Four new species of Heteromysis (Crustacea: Mysida) from public aquaria in Hawaii, Florida, and Western to Central Europe

Four new species of the subgenus Heteromysis (Olivemysis) were detected in material from (sub)-tropical aquaria in six public aquarium institutions around the globe. Modifi cations of pleopods by spines represent the strongest structural complex used for differentiation within this subgenus: male pleopods 1–4 modifi ed in H. smithsoniana sp. nov., male pleopods 2–4 plus female pleopod 2 in H. hornimani sp. nov. and H. waikikensis sp. nov. Additional important diagnostic characters are provided by the antennulae, uropods, and telson. The male of H. sixi sp. nov. represents a very rare case within the genus Heteromysis by having only pleopod 2 modifi ed by fl agellate spines. The defi nition of the subgenus Olivemysis is modifi ed in order to include H. sixi sp. nov. A summary of pleopod modifi cations in the genus Heteromysis and a key to the species of the subgenus Olivemysis are given. The here described new taxa more than double the number of Heteromysis species known from aquaria yet unknown in nature from three to seven.

The adjective 'fl agelliform' is here used for whip-like structures (Fig. 10G) continuously tapering, without differentiation into a whip handle and whipcord; 'fl agellate' for any bifi d structures (Fig. 12D) bearing a fl agellum in subapical or more proximal position. 'Whip seta' (Fig. 12E, M) is used following Wilson (1989) for setae with a basal part (handle) bearing a thin fl agellum (cord, sensory part) at its tip; here, the handle and the cord separated by an articulation, suture, or at least by an optically dense section upon standard microscopy. = adult BL = body length measured from anterior margin of carapace to posterior margin of telson without spines N1 to N4 = nauplioid larvae at substage N1 freshly hatched from the egg membrane, up to N4 for those shortly before the moult leading to the postnauplioid stage P1 to P3 = postnauplioid larvae at substage P1 freshly moulted, up to P3 that lasts until moult to juvenile stage upon or shortly after release from brood pouch # = sample numbers in Table 1 Results Class Malacostraca Latreille, 1802Order Mysida Boas, 1883Family Mysidae Haworth, 1825Subfamily Heteromysinae Norman, 1892 Tribe Heteromysini Norman, 1892

Patterns of modifi ed pleopods in the genus Heteromysis
Pleopods reduced to small bilobate or obscurely bilobate, setose plates with residual differentiation of pseudobranchial lobes, are among the main diagnostic characters of the genus Heteromysis. The monotypic subgenus Neoheteromysis Băcescu, 1976, is characterized by pleopods 2-4 modifi ed by elongate, smooth setae in both sexes. Modifi cations of male pleopods by spines or by attenuated setae are typical of the subgenus Olivemysis Băcescu, 1968, where they show a great species-specifi c diversity related to nine different morphological constellations. Unexpectedly, all four new species of Olivemysis described below are quite unusual by demonstrating a total of three different, rare combinations of pleopods modifi ed by spines. More common modifi cations are known in six insuffi ciently known species so far not assigned to any subgenus. Pleopods of both sexes are unmodifi ed (as far as known) unless listed in the following (data from fi rst descriptions of species; additional data sources given in square brackets):  Murano, 1988;H. tattersalli H. Nouvel, 1942[O.S. Tattersall 1967; H. (O.) tenuispina Murano, 1988;H. (O.) wirtzi Wittmann, 2008;H. (O.) zeylanica W.M. Tattersall, 1922 [in material from Australia according to Murano 1988].  Tattersall 1967]. O.S. Tattersall (1967) claimed modifi ed male pleopods 4 for H. gymnura W.M. Tattersall, 1922, but did not state whether she referred to the (meanwhile probably lost) types from the Gulf of Manaar (N Indian Ocean) or to non-types from Zanzibar. According to Wittmann (2013), materials from Zanzibar and Australia attributed to H. gymnura by O.S. Tattersall (1962Tattersall ( , 1967 and Murano (1988), respectively, do not belong to this species.

