Revision of the amphiamerican genus Mysidium Dana, 1852 (Crustacea: Mysida: Mysidae), with descriptions of two new species and the establishment of two new subgenera

. Revised definitions are given for the genus Mysidium Dana, 1852, and its eight previously known species, based on material from Curaçao, Bonaire and SE-Brazil, along with the evaluation of published data. Type material of Diamysis columbiae Zimmer, 1915, M. cubanense Băcescu & Ortiz, 1984 and M. rubroculatum Băcescu & Ortiz, 1984 is examined. A lectotype is designated for D. columbiae Zimmer, 1915, a senior synonym of Mysidium columbiae (Zimmer, 1915). Two new species are described, M. triangulare Wittmann sp. nov. from Curaçao and M. antillarum Wittmann sp. nov. from Curaçao and Bonaire. Known ranges are extended by first records of M. cubanense from Curaçao and Bonaire and of M. integrum W.M. Tattersall, 1951 from SE Brazil. Three morphologically different groups are established at the subgeneric level: (1) the nominotypical subgenus Mysidium Dana, 1852 with M. gracile (Dana, 1852), M. integrum , M. cubanense , M. rubroculatum and M. triangulare sp. nov. from the West Atlantic plus M. rickettsi Harrison & Bowman, 1987 from the East Pacific; (2) Occimysidium Wittmann subgen. nov. represented only by M. pumae Ortiz, Hendrickx & Winfield, 2017 from the Pacific coast of Mexico; and finally (3) Orientomysidium Wittmann subgen. nov. comprising M. columbiae and M. antillarum sp. nov. from the West Atlantic. The poorly known M. iliffei Băcescu, 1991 is not assigned to any subgenus. A key to the resulting three subgenera and ten nominal species of the genus Mysidium is given.

Due to the large numbers and sizes of swarms in near-shore habitats, together with their exclusively amphiamerican distribution range, it is unsurprising that these crustaceans constituted the first mysid genus described as endemic to America (Dana 1850).The original generic definition given by Dana for the junior homonym Mysidia in 1850, renamed by him as Mysidium in retrospective formulation (see Discussion) in 1852, was very brief as was usual for that period.Many more details became available by subsequent descriptions and supplementary definitions of the genus by Czerniavsky (1887) and Ortiz et al. (2017b).Detailed species descriptions were provided by Dana (1852), Zimmer (1915a), W.M. Tattersall (1951), Harrison & Bowman (1987) and Ortiz et al. (2017aOrtiz et al. ( , 2017b)).The first descriptions of M. cubanense Băcescu & Ortiz, 1984, M. rubroculatum Băcescu & Ortiz, 1984and M. iliffei Băcescu, 1991 were also rather brief, complicating the separation of M. cubanense and M. rubroculatum from M. gracile (Dana, 1852) and M. integrum W.M. Tattersall, 1951.In diagnoses and key to species, Ortiz et al. (2017aOrtiz et al. ( , 2017b) ) and Ortiz & Lalana (2018) introduced numbers of setae on the endopod of the fourth male pleopod as supplementary measures of discrimination.Nonetheless, the diagnostic resolution is lowered, but not suspended, by the below documented inter-individual variability of these numbers.Based on the study of types and of additional material from a wide distribution range, the present contribution improves the discriminatory performance by determining additional discriminatory features and by applying a consistent, clear-cut scheme of characters at the genus, subgenus and species level.This led to the detection of new species together with the establishment of new subgenera.

Material and methods
Abbreviations MINGA = Grigore Antipa, National Museum of Natural History, Bucharest NHMW = Naturhistorisches Museum Wien ZMH = Zoologisches Museum Hamburg bl = body length N1 to N4 = nauplioid larvae at substage N1 freshly hatched from the egg membrane, up to N4 for those shortly before the molt leading to the postnauplioid stage # = sample number in Table 1 Material A total of 13 samples were taken with hand net by snorkeling and scuba diving by one of us (Peter Wirtz) in littoral and sublittoral marine waters of Curaçao, Bonaire and SE-Brazil (Table 1).Additional material was obtained upon request from museum collections: the Zoological Museum of Hamburg kindly provided the types of Diamysis columbiae Zimmer, 1915.The Grigore Antipa, National European Journal of Taxonomy 495: 1-48 (2019) Museum of Natural History, Bucharest, provided types of Mysidium cubanense Băcescu &Ortiz, 1984, andM. rubroculatum Băcescu &Ortiz, 1984.Types of newly described species are deposited at MINGA, NHMW and ZMH.

