaries. Hooks bidentate, with upper tooth closely applied to main fang; shaft with constriction on upper part and lower part bent at right angle ( Fig. 21D-E).
Nototrochs from chaetiger 5 onwards, composed of single rows of cilia. On branchiate chaetigers, nototroch cilia long, arranged in transverse lines and extending onto branchiae; on posterior abranchiate chaetigers, cilia arranged in U-shaped bands, with arms directed posteriorly. Nototrochs on chaetigers 5 and 6 interrupted in the middle by caruncle; nototrochs on succeeding chaetigers complete. Intersegmental ciliation absent.
Pygidium bilobed, with two semi-oval lateral lobes, white due to numerous spindle-shaped glandular cells with striated contents. Glandular pouches in neuropodia from chaetiger 1, largest and paired in chaetigers 6 and 7, single in other neuropodia.
Digestive tract without gizzard-like structure and pigmentation.

MG staining
Weakly stained prostomium, caruncle, dorsal side of peristomium and ventral side of seven anterior chaetigers. Intensely stained ventral side from chaetiger 8 (Fig. 20A, E-F). As usual for other species, pale greenish staining on the anterior chaetigers and on the dorsum disappeared soon after placing the specimen into clean ethanol; intense violet staining on the ventral side from chaetiger 8, on the contrary, remained for hours (Fig. 20F).

Habitat
The only individual of P. multispinosa sp. nov. was found in a silty tube in a muddy sand intertidal habitat.

Reproduction
Pseudopolydora multispinosa sp. nov. is probably gonochoristic. The holotype is a female without oocytes but with glandular cells in the wall of the distal part of enlarged nephridia typical for female fertile chaetigers in Pseudopolydora worms. The enlarged nephridia open to the exterior via single middorsal nephridiopore, appearing first on chaetiger 13. The oocytes may thus develop from chaetiger 12 onwards.

Remarks
Pseudopolydora multispinosa sp. nov. is unusual among Pseudopolydora in having numerous spines in the notopodia of chaetiger 5 (up to 32 in the anterior row and 27 in the posterior row), well developed notopodial postchaetal lamellae on chaetiger 5 (as same as on chaetigers 4 and 6), and branchiae posteriorly arranged beyond the middle of the body. The majority of Pseudopolydora adults have notopodial postchaetal lamellae on chaetiger 5 totally lacking or greatly reduced, and branchiae limited to the anterior half of body. The only known individual of P. multispinosa sp. nov. has a unique pattern of MG staining: an intensely stained ventral side of the body from chaetiger 8 onwards, which remained for hours after placing the specimen into clean ethanol.

Pseudopolydora in the Arabian Gulf
Only two species of Pseudopolydora: P. antennata and P. paucibranchiata were reported from the Arabian Gulf and the Arabian Sea until recently (see review by Wehe & Fiege 2002;Joydas et al. 2015). Our previous work (Al-Kandari et al.  and the present study discovered seven species occurring in the intertidal and shallow waters of Kuwait. Pseudopolydora achaeta is reported for the Arabian Gulf for the first time. The records of P. antennata, originally described from the Tyrrhenian Sea, Gulf of Napoli, Italy, is herein confirmed for Kuwait (see comments below). The records of P. paucibranchiata were not confirmed but a new species, P. arabica, morphologically very similar but genetically different from P. paucibranchiata, was described by  instead. Four new species: P. auha sp. nov., P. kuwaiti sp. nov., P. melanopalpa sp. nov., and P. multispinosa sp. nov. are described in the present study. The distribution of the new species should further be investigated to understand whether they are endemics of the Gulf or occur over a wider range in the Indian Ocean or elsewhere.

Pseudopolydora antennata
Taxonomic confusions resulted from the opaque concepts of the identity of P. antennata. Morphological characteristics of this species were discussed in details by Simon et al. (2019) and Radashevsky (2021). The latter study for the first time raised a question about the origin of Pseudopolydora and suggested that early members of the genus might have "originated and diverged in the Indo-West Pacific and later either naturally arrived in the Atlantic Ocean and the Mediterranean (or vice-versa) or were transported to these regions by human activities (as was P. paucibranchiata)" (Radashevsky 2021: 19). Pseudopolydora antennata, which was first described outside of its native range, was suggested as one of those invaders in the Mediterranean. Pseudopolydora worms from the Arabian Gulf fit the morphology of P. antennata, but, in light of a series of sibling and cryptic species discovered within the genus, the ultimate conclusion about the conspecificity of the Mediterranean and Arabian populations should be inferred from a genetic comparison of the corresponding individuals. No sequences of P. antennata from the Tyrrhenian Sea or any other part of the Mediterranean are yet available.