Etymology
The species name is a Latinized adjective with female ending, referring to the Smithsonian Marine Ecosystems Aquarium (Fort Pierce, Florida).
MAXILLIPEDS. Coxa of fi rst maxilliped with small endite bearing one barbed seta at tip. Basis with large, prominent endite that is densely setose on inner margin. Ischium and merus each with one smaller but distinct, medially setose endite. Basis of second maxilliped with large, distinctly medially projecting endite. In both sexes, combined praeischium plus ischium are 0.7-0.9 times length of merus, carpopropodus plus dactylus 1.0-1.2 times merus. Dactylus very large, with dense brush formed by large numbers of normal setae and 10-12 modifi ed setae, the latter apically bent, bearing two symmetrical series of denticles (stiff barbs) on either side in subbasal to median portions. GNATHOPODS (thoracic endopods 3; Fig. 2L-Q, S). Ischium 1.5−2.0 as long as wide; merus 2.3−3.8 as long as wide and 1.5−1.7 length of ischium. Carpus 0.6-0.8 times length of merus, 1.1-1.3 times ischium. Claw 2.4-4.0 times length of dactylus and 41-43% of carpopropodus. Distal third of ischium with 3-7 short whip setae on mesial margin, each whip seta on tip of short tooth-like projection (Fig. 2L) in males, the latter projection shorter in females (Fig. 2O). Series of 2-5 unilaterally barbed setae in males (Fig. 2N), 6-9 in females (Fig. 2Q), on basal 50-60% of outer face of merus, plus single seta in subterminal position. These setae with series of normal barbs (cilia) along median to subapical portions, basal portions thickened. Subterminal seta without, remaining setae at end of thickened portion with 1-2 modifi ed, spine-like barbs; mainly two such 'spines' in males (Fig. 2N), only one in females (Fig. 2Q). Proximal 60-80% of merus with 3-7 short whip setae on mesial margin, whip setae alternating with longer smooth setae; no tooth-like projections. Carpus with fl agellate spines (Fig. 2M, P) arranged as two single spines and more distally two pairs of spines; all these spines with subapical fl agellum positioned on smooth posterior (proximal) face; four distal spines rugged to various degrees on anterior face.
MARSUPIUM. Oostegites 1-2 (derivates of thoracopods 7-8) without setae on upper (dorsal) margins. Lower margins from subbasal region up to the rounded tip bearing series of setae, most of which are barbed by fi ne cilia along subbasal to median portions. Oostegite 1 near basis with 7-10 setae, oostegite 2 with 2-4 setae which are microserrated along distal half. Ventral and rostral portions of outer face of only second oostegite with total of 7-11 slender whip setae, but shorter than barbed setae. Thoracopod 6 with rudimentary oostegite represented by small, rounded lobe terminally with two setae which are microserrated on distal half. PLEOPODS (Fig. 3H-O). Female pleopods increase in length caudally. Length of male pleopods increases in series of pleopods 1, 2 ≈ 5, 3, 4. For presence and numbers of spines on male pleopods 1-4, see 'Diagnosis' above. Stylet of pleopod 1 shorter than that of pleopod 2. Single smooth seta at outer terminal edge of male pleopods 3-4; all other setae of pleopods 1-5 are plumose or barbed in both sexes (not counting fl agellate spines).
FOREGUT (Fig. 3A-B). Gross structure as described by Wittmann & Abed-Navandi (2019) for H. domusmaris. Lateralia anteriorly with apically coronate spines with smooth shaft and long spines armed with loose series of small denticles along shaft. Lateralia more caudally with separate group of 4-6 centrally, unilaterally serrated spines (Fig. 3B). Dorsolateral infoldings with two longer, apically pronged spines furnished with series of acute teeth (Fig. 3A) along subbasal to subapical portions. Gut contents of three examined H. smithsoniana sp. nov. were mostly unidentifi able material, minor amounts of crustacean remains and even fewer mineral particles.

Distribution
The species is so far known only from tanks in the 'Smithsonian Marine Ecosystems Aquarium', Fort Pierce, Florida. Origin most likely in coastal marine waters of the subtropical NW-Atlantic (Table 1).