Laboratory methods
Preparation, measurements and examination of materials as in Wittmann (2008).Body length (bl) measured according to Tattersall & Tattersall (1951) from the anterior margin of the carapace to the posterior margin of the telson, excluding the spines.Note in the present context that Băcescu & Ortiz (1984) measured according to Băcescu (1954) from the tip of the antennal scale to the end of the exopod of uropods without setae.Length measurements of somites, appendages and segments do not include setae, laminae, spines, claws, etc.The length of the antennular trunk and its segments is measured along the dorsal midline.Size ratios within and between angular and oblique objects were calculated as linearized within a single observation plane; this is especially important for estimating the relative size of eyes.The easily confounded, large intersegmental joint between the basis and flagellum of the thoracic exopods was excluded from counts of segmental numbers.The statolith mineral was determined according to Wittmann et al. (1993).Juveniles and immatures are not represented in material lists below.

Terminology
Essentially as in Wittmann et al. (2014).In the present context, note that thorn-like projections with a basal suture are termed 'spines', those without suture are 'teeth' or 'denticles'; slender projections in dense series without suture are 'laminae'; slender, flexible projections with basal suture are 'setae', particularly if barbed or pinnate.The maxillula is obscurely three-segmented with inner lobe (endite) according to the interpretation by Tattersall & Tattersall (1951).Terminology of gross structures of the foregut follows Kobusch (1998); modified spines of the foregut according to Wittmann & Griffiths (2018), in addition, 'apically pronged, serrated' spines (Fig. 13G) are now distinguished.Marsupial stages are distinguished according to Wittmann (1981) and Wittmann et al. (2014).Post-marsupial stages are distinguished according to Wittmann et al. (2016).Drawings of sex-specific features are labeled by ♀-or ♂-symbols, respectively, in Figs 1-13.The absence of such labels implies missing or unapparent gender-specific differences.

Taxonomy
At subfamily to tribe levels adopted from Wittmann et al. (2014) with additions by Wittmann et al. (2016).Diagnoses and keys to species by Ortiz et al. (2017aOrtiz et al. ( , 2017b) ) provided a valuable starting point for the present revision of the genus Mysidium.

Diagnosis
The diagnosis of this tribe is slightly modified from Wittmann et al. (2014) due to certain spine-like cuticle structures of the telson being now interpreted as laminae (see 'Methods' and species descriptions below): Antennal scale setose all around (exceptionally with blunt spine on outer margin) and with short apical segment; carpopropodus of thoracic endopod 6 with 1-3 segments; two pairs of well-developed oostegites, rudimentary oostegite on thoracic endopod 6; pleopods rudimentary in both sexes, except male pleopod 4 and to a minor extent also pleopod 3; third male pleopod uniramous, unsegmented, mostly rudimentary as in females or reduced to small endopod fused with sympod; exopod of fourth male pleopod with total of 1-2 modified setae (1-2 on apical segment, 0-1 on subapical segment); uropods without spines; telson shorter than ultimate pleonite, terminally entire or with apical cleft, lateral margins bare or furnished with spines or laminae; spines (if present) on lateral margins arranged in continuous series, not arranged in groups of large spines with smaller spines in between; telson always devoid of setae.

Diagnosis
Compiled with the reservation that not every one of the following characters is known in Mysidium iliffei Băcescu, 1991 (see Discussion): Anisomysini with eyes normal, eyestalks and cornea well developed.Antennular trunk terminally with the usual two flagella in both sexes, no accessory flagellum developed.Appendix masculina large, setose.Antennal scale lanceolate, without spines, but setose all around, with small terminal segment.Mandibles normal, palp three-segmented, cutting edges with well-developed pars incisiva, lacinia mobilis and pars molaris.Distal segment of maxillary palp with setae, but without denticles.Thoracopods essentially normal, endopods with 2-or three-segmented carpopropodus.All pleopods of females and pleopods 1-3, 5 of males are vestigial, setose, uniramous, unsegmented.Male pleopod 3 larger than pleopods 1-2, 5. Male pleopod 4 even longer, modified, with distinct sympod, unsegmented endopod and 3-or 4-segmented, slender exopod.Terminal two segments of exopod each bearing a large modified seta, remaining segments usually without setae, at most with one small seta on the basal segment.Uropods setose all around, without spines; exopods longer than endopods.Telson longer than broad, with entire or incised terminal margin; with spines and laminae on distal half.

