Lophogastrida and Mysida (Crustacea) of the “DIVA-1” deep-sea expedition to the Angola Basin (SE-Atlantic)

Three species of Lophogastrida and eight Mysida are documented for samples from 5161–5497 m bottom depth in the Angola Basin. Previously known latitudinal ranges are extended southward for fi ve species, and bathymetric ranges extended beyond 5000 m for six species. Upon revision of the subfamily Petalophthalminae (Mysidae), four species previously attributed to the genus Petalophthalmus are integrated into Ipirophthalmus gen. nov. as I. liui gen. et comb. nov., I. caribbeanus gen. et comb. nov., I. oculatus gen. et comb. nov., and I. macrops gen. et comb. nov., mainly based on the structure of eyes and presence of setae on the telson. Petalophthalmus cristatus sp. nov. is described based on its reduced cornea and the structure of eyestalks, rostrum, mandibles, and telson. The structure of mouthparts, foregut and maxillipeds suggests an omnivorous mode of life. The diagnosis of the tribe Calyptommini (Mysidae: Erythropinae) is widened to cover the 3-segmented, uniramous fourth male pleopod and the non-incised eyeplate with horn-like rudiments of eyestalks in Abyssomysis cornuta gen. et sp. nov. The structure of mandibles, foregut, and second maxilliped suggest detritus feeding in this species. Keys to the Calyptommini and Petalophthalminae are given.


Introduction Terminology and taxonomy
Terminology and taxonomy as in Wittmann et al. (2014). Drawings of sex-specific features are labelled by symbols for females or males, respectively, in . The absence of such labels implies absent or unapparent sex-specific differences.

Description
All features of the diagnosis. Body moderately slender, cephalothorax contributes 34% total length, pleon 46%, telson 20%, carapace 29%. CarapaCe ( Fig. 1A-C). Leaving ultimate 1.5 thoracic somites mid-dorsally exposed; antero-lateral edges rectangular, terminally rounded; small bulge behind rostrum; cervical sulcus weak but distinct. Posterior margin of carapace emarginated; emargination unapparent upon inspection in situ (Fig. 1A-B), but well visible in carapace detached and expanded on slide (Fig. 1C). eyes ( Fig. 1A-C). No external corneal structures visible. Definite ocular papilla on mesial margin visible in dorsal view ( Fig. 1B-C) but not so in lateral view (Fig. 1A). Eyes 1⁄₅ carapace length; dorsoventrally weakly compressed with length 5⁄2 maximum width in dorsal view vs 7⁄2 maximum height in lateral view. Eyes appear wider in Fig. 1C due to pressure exerted by cover glass. antennula ( Fig. 1A-B, D-E). Trunk long and slender, 1.1 times length of carapace. Three-segmented, whereby segments 1-3 are 46-48%, 35-37%, and 17% total trunk length, respectively. Basal segment with small spine in subterminal position. Terminal margin of distal segment dorsally produced in a triangular, setose lobe. Width at basis of outer flagellum is 4⁄ 5 that of inner flagellum. Width of longitudinal crista near basis is ¼ width of outer flagellum, length is ⅔ width of outer flagellum. antenna ( Fig. 1A-B, F). Sympod on its mesial margin with small lobe bearing a minute spine at tip. Peduncle 3-segmented, whereby segments 1-3 contribute 9-11%, 40-44%, and 47-49% to total peduncle length. Mesial margin of basal segment with small lobe ending in a large spine. Flagellum tiny, short, 6-segmented. Antennal scale extends slightly beyond antennal peduncle, i.e., clearly beyond basal segment of antennular trunk. Scale 0.6 times length of carapace; scale length six times maximum width. Scale subdivided by a slightly oblique articulation; apical segment 4-5% total scale length. labrum ( Fig. 1G-H). Roughly trapezoidal in ventral view; caudal margin densely setose, with three bunches of slender spines facing the mandibles. mandibles ( Fig. 2A-C). Palpus 3⁄2 length of carapace; segments 1-3 contribute 8-9%, 58-60%, and 32-33% to total length, respectively. Basal (first) segment laterally with spine on the tip of a triangular lobe. Analogous, smaller structure set subbasally on outer margin of median segment; apart from that, outer margin smooth all along; inner margin almost all along with numerous smooth, in part spine-like, long setae with shorter ones in between. Terminal segment reflexed, together with seven very long, strong setae rendering the palpus a large prehensile appendage. Left mandible with large processus incisivus ending in three blunt teeth; lacinia mobilis well developed with four, mostly less blunt teeth; pars centralis reduced to a single finger-like process; processus molaris with weakly sclerotized ridges and with ventral and dorsal ridges of stiff bristles. Right mandible with large processus incisivus bearing three rounded lobes representing surrogates of teeth; no lacinia mobilis developed; pars centralis reduced to a finger-like process; processus molaris trilobate. labium (Fig. 2D). Normal, bilobate. Inner face of the lobes all along densely setose, with dense series of stiff bristles (short spines) extending from half to ⅔ length from basis. maxillula ( Fig. 1I-J). Small, distal segment shorter than exopod of maxilla. This segment terminally with nine unilaterally serrated spines, namely four smaller ones on caudal face plus five large ones on rostral face. No pores visible. Endite subterminally with three setae barbed along their median to subterminal portions, basal third smooth. Terminal margin of endite with two additional setae showing longer barbs from their basal to subapical portions; additional two such setae subterminally on ventral margin.   (Fig. 2E). Sympod with striking, very long, barbed seta on inner face; this seta more densely barbed along its distal half; sympod with well-projecting endite, the latter setose on its distal half. Palp 2-segmented, with stout proximal and slender distal segment; the latter contributing 60% to total length, its length three times maximum width; distal segment densely setose along distal and inner margins, outer margin smooth; its most distal setae plumose, longer than distal segment. Basal segment of palp with densely setose, weakly projecting endite. Exopod slender, length four times maximum width; exopod reaches to mid of distal segment of palp; exopod densely setose all along outer margin and on distal half of inner margin; proximal quarter of inner margin with dense series of minute hairs. Foregut ( Fig. 4A-E). Object in Fig. 4A dorsally opened, laterally unfolded and expanded on slide; posterior half distorted. Gross morphology as described by Kobusch (1998) and for Petalophthalmus armiger. Gastric mill restricted to anterior half of the cardiac portion of the foregut. Anterior part of each laterale with 29-30 smooth spines (Fig. 4C) increasing in length posteriorly. Spines in ribbon-like arrangement on posterior half of this part of the lateralia. Posterior part of the right lateralia broken in this object; left ones intact, with linear series of throughout slender spines (Fig. 4B), the spines smooth except for tuft of acute spinules at tip. Dorso-lateral infoldings of the cardia each with antero-posterior series of setae, anteriorly starting with numerous long, smooth setae, continued by shorter setae with stiff bristles on apical half, and finally by 12-14 spine-like setae with continuous gradient from basally widened, oblong setae ( Fig. 4D) all along with dense series of acute bristles (spinules) to basally smooth sheets ( Fig. 4E) with oblique terminal margin densely occupied by such bristles. Pyloric portion of foregut with two slender, posteriorly directed, soft processes resembling spines (below drawing plane in Fig. 4A). Foregut empty in this specimen. thoraCopods in general . Total length of exopods as well as their flagella increase from exopod 3 to 7, whereas that of exopod 8 ranges between nos 3 and 4. Basal plate of exopods slender, lateral margins parallel, disto-lateral edge rounded; length four times width in exopods 3-7, only three in exopod 8; basal plates 3-4, 8 with roughly equal length, plates 5-7 about 1.3 times longer. Flagella densely setose. Total length decreases from endopods 5 to 8. Dactylus 1-2, 5-8 not fused with claw (nail), endopods 3-4 without claw. Marsupium formed by seven pairs of oostegites emerging as setose epipods on thoracopods 2-8.