Phylogenetic relationships and complexes of species of Pseudopolydora
The results of the Bayesian analysis of the combined dataset of four genetic markers are congruent with the morphological characteristics of the species. The analysis showed that the worms sharing diagnostic features of P. antennata, P. diopatra and P. paucibranchiata form three monophyletic groups which we refer to as species complexes (Fig. 2). Members of the P. antennata complex share the following characters: 1) palps without chromatophores, 2) the prostomium anteriorly bifurcated, 3) occipital antenna present, 4) chaetiger 5 similar in size to chaetiger 4, dorsally not overlapping anterior part of chaetiger 6, 5) chaetiger 5 spines arranged in a U-shaped double row, 6) chaetiger 5 anterior-row notopodial spines with enlarged distal end having concavity on top and large triangular tooth on its side directed upwards, and fine bristles arising from concavity and forming long flag-like pointed geniculate transparent tip, 7) chaetiger 5 posterior-row notopodial spines falcate, with short rounded distal part geniculate, with subdistal bulbous swelling bearing very short fine bristles and facing towards the inside of the U-shaped row of spines, 8) the pygidium bilobed, with two semi-oval lateral lobes, and 9) glandular pouches paired in each neuropodium in chaetigers 6 and 7.
The specified complexes do not include all the species of Pseudopolydora described up to now. The characters annotated above are not apomorphies (or, at least, not all of them) but merely sets of features shared by members of each complex. The phylogenetic relationships between species of Pseudopolydora and the evolutionary transformation of characters within this genus will be investigated in a future analysis of their morphological, reproductive and ecological characteristics. Simon et al. (2019) for the first time used the proportion of branchiate chaetigers as one of the fixed diagnostic characteristics to delineate species of Pseudopolydora. However, the analysis of the distribution of branchiae in three species examined in the present study showed that this proportion changes during ontogenetic growth, and two different kinds of correlations can be distinguished (Fig. 5C-E). In P. antennata, new pairs of branchiae develop faster than new chaetigers appear in the prepygidial growth zone. Consequently, the proportion of branchiate chaetigers increases during ontogeny. In individuals with more than 30 chaetigers, branchiate chaetigers exceed the middle of the body (Fig. 5C). On the contrary, in P. arabica and P. kuwaiti sp. nov., new pairs of branchiae develop slower than new chaetigers appear in the prepygidial growth zone. Small individuals of these species have branchiae occurring until the middle of the body, while large individuals have branchiae on only the first ⅓-2/5 part of the body (Fig. 5D-E). It seems likely that the former kind of positive correlation is characteristic for the species of the P. antennata complex, while the latter kind of negative correlation is characteristic for the species of the P. diopatra and P. paucibranchiata complexes. This assumption should be verified in further studies. Simon et al. (2019) for the first time successfully used methylene green staining to delineate species of Pseudopolydora. They showed that staining patterns anterior to chaetiger 6 supported the division of the examined specimens into five species, although "all species had similar patterns in dorsal staining posterior to chaetiger 6" (Simon et al. 2019: 19). In the present study, we found unique, species specific patterns of MG staining posterior to chaetiger 6 in P. arabica and P. multispinosa sp. nov. The MG staining in other species was either weak or similar to patterns revealed by Simon et al. (2019). No stain was applied to P. kuwaiti sp. nov.

Habitat
All previously studied species of Pseudopolydora live in sandy or silty tubes in soft sedimens or attached to hard substrata. Adult P. kuwaiti sp. nov. inhabit silty tubes in muddy sand intertidally and in shallow waters and bore in live mollusc shells, dead corals and shell limestone encrusted by coralline algae. No apparent morphological differences were found between the tube-building and shell-boring worms and the genetic analysis confirmed their conspecificity ( Fig. 2; Table 2). Pseudopolydora auha sp. nov. was found in burrows in shell limestone encrusted by coralline alga but it was not certain if the worms made the burrows themselves or secondarily occupied empty holes made by other organisms.
Among spionids, species able to bore in hard substrata are present only among some genera of the tribe Polydorini Benham, 1896. Because (1) Polydorini is an advanced clade in the phylogeny of Spionidae, (2) all the other spionids are either free crawlers or tube dwellers (TD), and (3) the shell borers (SB) occur in every polydorin genus, Radashevsky & Pankova (2013) suggested that the ability to bore evolved more than once within Polydorini. The unique example of shell-boring P. kuwaiti sp. nov. among predominantly tube-dwelling species of Pseudopolydora supports this suggestion.
Remarkably, all shell-boring polydorins are innately able to build tubes. Settled larvae of those species first make silty tubes and then start boring into the substratum. Adults make pairs of tubes that extend from two joined apertures of their burrow. Nevertheless, the majority of polydorins are strictly constrained to only one mode of life, which attribute can be used as an additional character to facilitate identification of species. Conspecific SB and TD individuals were reported in several species of Polydorini based on their morphological characteristics (see Radashevsky & Pankova 2013). Genetic support for their conspecificity is available to date only for Dipolydora carunculata (Radashevsky, 1993), Polydora