Diagnosis
Subdivision of the genus Mysidium Dana, 1852, characterized by three-segmented carpopropodus of third thoracic endopod.Merus of this endopod with non-serrated setae exclusively, with the reservation that this feature is unknown in M. rickettsi Harrison & Bowman, 1987.Third male pleopod with medial widening at 40-60% distance from basis.Sympod of fourth male pleopod without setae on rostral face.Its exopod four-segmented; the two proximal segments without setae; the two distal segments each with one unbranched modified seta.Telson entire, or at most with a rounded, shallow indentation.

Distribution
Littoral and sublittoral waters of Atlantic and Pacific coasts of subtropical to tropical America (32° N to 24° S), including Caribbean, Gulf of Mexico and Gulf of California.(Dana, 1852) Fig. 1 Macromysis gracilis Dana, 1852: 653-655 (senior synonym in combination with generic, subjective junior synonym).

Type locality
(Sub)tropical SW-Atlantic, harbour of Rio de Janeiro (Dana 1852), about 22.90° S, 43.17° W. The present sample off Cabo Frio is at about 145 km east along the shoreline from this harbour.

Revised definition
All features as diagnosed for the genus and its subgenus Mysidium Dana, 1852.Cornea globose in lateral view, calotte-shaped in dorsal view, with diameter 1.7-3.2times as long as terminal segment of antennular trunk.Eyestalks smooth.Rostrum short, triangular, apically rounded to obtusely pointed, not extending beyond basis of eyestalks.Antero-lateral edges of carapace rounded.Only males with anterior margin of antennular trunk dorsally with rounded, shield-like, mediodistal extension (Fig. 1A-B); plumose seta at anterior margin of this extension showing 0.9-1.4times extension length; longitudinal series of 6-8 shorter (partly minute) setae all over this extension plus a short proximal stretch.Appendix masculina bilobate, 1.6-2.7 times as long as terminal segment of antennular trunk.Length of antennal scale 4-6 times maximum width; scale clearly reaching beyond antennular trunk.Median segment of mandibular palp with setae on both margins.About evenly rounded hump on the outer face of the distal segment of the maxillula.Carpopropodus of first to eighth thoracic endopods with 2, 2, 3, 3, 3, 3, 3-2, and 2-3 segments, respectively.Basal segment occupies half total length of carpopropodus of endopod 3. Pleopod 1 rod-like to indistinctly bilobate in both sexes.Sympod of pleopod 4 with endite missing or reduced to weak medial hump.Basal segment occupies 56-69% total length of exopod.Endopod reduced to lobe with 12-18% sympod length; apically with one long seta and more proximally additional 4-7 shorter, barbed setae.Endopod of uropods 0.6-0.8times length of exopod.Telson subrectangular, length 1.4-2.0times maximum width near basis; lateral margins slightly tapering or parallel.Proximal 46-64% of lateral margins smooth, distal portion of each margin with dense, continuous series of 11-15 acute spines.Terminal margin concave, densely furnished with 15-21 apically blunt laminae.

Distribution and habitat
Reported from euhaline coastal waters of Bermuda, Bahamas, Caribbean and Gulf of Mexico (Brattegard 1970(Brattegard , 1973;;Ortiz et al. 2017b).The present record from the Brazilian coast off Cabo Frio represents WITTMANN K.J. & WIRTZ P., Revision of the genus Mysidium a strong extension of the known distribution to the south, yielding a total range from 32° N to 23° S along the coasts of the West Atlantic.According to Zoppi de Roa & Alonso (1997) these mysids form swarms close to the bottom of diverse coral reef and mangrove habitats, also found among spines of the sea urchin Diadema antillarum.The present data from Curaçao and SE-Brazil were recorded during the daytime at a depth of 0-35 m, on swarms mostly hovering around, between or above rock and coral blocks, also from the entrance area of a cave.Most surprising was sample C9 (Table 1) from the water edge where the mysids showed a striking green body color upon swarming above algae at a depth of only 5 cm.Băcescu & Ortiz, 1984 Fig. 3 Mysidium cubanense Băcescu & Ortiz, 1984: 18-20