Etymology
The generic name is a Latinized noun with masculine ending, formed as a condensate of the Ancient Greek adjective ύπηρετικός (functional) with the noun ὀφθαλμὸς (eye), underlining the difference from the reduced visual elements in the genus Petalophthalmus.

Diagnosis
Petalophthalminae with antennal scale setose all around; eyes with well-developed, functional cornea; anterior margin of carapace with rostrum flanked by a pair of processes; thoracic endopods 5-8 slender, each with more than three segments; flagellum of thoracic exopods 3-8 with 10-14 segments; male pleopods biramous, female pleopods uniramous or biramous; telson large, subquadrangular, terminal margin truncate without or with distinct mid-terminal emargination, terminal margin with two submedian groups of small teeth, with 7-8 large, toothed spines and 1-2 pairs of long setae, these setae arising from the emargination, if present, otherwise from homologous submedian position.

Type locality
Not specified by H. Milne Edwards (1837), who reported this species upon first description from the North Atlantic anywhere between Madeira and the Antilles.

Nomenclatorial note
W.M.  did not explicitly state a derivatio nominis, however he noted on page 45 "From the type it differs rather markedly in the form of the telson...". This suggests that he derived the species name from the narrow telson. Be that as it may, 'cauda' is a Classical Latin noun, while the respective adjective would be 'caudalis'. Therefore, the species name 'tenuicauda' is to be treated as an amalgamated noun in apposition rather than an adjective. According to Article 31.2.1 of the Nomenclatorial Code (ICZN 1999), a noun in apposition is to be retained, with gender ending unchanged.

Type locality
Not stated by W.M. . Upon first description, he reported materials from six stations off Ireland, NE-Atlantic, 50-52° N, 11-12° W, depth 447-770 fathoms (817-1408 m). One additional sample was stated by him on page 44, but accidentally omitted from the station data on page 69, finally indirectly quoted by Tattersall & Tattersall (1951) as pertaining to the samples from the Faroe Islands.

Type locality
Not stated by Holt & Tattersall (1905). Upon first description, they reported material from seven stations at the NE-Atlantic slope, depth 181−382 fathoms (331−699 m). A rough estimate by the present author suggests that the stations are in the range of 50-54° N, 10-13° W.

Type locality
Not stated by Holt & Tattersall (1905). They reported materials from four stations off Ireland, depth 199-382 fathoms (363-696 m). A rough estimate by the present author suggests that the stations are in the range of 52-54° N, 12-13° W.