Larvae
In the ethanol-fi xed material, three out of six adult females examined had 1-2 nauplioid larvae in the brood pouch; three lacked a brood. A female with 4.2 mm body length carried two nauplioid larvae at substage N1 with 0.66-0.69 mm, a female of 3.9 mm had one N3 with 0.78 mm, and a female of 3.6 mm had one N4 with 0.84 mm. Larvae at substages N3 and N4 with smooth cuticle all around. The two N1 larvae (Fig. 3S) also with smooth cuticle except for minute down-like structures (Fig. 3T)

Etymology
The species name is a masculine noun in the genitive singular, referring to the Horniman Museum Aquarium, London.
MAXILLIPEDS. As described above for H. smithsoniana sp. nov., coxa of fi rst maxilliped with small endite bearing one barbed, basally thick seta at tip. In both sexes, combined praeischium plus ischium of second maxilliped are 0.6-0.9 times length of merus, carpopropodus plus dactylus 1.0-1.2 times merus. Dactylus 2 with dense brush formed by large numbers of normal setae and 13-14 modifi ed setae; modifi cation as in above-quoted species.
MARSUPIUM. As described above for H. smithsoniana sp. nov. Oostegite on thoracopod 7 near basis with 6-9 microserrated setae, larger oostegite on thoracopod 8 with 3-5 such setae. Ventral and rostral portions of outer face of only larger oostegite with total of 14-19 fl agelliform setae which are shorter than barbed setae present in dense series along lower margin. Thoracopod 6 with rudimentary oostegite represented by small, rounded lobe terminally with three setae which are microserrated on distal half. PLEOPODS (Fig. 7). Size increases in series of pleopods 1, 3, 4, 5, 2 in females, and 1, 5, 2, 3, 4 in males. For presence and numbers of spines on pleopods 2-4, see 'Diagnosis' above. All setae of pleopods 1-5 plumose or barbed in both sexes (not counting fl agellate spines). Most specimens of both sexes, including holotype, show peculiar perpendicular orientation of only second pleopod (as in Fig. 8C).
FOREGUT (Fig. 6A-B) essentially as described above for H. smithsoniana sp. nov. Caudal portion of lateralia in H. hornimani sp. nov. with group of 3-5 small, apically pronged spines serrated along distal ¾ of shaft (Fig. 6B). Dorsolateral infoldings with group of three longer, apically smooth, subbasally to subapically serrated spines (Fig. 6A). Gut contents of three specimens examined were mostly unidentifi able material and mineral particles, a few fragments of fi liform algae, no arthropod remains.   (Fig. 8A-B). General appearance transparent with yellow to orange-brown portions, rendering contents of foregut and alimentary canal visible. Small orange to brown chromatophore spots scattered over the thorax, with greatest density above the foregut. The refl ecting spots above the foregut may be due to small oil globules (usually disappearing in ethanol-preserved material). Eggs are green, as often found in species of Heteromysis. PHOTO FROM LONDON (Fig. 8C). Compared with the fi gures from Brest, the fi gure from London shows an essentially similar density of chromatophores, but more 'expanded' ones, giving the animal a more intensive orange to red tinge. The differences between Fig. 8A-B and Fig. 8C are well within the range of normal colour change in species of the family Mysidae.

Distribution
The species is so far known only from tanks in the 'Horniman Museum Aquarium London' (UK), the 'Oceanopolis Aquarium' in Brest (France), the 'Aquarium de Paris' (France), and the 'Zoo Wroclaw' (Poland). The data from Paris suggest a Caribbean origin (Table 1: # 4).