Revised definition
All features diagnosed above for genus and subgenus Mysidium Dana, 1852.Cornea large, globose to ellipsoidal in lateral view, calotte-shaped to crescent-like in dorsal view, with 'diameter' 2.6-4.1 times as long as terminal segment of antennular trunk.Eyestalks smooth.Rostrum (sub)triangular, apically bluntly pointed to well rounded, not extending beyond basis of eyestalks.Antero-lateral edges of carapace rounded.Only males with anterior margin of antennular trunk dorsally with rounded, shield-like, mediodistal extension (Fig. 3A-B); longitudinal series of 4-6 setae extending all over this extension plus a short proximal stretch.The largest seta 0.2-0.4times extension length.Appendix masculina bilobate, all along inner margins fringed by large setae; its length 1.1-2.3times terminal segment of antennular trunk.Length of antennal scale 4-6 times maximum width; scale reaching beyond antennular trunk.Median segment of mandibular palp with setae on both margins.Large, about evenly rounded hump on outer face of distal segment of maxillula.Carpopropodus with two segments European Journal of Taxonomy 495: 1-48 (2019) in thoracic endopods 1-2, 8, versus three segments in endopods 3-7.Basal segment occupies half total length of carpopropodus of endopod 3. Pleopod 1 bilobate in both sexes.Sympod of male pleopod 4 with endite reduced to weak medial hump or missing.Basal segment of exopod occupies 54-61% total length.Endopod reduced to lobe with 14-18% sympod length; apically with one long seta and more proximally additional 4-6 shorter, barbed setae.Endopod of uropods 0.6-0.8times as long as exopod.Telson subrectangular to trapezoid; length 1.6-2.1 times maximum width near basis; its lateral margins slightly concave to almost straight; latero-terminal corners rounded; terminal margin traverse to convex.

WITTMANN K.J. & WIRTZ P., Revision of the genus Mysidium
Proximal 48-73% of lateral margins smooth, distal portion of each margin with dense, continuous series of 7-16 acute spines.This series extending up to the rounded, latero-terminal corner.Terminal margin lined by 12-20 densely set, apically blunt or weakly pointed laminae.

Revised definition
All features diagnosed above for genus and subgenus Mysidium Dana, 1852.Cornea globose in lateral view, calotte-shaped in dorsal view, with diameter 1.9-2.9times as long as terminal segment of antennular trunk.Eyestalks smooth.Rostrum (sub)triangular, with rounded to acute apex, rostrum not extending beyond basis of eyestalks.Antero-lateral edges of carapace rounded.Only males with anterior margin of antennular trunk dorsally with rounded, shield-like, mediodistal extension (Fig. 4A-B) bearing a longitudinal series of 4-5 setae.The largest seta 0.1-0.3times extension length.Appendix masculina bilobate, along inner margins fringed by large setae; its length 1.3-2.0times terminal segment WITTMANN K.J. & WIRTZ P., Revision of the genus Mysidium of antennular trunk.Length of antennal scale 4-5 times maximum width; scale reaching well beyond antennular trunk.Median segment of mandibular palp with setae on both margins.Large, moderately unevenly rounded hump on outer face of distal segment of the maxillula.Carpopropodus with two segments in thoracic endopods 1-2, 8, versus three segments in endopods 3-7.Basal segment occupies half total length of carpopropodus of endopod 3. Pleopod 1 minute, rod-like (Fig. 4D) in both sexes, not considering the weak median widening at its vestigial endopodal portion.Sympod of male pleopod 4 with endite reduced to a weak medial hump or missing.Exopod with basal segment occupying 52-63% total length.Endopod reduced to lobe with 13-17% sympod length; apically with a long seta plus a minute seta and more proximally 2-4 additional small barbed setae.Endopod of uropods 0.7-0.8times as long as exopod.Telson subrectangular, caudally narrowing by 30-54%; length 1.5-1.8times maximum width near basis; lateral margins slightly concave to almost straight; latero-terminal corners rounded; terminal margin slightly convex to traverse.Proximal 50-60% of lateral margins smooth, distal portion of each margin with dense, continuous series of 8-12 acute spines.This series extending up to the rounded, latero-terminal corner.Terminal margin lined by 11-18 strong, apically blunt laminae; these last stouter compared to the neighboring latero-terminal spines.