Diagnosis
Revised from Wittmann et al. (2014) in order to receive Abyssomysis gen. nov. Eyeplate without distinct median cleft, plate mainly formed by a transverse median bridge, eyes rudimentary or absent. Antennal scale well developed, its smooth outer margin ending in a tooth; antennal gland hypertrophic. Labrum normal, symmetrical. Females with 2-3 pairs of oostegites. Pleopods uniramous, reduced to unsegmented endopods in both sexes, except that the fourth male pleopod may be sub-segmented in certain species. Uropods normal. Telson entire, lateral margins not serrated but for the most part or entirely smooth, terminal portions with spines, no setae.

Type species
Abyssomysis cornuta gen. et sp. nov. by monotypy and present designation.

Diagnosis
Calyptommini with eyeplate formed by a transverse, median lamina (= ocular symphysis) connecting separate, rudimentary eyestalks; lamina not incised, visual elements absent. Carapace normal, anteriorly rounded. Antennula with well-developed, setose appendix masculina; proximal segment of trunk without ventral carina. Antennal sympod without spiniform extension; antennal scale well developed, entire, its smooth outer margin ending in a prominent tooth, terminal lobe well-developed. Labrum with small, knob-like rostral process. Mandibles with distinct molar part; lacinia mobilis present on left mandible, absent or vestigial on right mandible. First thoracopods normal; basis of endopod with long, setose endite. Females with three pairs of functional oostegites. Penes normal. Pleopods of both sexes reduced to unsegmented, plate-or rod-like endopods, except that the fourth male pleopod is sub-segmented. Both rami of the uropods unsegmented, setose all around, without spines. Telson short, terminally truncate; lateral margins smooth, terminal margin with spines.

Etymology
The generic name is an amalgamated noun with female ending, referring to the occurrence of this mysid in the abyssal plain.

Diagnosis
Abyssomysis gen. nov. with eyeplate in the form of a narrow, transverse band with lateral, apically acute or narrowly rounded rudiments of the eyestalks resembling horns. Anterior margin of carapace evenly rounded, upturned on the edge, forming a weakly separated rim as a rostral plate. Only distal portions of the eye rudiments extend beyond the rostral plate. Antennular trunk longer than antennal scale in males, shorter than scale in females. Male trunk with comparatively large appendix masculina bearing a dense brush of long setae. Antennal peduncle not reaching beyond antennal scale in females, whereas reaching well beyond in males. Left mandible with lacinia mobilis well developed in both sexes. Right mandible without lacinia mobilis in females, but with lacinia reduced to a tiny, slender, apically toothed spine in males (Fig. 6D). Median segment of palpus of both mandibles with only three setae in terminal position, remaining portions of palpus smooth in females, whereas distal ⅔ of the median segment showing dense series of at least 14 setae in males. Females with pleopods 1-5 continuously increasing in length caudally. Males with pleopods 1-3, 5 increasing in length caudally, pleopod 4 is not in series by being the longest; fourth pleopod 3-segmented, its basal segment contributes ⁴⁄₅ pleopod length, the two apical segments each 1⁄ ₁₀. Pleomere 6 longer than combined pleomeres 4-5. Telson 0.3-0.4 times length of pleomere 6; telson length 1.0-1.1 times maximum width near basis; terminal margin with two pairs of large, robust spines; outer robust spines shorter than inner ones.

Etymology
The species name is a Latin adjective with female ending, referring to the pronged eyeplate, well contrasting from that in the remaining Calyptommini so far known.

Type locality
Station #350 (Table 1) in the abyssal plain of the Angola Basin, 16°14′ S, 5°27′ E, depth 5433-5434 m. (Fig. 5A) All diagnostic features of males. Body length 6.6 mm, specimen not dissected. Cephalothorax without rostrum measures 40% body length, pleon without telson 51%. The great contribution by the pleon is largely due to the elongate sixth pleomere. Carapace including rostrum measures 38% body length. Carapace weakly inflated, with distinct cervical sulcus, anterior third of cephalic portion with knobbly cuticle. Carapace posteriorly emarginated, leaving half of ultimate thoracomere dorsally exposed. Eyes without visual elements. Small, horn-like rudiments of eyestalks widely set apart on a common eyeplate probably homologous to the ocular symphysis. Eyeplate dorsally covered by rostral plate, only part of eyestalk rudiments projecting from behind the rostrum in dorsal view.