Eggs and larvae
MATERIAL FROM LONDON. Two females with empty brood pouch, while another female with 5.3 mm body length carried three postnauplioid larvae at substage P2 with 1.28-1.35 mm length, one female 6.1 mm carried three P2 with 1.23-1.35 mm. A crude estimate suggests that the larvae attain ¼ parent length shortly before the moult to the free-living juvenile stage. MATERIAL FROM WROCLAW. Female with 6.1 mm body length carried one nauplioid larva at substage N1 (0.92 mm length); female 6.5 mm carried two eggs with diameter 0.49 mm and eight N1-larvae with 0.87-1.11 mm. The co-occurrence of eggs and N1-larvae in the same brood pouch is rarely found. This indicates that the embryos were asynchronously hatching from the egg membrane shortly before and during fi xation. The observed strong size differences between the hatchlings are clearly due to the obligatory stretching of the abdomen during and shortly after hatching. Adoption of eggs or larvae from other mothers appears unlikely in this case: in experiments of Wittmann (1978) with mysids from other subfamilies, the breeding females did not introduce offered eggs (embryos) into their brood pouch, but a few mothers did so with up to two nauplioid larvae, never with more. The here studied N1-larvae show a smooth cuticle all around.

Etymology
The species name is a feminine adjective formed by the addition of location suffi x, related to the 'Waikiki Aquarium' (Honolulu, Hawaii) where the new species was detected.
GNATHOPODS (thoracic endopods 3; Fig. 10I-M). Ischium 1.2−2.2 times as long as wide; merus 2.0−3.6 as long as wide and 1.5−2.2 length of ischium. Carpus 0.5-0.8 times length of merus, 0.9-1.1 times ischium. Claw 2.6-3.9 times dactylus length, and 45-50% carpopropodus. Both sexes with series of basally thickened, unilaterally barbed setae (Fig. 10M) on distal 71-80% of outer face of merus. These setae with series of normal barbs (cilia) along median to subapical portions, basal portions thickened. Distal half of ischium with 4-5 short whip setae on mesial margin, each whip seta on tip of a short rounded projection in males, no such projections in the only female available in the present material. Proximal ⅔ of merus with 3-6 short whip setae on mesial margin, the whip setae alternating with longer smooth setae; these setae not implanted on projections. Carpus 3 with proximal 1-2 fl agellate spines (Fig. 10K), in case of two spines one behind the other; distal spines arranged in 2-4 pairs (one pair after the other); spines of only the most distal 2-3 pairs with rugged anterior margins (Fig. 10J).

Distribution
The species is so far known only from an aquarium tank of the 'Waikiki Aquarium', Honolulu, Hawaii. Origin most likely in coastal marine waters of the Central Pacifi c (Table 1).

Etymology
The species name is a masculine noun in genitive singular, dedicated on the occasion of his retirement to Franz Six, citizen scientist and 55 year-long promoter of the 'Haus des Meeres', a public marine aquarium in Vienna.

MOUTHPARTS (
MAXILLIPEDS. First maxilliped as described above for H. smithsoniana sp. nov. Basis of second maxilliped with large, distinctly medially projecting endite. Combined praeischium plus ischium 0.9 times length of merus, carpopropodus plus dactylus 1.1 times merus. Dactylus very large, with dense brush formed Heteromysis (Olivemysis) mclellandi Price & Heard, 2011, from sponges in fringing reefs off Turks and Caicos Islands, NW-Atlantic, differs from the new species by the carpopropodus of the thoracic endopod 3 with more (7-9) large fl agellate spines, the carpopropodus 8 with fewer (6) segments, all fi ve pairs of pleopods modifi ed by attenuated setae in both sexes, the endopod of the uropods with more (4-5) spines, telson with apical cleft whose margins are shorter (15-20% telson length) and bear fewer (12-15) laminae all along their length.
The male pleopods are unknown in three species of Heteromysis sharing a fl agellate spine (as far as known) at the disto-mesial edge of the antennular trunk and series of laminae along > 70% of the margins of the telson cleft with H. smithsoniana sp. nov.: Only a single female of Heteromysis bredini Brattegard, 1970 is known from Tobago, Caribbean. It differs from the new species by a stouter carpopropodus of the thoracic endopod 3 with more (10) fl agellate spines, the endopod of the uropods with more (5) spines, and the lateral margins of the telson with more (19-20) spines.
Only an adult female and one immature male of Heteromysis (Olivemysis) sexspinosa Murano, 1988 are known from a reef fl at near Darwin, Australia. It differs from the new species by an undivided antennal scale, the carpopropodus of the thoracic endopod 3 less than twice as long as broad, each lateral margin of the telson all along with more (18-19) spines.
Only two females of Heteromysis kossmanni Nouvel, 1964 are known from the Red Sea (Kossmann 1880). This poorly known species differs from the new species by the female pleopod 1 (if correctly described) terminally with several spines and many setae, the carpopropodus of the thoracic endopod 3 about twice as long as wide, the endopod of the uropod without a spine, the telson with spines all along the lateral margins. Females with pleopod 2 modifi ed by attenuated setae or by spines were previously found only in H. mclellandi and H. kushimotensis, respectively, and now also in H. hornimani sp. nov. and H. waikikensis sp. nov. Interestingly, the latter three species are known only from public aquaria.