Definition
All features diagnosed above for genus and subgenus Mysidium Dana, 1852.Cornea globose in lateral view; calotte-shaped in dorsal view, with diameter 1.7-2.5 times as long as terminal segment of antennular trunk.Eyestalks smooth.Rostrum triangular, apically pointed to well rounded, not extending beyond basis of eyestalks.Antero-lateral edges of carapace rounded.Only males with anterior margin of antennular trunk dorsally with rounded, shield-like, mediodistal extension (Fig. 5A-B); longitudinal series of 6-10 setae extending all over this extension plus a short proximal stretch.The largest seta 0.1-0.3times extension length.Appendix masculina bilobate, densely setose; its length 1.5-2.0times terminal segment of antennular trunk.Length of antennal scale 5-7 times maximum width, scale reaching far beyond antennular trunk.Median segment of mandibular palp with setae on both margins.Almost evenly rounded hump on outer face of distal segment of maxillula.Carpopropodus two-segmented in thoracic endopods 1-2, 8, or three-segmented in endopods 3-7, except that endopods 6, 7 may be twosegmented in some females.Basal segment occupies 0.4-0.5 times total length of carpopropodus of endopod 3. Pleopod 1 representing a stout, bilobate plate in both sexes.Sympod of male pleopod 4 with endite reduced to a weak medial hump or missing.Exopod with basal segment occupying 56-63% total length.Endopod reduced to lobe with 10-16% sympod length; lobe apically with one long, barbed seta and more proximally with additional 4-6 shorter, barbed setae.Endopod of uropods 0.6-0.8times as long as exopod.Telson spatulate, length 1.9-2.1 times maximum width near basis; median portion with concave lateral margins, terminal portion triangular with rounded tip.Proximal 52-64% of lateral margins smooth; distal portion of each margin with continuous series of 5-11 acute spines, this series extending up to the corner with the triangular apical portion.Margin of the triangular portion densely furnished with a total of 21-24 strong, apically blunt laminae.
AntennAl AppendAges (Fig. 5A-B, F-G).Antennular trunk extends 10-50% its length beyond (artificially aligned) eyes.First to third segments occupy 46-57%, 15-18% or 28-36% total trunk length, respectively.Trunk dorsally with forward directed small, setose lobes near terminal margin of each segment: two lobes from basal, one from median, and one from terminal segment (not counting the mediodistal extension of anterior margin in males).Appendix masculina 0.4-0.5 times total trunk length, antennal scale 1.1-1.5 times trunk.Sympod of antenna produced into spiniform extension on outer distal corner.Antennal scale with terminal segment occupying 13-24% total length and bearing five plumose setae.
Mouth pArts (Figs 5h, J, 6A-C).Mandibular palp without spines; terminal segment with strong, modified, bent seta at apex, and barbed setae along inner and outer margins.Median segment of palp with angular, medially directed dilatation, both its margins setose.Proximal segment normal, with smooth margins.Distal segment of maxillula terminally with strong spines, subterminally with one barbed seta; tip of endite of maxillula with apically modified setae (armed with stiff barbs) plus several shorter setae with normal, fine barbs.Maxilla with well-developed exopod, moderately large, two-segmented palp, WITTMANN K.J. & WIRTZ P., Revision of the genus Mysidium WITTMANN K.J. & WIRTZ P., Revision of the genus Mysidium and three apically setose endites.Exopod with maximum width in median portions; its outer margin all along with series of plumose setae.Basal segment of palp with three barbed setae.Apical segment about two times as long as basal segment.Length of apical segment 2.0-2.5 times maximum width, densely setose on terminal margin but lined by small hairs along more than basal half of inner margin.Apex of palp with two strong, modified setae bearing strong, spine-like barbs along distal third of their inner margin.
nAuplioid lArvAe (Fig. 8M-N).Smooth cuticle all around, except for a pair of minute furcal processes and a number of minute setae on the blunt end of the abdomen.

Distribution and habitat
So far only known from euhaline, sublittoral waters of Curaçao (12° N), where the mysids occur in swarms hovering during daytime around and between corals.

Material examined
None.