Description of the holotype
antennae sensu lato (Fig. 5A). Antennular trunk is 15% body length. Trunk 1.1 times antennal scale length. Trunk conical with distally increasing width. Terminal segment is 1.4-1.5 times width of basal segment. Transverse articulations between the three segments of the trunk. Segments 1-3 contribute 35%, 20%, and 45% to total trunk length, respectively. Appendix masculina inserts ventrally behind inner flagellum, appendix ⅓ trunk length, its setae up to ⅔. Basal segment of antennular trunk not produced at outer distal corner. Setose lobe dorsally in about median position shortly behind the anterior margin of each segment. Lobe of distal segment with four barbed setae and transverse series of four forward projecting teeth. Tooth size increases laterally. Antennal scale 1⁄₇ body length; its length four times maximum width. Apical lobe contributes 7% to total scale length, lobe wider than long, with seven plumose setae. Antennal peduncle ⁶⁄ ₅ scale length, segments 1-3 are 0.1, 0.4, and 0.5 times peduncle length, respectively. Sympod with long, linguiform, terminally rounded lobe on dorsal face.
mouthparts. Mandibular palpus slender. Its proximal segment, short, smooth all around. Median segment contributing 3⁄ ₅ to palpus length. Distal ⅔ of median segment with dense series of obliquely outward-backward projecting setae. Distal segment with dense comb of setae. Terminal segment three times as long as wide, ⅓ palp length.
thoraCopods. Endopod 2 large, carpus and propodus fused, dactylus with medially projecting brush of barbed setae. Sympods 3-8 with anteriorly facing, terminally rounded lobe close to (broken) insertion of praeischium of endopod (basis fused with sympod). Paired penes short, knob-like. pleon (Fig. 7D). Sixth pleomere 1⁄ 6 body length. Pleomeres 1-5 are 0.3-0.4 times length of pleomere 6. Scutellum paracaudale sinusoid, evenly rounded. Pleopods with setose edge as found in most species of the subfamily Erythropinae. Pleopods 1-3 reduced to setose plates, pleopods 4-5 to setose rods. Pleopod 4, when stretched posteriorly, reaches to the distal seventh of pleonite 6. Tip of pleopod 4 with two setae. The larger seta about as long as the combined two distal segments. Median and basal segments each ending in a pair of shorter setae. Basal segment with two additional setae in subterminal position and other about eight setae ⅓ segment length from basis. Uropods setose all around, exopod terminally rounded, endopod about as long as exopod, each 1.2 times length of sixth pleomere, and extending 0.7 times their length beyond telson. Telson with smooth lateral margins. Potential presence of a minute median tooth (as in some paratypes, Fig. 7I) on terminal margin could not be checked without dissection. The inner robust spines on terminal margin measure 45% telson length, the outer ones 37%. For additional details of telson see above Diagnosis and below Description of paratypes.
antennae sensu lato (Fig. 5A-C, H). Antennular trunk strongly dimorphic, size is 10% body length in females, 15-17% in males. Trunk 0.7-0.8 times antennal scale length in females, 1.1-1.2 in males. Trunk almost cylindrical in females, whereas conical with distally increasing width in males. Terminal segment is 0.8-1.0 times width of basal segment in females, 1.3-1.4 in males. Segments 1-3 contribute 43-46%, 19-20%, and 35-39% to total trunk length in females; 32-38%, 18-20%, and 43-50% in males, respectively. Antenna (Fig. 5H) non-dimorphic, scale length 3-6 times maximum width. Apical lobe contributes 5-9% to total scale length, lobe with 6-8 plumose setae. Antennal peduncle 0.9-1.0 times scale length in females, 1.1-1.3 in males. mouthparts (Fig. 6A-H). Labrum with frontal lobe bearing a small sinusoid rostral extension; the two posterior lobes terminally with median field of minute, stiff bristles, dorsally with cuticular ridges (dashed lines in Fig. 6A). Median segment of mandibular palpus 5-6 times as long as wide. Pars incisiva of both mandibles with 5-6 moderately sized teeth, processus molaris strongly cuticularized. Lacinia mobilis of left mandible with six teeth; for right lacinia see Diagnosis. Pars centralis of left mandible with 6-7 'spines' bearing stiff bristles or denticles. Right centralis with 8-9 'spines' of that kind distally projecting from a large median lobe. Paired labia mostly smooth, inner terminal edge with small field of stiff setae, lacking spines or teeth. Lateral portions of labia locally with knobbly surface similar to that present in anterior portions of carapace. Distal segment of maxillula (Fig. 6F) ending in 10-14 slender, acute, smooth spines; subterminally with 4-6 small, basally barbed setae on caudal face. Mesial margin of endite with dense set of setae, 3-4 of which are strong, extending beyond the distal segment, the latter setae on their distal half barbed by stiff bristles. Maxilla (Fig. 6G-H) with well-developed exopod, 2-segmented palp and four strongly setose endites arising from trunk; exopod elliptical, extending only slightly beyond basal segment of palp, terminally with five plumose setae that decrease in size laterally; apical segment of palp is 1.6-1.8 times length of basal segment; terminal margin of apical segment weakly convex, almost truncate, distally with 4-5 modified spine-setae armed with various numbers of spinules and barbs (Fig. 6H); outer edge with a subbasally barbed seta which is longer than the apical segment; inner margin of this segment mostly smooth, with spines and setae only along its distal third, outer margin well setose; first (basal) endite with long, terminally barbed seta overreaching the palp (measured without setae and spines); endites 2-3 each with normal setae plus 3-6 modified setae whose barbs are to varying degree transformed to denticles; endite 4 with smooth or barbed setae only. Foregut (Fig. 8E-H). Entire foregut with setae and moderately-sized spines only, no clavate spines or heavily armed spines or teeth. Primary cardiac filter formed ventrally by combs of stiff setae at inlet of oesophagus. Cardiac part of foregut with series of short, smooth spines with setae in between, on mesial and terminal margins of lateralia; with slender, microserrated spines (Fig. 8F) on disto-lateral portions of lateralia; and with apically pronged, proximally smooth, slender spines (Fig. 8G) on lateral portions. Stouter, centro-apically serrated spines (Fig. 8H) of various sizes on posterior part of lateralia, on dorso-lateral infolding, and on disto-lateral edges of superomedianum. Lateralia with weak and superomedianum with even weaker but distinct setation in addition to armature of spines. No setae present in pyloric part of foregut.