Validity of
Only Heteromysis (Olivemysis) kushimotensis Murano & Fukuoka, 2003, from exhibition tanks in the Aquarium of Kushimoto Marine Park Center, Wakayama, Japan, shares a fl agellate spine at the distomesial edge of the antennular trunk and modifi ed male pleopods 2-4 in combination with the modifi ed female pleopod 2 with both new species. It differs from both species by the undivided antennal scale, fewer fl agellate spines (one in males, two in females) on the mesial margin of the pleopod 2, and by the modifi ed male pleopod 5. In addition, it differs from H. hornimani sp. nov. by only one spine on the endopod of the uropods and by smooth basal half of the lateral margins of the telson.
The modifi ed male pleopods 2-4 of Heteromysis tattersalli Nouvel, 1942, from the Cape Verde Islands (NE Atlantic) were initially overlooked by Nouvel (1942Nouvel ( , 1943 and later communicated to O.S. Tattersall (1967: 190) without indicating the kind of modifi cation. Differences of this damaged, single male specimen from both new species are the telson cleft with fewer (10-12) laminae. In addition, it differs from H. waikikensis sp. nov. by the endopod of the uropods with more (3-4) spines, and from H. hornimani sp. nov. by the lateral margins of the telson with fewer (8-9) spines, which are distributed only on the distal half.

Validity of Heteromysis (Olivemysis) sixi sp. nov.
The only adult male known is nearly unique within its genus by having only the pleopod 2 modifi ed by spines: the remaining pleopods have setae only. This pattern was previously found only in small adult males of H. (O.) dardani, whose pleopods 3-4 are also modifi ed in normal, larger males (Wittmann 2008). Wittmann, 2008 is associated with diogenid hermit crabs on the Island of Madeira (NE Atlantic). It differs from H. sixi sp. nov. by one apically barbed spine (modifi ed seta) at the dorsal apophysis of the basal segment of the antennula, by a subdivided antennal scale, by the absence of a large smooth spine (stylet) on the apex of the male pleopod 2, by pleopods 3-4 modifi ed in most males, by more (3) spines on the endopod of the uropods, and fewer (21-23) laminae in the telson cleft.

Heteromysis (Olivemysis) dardani
Male pleopods are unknown in three species of Heteromysis sharing the fl agellate spine (as far as known) at the disto-mesial edge of the antennular trunk and a series of laminae along > 70% of the margin of the telson cleft with H. sixi sp. nov.: Only a single female of Heteromysis bredini Brattegard, 1970 is known from Tobago, Caribbean. It differs from the new species by the subdivided antennal scale, the stouter carpopropodus of the thoracic endopod 3 with more (10) fl agellate spines, the endopod of the uropods with more (5) spines, and the lateral margins of the telson with more (19-20) spines distributed all along the lateral margins.
Only one adult female and one immature male of Heteromysis (Olivemysis) sexspinosa Murano, 1988 are known from a reef fl at near Darwin, Australia. It differs from the new species by the carpopropodus of the thoracic endopod 3 less than twice as long as broad, the endopod of the uropods with more (2) spines, the lateral margin of the telson all along with more (18-19) spines.
Only two females of Heteromysis kossmanni Nouvel, 1964 are known from the Red Sea (Kossmann 1880). This poorly known species differs from the new species by the carpopropodus of the thoracic endopod 3 about twice as long as broad, the endopod of the uropod without spine, and the telson with spines all along the lateral margins.