Revised definition
Adapted to the scheme in Table 2 by using published data (Harrison & Bowman 1987;Ortiz et al. 2017b) on adults of both sexes: all features diagnosed above for genus and subgenus Mysidium Dana, 1852, with the reservation that the detailed structure of the setae on the merus of thoracic endopod 3 is unknown.Cornea calotte-shaped in dorsal view, its diameter 2.1 times as long as terminal segment of antennular trunk.Eyestalks smooth.Rostrum round-triangular, not extending beyond eyestalks.Appendix masculina obscurely bilobate, all along inner margins fringed by large setae; its length 1.5 times terminal segment of antennular trunk.Length of antennal scale four times maximum width; scale reaching well beyond antennular trunk.Median segment of mandibular palp with setae all along inner margin, whereas setae lacking on outer margin.Carpopropodus with two segments in thoracic endopods 1-2, 7-8, versus three segments in endopods 3-6.Basal segment is 0.6 times total length of carpopropodus of endopod 3. Male pleopod 1 stout, allusively bilobate.Sympod of male pleopod 4 without endite.Its exopod with basal About evenly rounded (1), moderately unevenly rounded (2) or distally angular (3) hump on outer face of terminal segment of maxillula Carpopropodus of third thoracic endopod (n)-segmented Basal segment is (x)-times total length of carpopropodus of third thoracic endopod 0.5 0.5 0.5 0.5 0.4-0.5 0.6 0.8 0.5 0.5 0.4 Numbers of serrated setae along merus of third thoracic endopod 0 0 0 0 0 ?0 2-6 2-7 ?
Penultimate segment of exopod of fourth male pleopod with entire (1) versus bifid (2) modified seta Endopod of fourth male pleopod is (x)-times length of sympod

Distribution and habitat
East Pacific coast of Mexico: Gulf of California, Islas Tres Marías, 21-26° N. Occurring in shallow, coastal marine waters where it forms dense epibenthic swarms (Gómez-Gutiérrez et. al. 2014).Also collected with night light at surface and from fish stomachs (Harrison & Bowman 1987;Ortiz et al. 2017b).

Etymology
Noun in nominative singular with neutral ending, formed by amalgamation of the Latin noun 'occidens' with the generic name Mysidium, referring to the occurrence on the west coast of America.

Material examined
None.

Revised definition
Adapted to the scheme in Table 2 by using published data (Ortiz et al. 2017a(Ortiz et al. , 2017b) ) on adults of both sexes: all features diagnosed above for the genus Mysidium Dana, 1852, and its new subgenus Occimysidium.Cornea crescent-like in dorsal view, with 'diameter' 1.5 times as long as terminal segment of antennular trunk.Eyestalks smooth.Rostrum triangular, apically bluntly pointed, not extending beyond eyestalks.Appendix masculina with separate inner and distal tufts of setae; its length 2.0-2.2 times terminal segment of antennular trunk.Length of antennal scale six times maximum width; scale reaching well beyond antennular trunk.Median segment of mandibular palp with setae on both margins.

Distribution
So far known only from type locality.

Etymology
Noun in nominative singular with neutral ending, formed by amalgamation of the Latin adjective 'orientale' with the generic name Mysidium, referring to the occurrence on the east coast of America.

Diagnosis
Subdivision of the genus Mysidium Dana, 1852, characterized by three-segmented carpopropodus of third thoracic endopod; merus of this endopod with serrated setae.Third male pleopod with strong endite at about 50-60% distance from basis of medial margin.Sympod of fourth male pleopod with series of setae on rostral face.Its exopod three-segmented; proximal segment without seta; the two distal segments each with one unbranched modified seta.Telson with terminal cleft.

Type locality
Cartagena at the Caribbean coast of Colombia (Zimmer 1915a).Coordinates estimated by present authors are about 10.37° N, 75.52°W.

Notes on additional material (Figs
Foregut (Fig. 13E-G) closely similar to that of M. antillarum sp.nov.(Fig. 11A-E).As main differences from this species, M. columbiae shows different structure and variable numbers of large spines: on each lateral half there are 2-4 apically pronged spines on posterior part of lateralia, the teeth of these spines with dense sets of secondary denticles (Fig. 13F); and there are 1-2 larger, apically pronged, serrated spines on dorso-lateral infolding, the latter spines flatter, more clavate, and with more slender teeth on basal ⅔ (Fig. 13G).

Notes on individual development
Nauplioid larvae (Fig. 13H) with smooth cuticle all around, except for a pair of minute furcal processes and a number of minute setae on the blunt end of the abdomen (Fig. 13J).Davis (1966: fig. 4) already reported apical, furcal 'spines' in nauplioids of M. columbiae from Jamaica.In non-types of the present material, the differentiation of male sexual characteristics is visible in early subadults as small rudiments of penis and appendix masculina and of a bifid fourth pleopod.At this stage the exopod of pleopod 4 is less than 3/2 as long as endopod and the endite of the sympod is missing or indistinct.In the course of further development a distinct endite (representing an important diagnostic character) becomes visible as soon as the exopod exceeds two times the length of the endopod.