Distribution
Recorded at stations #344, #348 and #350 in the abyssal plain of the Angola Basin, range 16°13′ S to 16°17′ S, 4°40′ E to 5°27′ E, depth 5430-5460 m ( Table 1). All positive samples were taken with selfclosing epibenthic sledge. At least one sample was captured in the lower, another one in the upper net, suggesting that at least some specimens were positioned shortly above the sediment.

Eggs and larvae
Considering only non-damaged specimens, the female with 8.0 mm body length carried eight eggs with diameters of 0.47-0.50 mm, the female with 7.9 mm carried six nauplioid larvae at substage N2 with 1.3-1.5 mm body length. Nauplioids (Fig. 8A-D) with smooth cuticle all around, except for 5-10 slender spines at tip of antennula and a pair of cercopods in subapical position on abdomen. Each cercopod armed with a fan of numerous acute, slender spines.

Gut contents
Two dissected foreguts contained mainly fine, unidentifiable material, mineral particles and fragments of detritus; maximum size of particles 12 μm. These components also found in midgut and hindgut, with greater fraction of mineral particles also in oesophagus. There were no traces of meiofauna or macrofauna in oesophagus and foregut.

Horizontal distribution
The three species of Lophogastrida and one species of Mysida collected by "DIVA-1" in the Angola Basin show panoceanic or circumtropical distribution, respectively, with roughly symmetrical latitudinal limits: 66° N-74° S in Neognathophausia gigas, 66° N-64° S in Eucopia grimaldii, 45° N-40° S in Siriella thompsonii, and 34° N-22° S in Fagegnathophausia gracilis. The new record at 22° S made the latitudinal limits of the latter species roughly symmetrical. The limits of 51° N-54° S are also symmetrical in the Mysida species Boreomysis bispinosa but confined to the Atlantic Ocean including the Atlantic sector of the Southern Ocean.
Four of the Mysida species reported here have previously been known only from the Northern Hemisphere. Their latitudinal range is now extended southward to the Angola Basin in the SE-Atlantic. The resulting limits are 60° N-22° S in the E-Atlantic for Amblyops tenuicauda, 61° N-22° S in the E-Atlantic and Mediterranean for Paramblyops rostratus, 62° N-17° S in the E-Atlantic and Mediterranean for Pseudomma nanum, and 58° N-22° S in the E-Atlantic, W-Mediterranean and NW-Pacific (off Japan) for Parapseudomma calloplura. The data for the here first-described species from the Angola Basin are 20° S for Petalophthalmus cristatus, 16-17° S for Abyssomysis cornuta gen. et sp. nov.
Among the five here listed, non-panoceanic Mysida species from the N-Atlantic, only Boreomysis bispinosa has also been previously reported from the S-Atlantic. The new southern limits encountered in the Angola Basin are in line with the conclusions by Brandt et al. (2005) that the abyssal isopod fauna of the Angola Basin appears to be isolated from the subantarctic and South African deep fauna; and that the Walvis Ridge may represent a main distribution barrier for the abyssobenthos in the south of the Angola Basin.
The epibenthic sledge samples yielded smaller numbers of abyssobenthic Mysida species in the southern study area no. 1 compared to the northern areas 5-6 (Table 1). However, the numbers of positive stations and species were insufficient to test the significance of that pattern. Based on relatively abundant peracarid taxa -mostly isopods and cumaceans (but only a few mysids) - Brandt et al. (2005) found significantly lower species diversity in the southernmost station compared to the northern stations. They argued that the difference may reflect higher food concentrations as concluded from higher biomass and taxa numbers recorded by Kröncke & Türkay (2003) in box corer samples at the more northern stations. According to the latter authors the different yields appear to be related to different water masses north and south of the Angola-Benguela Front crossing the study area, whereby the production in the north appears favoured by the input of nutrients and suspended matter from rivers.
In analogy, the "Valdivia" expedition yielded in the Gulf of Guinea a higher diversity of Lophogastrida and Mysida with vertical hauls from 600-4000 m depth to the surface in the northern, warmer waters of the Guinea Current compared with hauls from 3000-4000 m to the surface in the more southern, colder waters of the Benguela Current (see yields in samples listed by Illig 1930: 485-486; species list supplemented by Wittmann in press).

Vertical distribution
The circumtropical lophogastrid Fagegnathophausia gracilis is essentially bathypelagic, previously known from a depth range of 500-5225 m. The panoceanic Neognathophausia gigas is meso-to bathypelagic, range 650-4023 m; Eucopia grimaldii is also essentially meso-to bathypelagic, range 200-4829 m. The bottom depths, 5161-5430 m, of the new samples in the Angola Basin are mostly greater than the previously published limits, but the actual sampling depths may have been shallower due to the use of a non-closing device (Agassiz Trawl). All five Lophogastrida species sampled by the combined "Valdivia" and "DIVA-1" expeditions in waters of the Angola Basin show a circumtropical to panoceanic distribution and are essentially meso-to bathypelagic; none are epipelagic.
In the Angola Basin the normally epipelagic Mysida species Siriella thompsonii was sampled with the Bongo net at the surface, while one juvenile specimen was taken with the box corer at 5493 m bottom depth. Catching this species with a box corer sounds unlikely but has its precedence in a single specimen recorded by Price et al. (1986) in a daytime grab sample from 46 m depth in the Gulf of Mexico. The precise sampling depth cannot be judged for Petalophthalmus cristatus taken at or over 5487-5497 m bottom depth in the Angola Basin because it was captured with a non-closing device (Agassiz Trawl).
The remaining six Mysida species in the present collection were taken on and a short distance above the sea floor of the Angola Basin with a self-closing epibenthic sledge that gives reliable information on sampling depth. Among these all five previously described species were known only from less than 5000 m depth. Among the species taken with epibenthic sledge, Pseudomma nanum was present only in the epinet, despite the fact that this species was considered suprabenthic by . Boreomysis bispinosa, Amblyops tenuicauda, Paramblyops rostratus, Parapseudomma calloplura and Abyssomysis cornuta gen. et sp. nov. appeared in both epinet as well as supranet, pointing at least partly to a suprabenthic habit. In accordance with this, B. bispinosa has already previously been taken with both benthic and pelagic devices. Sorbe & Elizalde (2013) considered Paramblyops rostratus as suprabenthic. Frutos & Sorbe (2013) sampled Parapseudomma calloplura in the near-bottom water layer.
All seven Mysida species (listed in the Introduction) sampled by the "Valdivia" expedition in waters of the Angola Basin show a circumtropical to panoceanic distribution. Among these species six are essentially meso-to bathypelagic, only Arachnomysis megalops is mesopelagic, and none are epipelagic. These seven species were not found in the "DIVA-1" material, most likely due to greater sampling depth and exclusion of pelagic species by the prevailing use of devices that keep the nets open only during contact with the sea floor.

Feeding and deep-sea adaptation
The weak grinding surface of the processus molaris in both mandibles, the reduced lacinia mobilis in the right mandible and the absence of large, heavily armed spines or teeth in the foregut (Fig. 8E−H) suggest weak abilities of Abyssomysis cornuta gen. et sp. nov. for macerating large and/or hard particles. The second thoracic endopod has no claw, but resembles a brush with a long, slender broomstick. These features point to sweeping for soft food rather than predation. Finely particulate organic material together with minute mineral particles in the foregut (examined in two specimens) confirm that the mouth-parts are used for sweeping detritus from the sediment surface. The absence of the secondary filter also favours infiltration of mineral particles from the foregut into the midgut. The maximum particle size of 12 μm in the foregut fits with sediment parameters in the study area, where Kröncke & Türkay (2003) obtained 95−99% of the sediment samples by passing through a sieve with 63 μm mesh size. As a striking analogy, the masticatory part with lacinia mobilis absent in the right mandible of Mysimenzies hadalis Băcescu, 1971 (tribe Mysimenziesini) shows an astounding similarity, almost identical with that found in A. cornuta gen. et sp. nov. females. Details about feeding are not known in the former species.
Reduction of the lacinia mobilis on one of the two mandibles is also found in at least nine bathyal to abyssal species of the genera Hansenomysis Stebbing, 1893, Bacescomysis Murano & Krygier, 1985, Petalophthalmus and Ipirophthalmus gen. nov. These three genera belong to the Petalophthalmidae and are morphologically remote from Abyssomysis gen. nov. and Mysimenzies Băcescu, 1971. In the respective species of Petalophthalmidae, the lacinia mobilis of the left or the right mandible is reduced to varying degrees (Birstein & Tchindonova 1958;Băcescu 1971aBăcescu , 1971bSan Vicente & Corbari 2015;present study). Based on the strong setation of perioral structures Băcescu (1971b) argued that Hansenomysis species are detritus feeders. This was confirmed by SEM-studies that De Jong-  performed on the processus molaris in Bacescomysis abyssalis Murano & Krygier, 1985, a species closely related to Hansenomysis. They found a smoother and more concave processus compared to that in omnivorous Mysida from bathyal to sublittoral environments, which show a phytophagous and carnivorous tendency.
The feeding type is less clear for the above-described Petalophthalmus cristatus sp. nov. because it has a trilobate processus molaris with a weaker setation in the right mandible. The broom-like thoracic endopod 5 may hint at detritus feeding. In line with this, the spines of the gastric mill are small in relation to the size of the foregut (Fig. 4A−E). The numbers and absolute sizes of the spines are not larger than those found in much smaller sized, sublittoral species of the family Mysidae, many of which show a predatory mode of life (e.g., three species of Mysidopsis G.O. Sars, 1864, studied by Wittmann & Griffiths 2018). At the same time, the prehensile, heavily armed thoracic endopods 1-2 (found in all species of Hansenomysis, Petalophthalmus and Ipirophthalmus gen. nov.) and the giant mandibular palps (in Petalophthalmus and Ipirophthalmus gen. nov.) are indicative of a predatory habit. As a possible synthesis of the divergent characteristics, P. cristatus sp. nov. may be omnivorous, probably advantageous for survival in oligotrophic, deep oceanic waters.
As an additional, striking analogy of Abyssomysis cornuta gen. et sp. nov. with species of Petalophthalmidae, all seven currently acknowledged bathyal to abyssal species of Bacescomysis and seven species of Hansenomysis show a non-incised, transverse eyeplate with well-developed horn-like extensions lacking visual elements. The remaining eleven species of Hansenomysis exhibit rounded, lappet-like or inconspicuous extensions (Faxon 1895;Băcescu 1971b;Lagardère 1983;Murano & Krygier 1985;Bravo & Murano 1997b;Casanova & de Jong 2005;San Vicente & Sorbe 2008). Due to the lack of visible sensory structures the horns and lappets are here interpreted as remnants of convergent eye reduction in dark, deep waters. This is also valid for the larger eyes with reduced cornea in the here revised genus Petalophthalmus.