Distribution and habitat
From 24° N to 8° S in coastal waters of Bahamas, Caribbean, southern Gulf of Mexico, and Brazil (Price & Heard 2004;Miyashita & Calliari 2014).The present samples from Curaçao and Bonaire fit within this geographic range.The present material was encountered at a depth of 3-26 m, during daytime in swarms hovering around and between corals.During the night dispersed over the sea floor.

Etymology
The species name is a Latinized Spanish noun in the genitive plural, referring to the occurrence at islands of the Antilles.

Definition
All features diagnosed above for the genus Mysidium Dana, 1852, and its new subgenus Orientomysidium.Cornea globose in lateral view, calotte-shaped in dorsal view, with diameter 1.4-1.7 times as long as terminal segment of antennular trunk.Eyestalks smooth.Rostrum triangular, apically rounded or pointed, not extending beyond basis of eyestalks.Antero-lateral edges of carapace rounded.Only males with anterior margin of antennular trunk dorsally with rounded, shield-like, mediodistal extension (Fig. 10A-B) bearing 2-3 setae.The largest seta 0.1 times extension length.Appendix masculina 2.2-3.9 times terminal segment of antennular trunk; its shorter, dorsal lobe with brush of long setae, its longer ventral lobe with less dense set of shorter setae.Length of antennal scale 7-11 times maximum width, scale clearly reaching beyond antennular trunk.Median segment of mandibular palp with setae on both margins.Distally angular hump on outer face of terminal segment of maxillula.Carpopropodus of thoracic endopods 1-8 with 2, 2, 3, 3, 2-3, 2-3, 2-3 or 2 segments, respectively.Basal segment occupies half total length of carpopropodus of endopod 3. Series of 2-7 serrated setae (Fig. 12B) along merus of third thoracic endopod.Pleopod 1 rod-like in both sexes, not considering the weak (sub)median widening at its vestigial endopodal portion (Fig. 12D, G).Sympod of male pleopod 4 with longitudinal series of 3-7 plumose setae on rostral face; without or with an indistinct medial widening (endite) at ⅓ of sympod length from basis.Exopod with basal segment occupying 69-73% total length.Endopod reduced to a terminally rounded lobe with 29-41% sympod length; endopod with 5-12 mostly large, plumose setae of various sizes.Endopod of uropods 0.6-0.8times as long as exopod.Telson subrectangular with apical cleft.Telson length 1.4-1.7 times maximum width near basis.Its lateral margins convex along proximal third and smooth along proximal 51-58%; distal portion of each margin with dense, continuous series of 12-17 acute spines.Subtriangular apical cleft penetrating 11-14% telson length; cleft separating two broadly rounded, apical lobes; cleft and terminal portion of lobes densely lined in continuous series by a total of 19-23 (sub)acutely pointed, stout laminae.
AntennAl AppendAges (Fig. 10A-B, F-G).Antennular trunk extends 35-56% its length beyond (artificially aligned) eyes.First to third segments occupy 47-52%, 14-18% or 30-37% total length of trunk, respectively.Trunk dorsally with forward-directed small, setose lobes near terminal margin of each segment: two lobes from the basal, one from the median, and one from the terminal segment (not counting the mediodistal extension of the anterior margin in males).Appendix masculina 0.7-1.4times total trunk length, antennal scale 1.2-1.6 times trunk.Sympod of antenna (Fig. 10G) produced into spiniform extension on outer distal corner.Antennal scale with terminal segment occupying 10-14% total length and bearing five plumose setae.Mouth pArts (Figs 10h,J,.Mandibular palp (Fig. 10J) without spines; terminal segment with strong, modified, bent seta at apex, and barbed setae along inner and outer margins.Median segment of palp with angular, medially directed dilatation, both its margins setose.Proximal segment normal, with smooth margins.Distal segment of maxillula (Fig. 11G) terminally with strong spines, subterminally with one barbed seta; tip of endite of maxillula with apically modified setae (armed with stiff barbs) plus several setae with normal, fine barbs.Maxilla (Fig. 11H) with well-developed exopod, moderately large, two-segmented palp and three apically setose endites.Exopod with maximum width in submedian portions; its outer margin all along with continuous series of plumose setae.Basal segment of palp with three barbed setae.Apical segment about two times as long as basal segment.Length of apical segment 1.9-2.5 times maximum width.Apical segment densely setose on terminal margin but lined by small hairs along more than basal half of inner margin; apex with two strong, modified setae bearing strong, spine-like barbs along distal third of their inner margin.