Morphology of the Petalophthalminae and establishment of Ipirophthalmus gen. nov.
Only five species today, considered valid, have been described between 1875 and 1998 as pertaining to the genus Petalophthalmus. This contrasts with as many as three additional species, including the new one, in the period 2014-2020. The recent increase to a total of eight species has highlighted a previously unnoticed morphological divergence within this genus. This first group includes the type species P. armiger together with P. papilloculatus, P. papuaensis and P. cristatus sp. nov., characterized by reduced eyes, anterior carapace margin without pair of distinct submedian processes, and terminal margin of the telson with spines but without setae. The second group comprises P. liui, P. macrops, P. oculatus and P. caribbeanus with well-developed, functional cornea, anterior carapace margin with a pair of submedian processes flanking the rostrum, and terminal margin of the telson with setae and spines. The latter set of characters is shared with both so far known species of Pseudopetalophthalmus, suggesting an inconsistency between the former morphological concepts of Petalophthalmus and Pseudopetalophthalmus. In order to eliminate this supposed inconsistency (polyphyly) of the two character sets within the genus Petalophthalmus, the species P. liui, P. macrops, P. oculatus and P. caribbeanus are transferred to a separate genus, here named Ipirophthalmus gen. nov. The redefined Petalophthalmus and the new Ipirophthalmus gen. nov. share an antennal scale that is setose all around, differing by that feature from the remaining two genera of the subfamily Petalophthalminae, namely from Pseudopetalophthalmus and Parapetalophthalmus characterized by a smooth outer portion of the antennal scale ending at a single spine.
The type species Ipirophthalmus liui and I. macrops along with all four now known species of Petalophthalmus share thoracic endopods 3-4 reduced to only 1-2 segments, and endopod 5 elongate, ending in a brush of setae (Fig. 3C). These legs were reported as being different in the remaining two species here also assigned to Ipirophthalmus gen. nov.: Pillai (1968) reported an elongate endopod 4 ending in a brush of setae upon redescription of P. oculatus (now I. oculatus) based on a single specimen. San Vicente & Corbari (2015), however, concluded that Pillai (1968) may have confounded endopod 5 with endopod 4, due to the habitus of this specimen illustrated by Pillai (1968: fig. 1) with thoracopods 3-4 broken. Upon first description of P. caribbeanus (now I. caribbeanus), O.S. Tattersall (1968) noted without presentation of figures: "Third thoracic appendage long and very slender with the distal end armed with a cluster of setae surrounding a very minute nail". According to Lagardère (1983) and Fukuoka (2009), any (up to all) of the endopods 3-5 ending in a brush are also found in a number of species of Hansenomysis Stebbing, 1893, Ceratomysis Faxon, 1893 and Bacescomysis Murano & Krygier, 1985, all belonging to the subfamily Hansenomysinae Wittmann, Ariani & Lagardère, 2014, within the family Petalophthalmidae. That type of endopod was so far reported from diverse species in five out of a total of seven genera; this wide distribution suggests symplesiomorphy for both subfamilies of the Petalophthalmidae.

Validity of Petalophthalmus cristatus sp. nov.
The new species fits perfectly with the above-revised diagnosis of the genus Petalophthalmus. Accordingly, its validity requires a detailed discussion solely of the differences from the remaining three species of its genus: The characters reported by Hendrickx & Hernández-Payán (2018) upon redescription of Petalophthalmus armiger Willemoës-Suhm, 1875 differ from the new species in the outer flagellum of antennular peduncle without subbasal ridge; antennal flagellum of female figured by these authors with more (> 11) segments; merus of thoracic endopod 2 with outer margin smooth, carpopropodus with only three setae on outer margin; flagellum of thoracic exopods 3-4 with more segments (19-21); endopod 5 with subequal praeischium, ischium and merus (vs much longer merus in Fig. 3C), carpus not shorter than propodus; female pleopods terminally without pair of spines (vs present in Fig. 3O-P); basal segment of uropodal exopod without basally barbed seta (in addition to spines) on disto-lateral corner. San Vicente, Frutos & Cartes, 2014 differs, according to its first description, from the new species by a small, acute tooth behind anterior margin of carapace, no rostrum developed; outer flagellum of antennular peduncle without subbasal ridge; antennal flagellum with fewer (5) segments in females; pars centralis of right mandible without digitus; distal segment of maxillula with few (7) serrated spines; claw of thoracic endopods 1-2 fused with dactylus; coxa of endopod 1 without endite; merus of endopod 2 with smooth outer margin, carpopropodus without barbed setae apart from the usual smooth setae; flagellum of exopods 3-4 with more segments (19-20); endopod 8 shorter than exopod; endopod of uropods not extending beyond telson; basal segment of exopod with three spines but no barbed seta on disto-lateral corner.

Petalophthalmus papilloculatus
Petalophthalmus papuaensis San Vicente & Corbari, 2015, differs according to its first description, from the new species by the presence of some scattered visual elements in the globular cornea; small, acute tooth behind anterior margin of carapace, no rostrum developed; outer flagellum of antennula without subbasal ridge; antennal flagellum of female with more (7) segments; processus molaris of right mandible not trilobate; distal segment of maxillula with fewer (7) serrated spines; coxa of thoracic endopod 1 without endite; carpopropodus of endopod 2 without barbed setae apart from the usual smooth setae; endopod 5 without apical claw, terminal margin of propodus armed with two short cuspidate setae in addition to the pair of microserrated setae, the latter as in Fig. 3D; flagellum of thoracic exopods with few segments: exopods 3-4 with 13-15, exopod 5 with 16, exopods 6-8 with 13-16; female pleopod 1 unsegmented; female pleopods 1-4 (possibly also no. 5) terminally without pair of spines as in Fig. 3O-P; basal segment of uropodal exopod with three spines but no barbed seta on disto-lateral corner.

Historical outline of the tribe Calyptommini W.M. Tattersall, 1909
Based on the undivided eyeplate and the rod-like pleopods 1-5 in both sexes, W.M. Tattersall (1909) established the subfamily Calyptomminae as a monotypic taxon upon first description of Calyptomma puritani W.M. Tattersall, 1909. Hansen (1910 did not list this subfamily in his key to subfamilies and tribes of the Mysidae. Shortly thereafter, W.M.  redefined the Calyptomminae in order to receive the most similar genus Michthyops W.M. , newly established by him for two species previously attributed to the genus Pseudomma G.O. Sars, 1870, by Vanhöffen (1897) and Ohlin (1901). Without indication of details, Illig (1930) and Tchindonova (1981) listed Calyptomma and Michthyops within the tribe Erythropini, whereas Mauchline (1980) placed Calyptomma within the Leptomysini but Michthyops within the Erythropini. Finally, Nouvel et al. (1999) reinstituted the Calyptomminae at tribe level as Calyptommini and followed W.M.  by inclusion of Calyptomma as well as Michthyops. A close relationship of the two genera was confirmed with sequencing of nuclear small-subunit ribosomal DNA by Meland & Willassen (2007) and Chevaldonné et al. (2015), who equally obtained Michthyops parva (Vanhöffen, 1897) (now as M. parvus) and Calyptomma sp. as closely-related species within the subfamily Erythropinae. The reinstitution proposed by Nouvel et al. (1999) was acknowledged by Wittmann et al. (2014), Meland et al. (2015) and San Vicente (2017). In accordance with the resulting system, Wittmann et al. (2014) provided revised diagnoses of the tribes Erythropini and Calyptommini, the latter now revised further as explained below.

Monotypy, validity and affiliation of Abyssomysis cornuta gen. et sp. nov.
The sexual dimorphism among the ten specimens studied here goes beyond published data for most species in the order Mysida. Unusual dimorphisms of the new species are the structure and relative dimensions of the antennal peduncle, lacinia mobilis, and by the setation of the mandibular palpus. Importantly, mandible structure has so far been poorly checked for dimorphisms, and the mandibles are particularly scantly known in the tribe Calyptommini. Be that as it may, the strong differences pose the question whether the five females and five males studied here may represent more than a single species. Arguments for monotypy are supported by the fact that both sexes were found together at three stations, no other specimens were found elsewhere, and based on the detailed structure of eyeplate, maxillula, maxilla, first and second thoracic endopods, and telson.
Within the Erythropinae the new genus and species fits with the tribe Calyptommini W.M. Tattersall, 1909, by eyeplate without median cleft, by uniramous pleopods, reduced to endopods in both sexes, unsegmented in female pleopods 1-5 and male pleopods 1-3, 5, and by non-serrated lateral margins of the telson. However, it differs from all four so far known species of this tribe by rudimentary eyestalks laterally on the eyeplate and by segmentation of the uniramous male pleopod 4.
Uniramous pleopods reduced to unsegmented endopods in both sexes (with the exception of the segmented [uniramous] fourth male pleopod) are shared by the new taxon with Inusitatomysis insolita Ii, 1940, the only representative of the tribe Inusitatomysini Wittmann, Ariani & Lagardére, 2014. However, this species differs strongly by well-developed eyes, serrated outer margin of the antennal scale, and a pair of setae arising from the bottom of the telson cleft.