Distribution and habitat
In euhaline, sublittoral to littoral waters of Curaçao and Bonaire, both islands at 12° N. The new species forms swarms during the day, hovering closely above the sea floor at a depth of 0-26 m.Also found near shore boulders, around and off piers.

Material examined
No material available, not assigned to any subspecies: see Discussion.Băcescu (1991) indicated "Jamaica, St. D, 90.026 Sa Rba" as sampling site of the types.

Revised definition
Adapted to the scheme in Table 2 by using published data (Băcescu 1991) on adult females and subadult males: Mysidium with cornea calotte-shaped in dorsal view; diameter 1.5 times as long as terminal segment of antennular trunk.Anterior margin of carapace rounded.First to third segments occupy 57%, 13% and 30% total length of antennular trunk, respectively.Length of antennal scale six times maximum width, scale not reaching beyond antennular trunk.Carpopropodus three-segmented in WITTMANN K.J. & WIRTZ P., Revision of the genus Mysidium thoracic endopod 3, and two-segmented in endopod 8. Basal segment occupies 0.4 times total length of carpopropodus of endopod 3. Uropodal endopod 0.8 times as long as exopod.Telson subrectangular with apical cleft.Telson length 1.6 times maximum width near basis.Its lateral margins slightly tapering and smooth along proximal 56-63%; distal portion of each margin with dense continuous series of about 9-11 acute laminae.Subtriangular apical cleft penetrating 11% telson length; cleft separating two broadly rounded, apical lobes; cleft and terminal portion of lobes densely lined in continuous series by about 17-22 acute laminae.Telson with a total of about 35-42 laminae.

Distribution and habitat
Only known from type locality.Băcescu (1991) reported T. Iliffe as collector, so the positional data are interpreted by the present authors according to a copy of the sampling protocol submitted by courtesy of Tom Iliffe: Jamaica, Westmoreland Parish, Negril, Joseph's Caves, station 90.026, sea cave with swarm of mysids congregated at the cave entrance, collected from water column and sand bottom at depths of 1-3 m, 26 June 1990, leg.Tom Iliffe.Coordinates estimated by the present authors are 18.2680° N, 78.3681° W at the entrance of this touristic cave.Băcescu (1991) reported M. cubanense to co-occur with M. iliffei at this station.

Differentiation within the genus Mysidium Dana, 1852
Most interspecific differences belong to an ensemble of the 25 morphological criteria listed in Table 2 as being sufficient for the distinction of all the here acknowledged species of the genus.The numbers of morphological differences per species pair are summarized in Table 3 as a derivate from Table 2.Not considering the poorly known M. iliffei Băcescu, 1991, three species groups are distinguished whereby the respective numbers of differences range from 1-6 for within-group differences and 9-15 WITTMANN K.J. & WIRTZ P., Revision of the genus Mysidium for between-group differences.The great number of between-group differences strongly supports three different morphological entities, here established as the subgenera Mysidium Dana, 1852, Occimysidium subgen.nov.and Orientomysidium subgen.nov.The main differences between these taxa are emphasized in the above diagnoses of subgenera.

Differentiation within the nominotypical subgenus Mysidium Dana, 1852
Within this cluster of six very similar species, M. gracile is essentially distinguished by a weak, but distinct indentation of the terminal margin of the telson and by large setae on the mediodistal extension of the male antennular trunk, M. cubanense by the enormous eyes, M. rickettsi by the absence of setae on the outer margin of the median segment of the mandible in combination with a comparatively long basal segment of the carpopropodus of the third thoracic endopod, M. integrum by a traverse to weakly convex terminal margin of the telson in combination with a great number of spines on the lateral margins and M. rubroculatum by a more unevenly rounded hump on the outer face of the terminal segment of the maxillula and more rod-like first pleopod.Within its subgenus, M. triangulare sp.nov. is unique by a triangular apical portion of the telson, and shows on the average the greatest numbers of setae on the mediodistal extension of the male antennular trunk and of laminae on the terminal margin of the telson.
Conversely, it shows the fewest spines on the lateral margins of the telson.

Differentiation of the new subgenus Occimysidium
In addition to the ensemble of diagnostic characters indicated above for this monotypic subgenus, its type species, M. pumae, differs from all species of the remaining subgenera by acute laminae rather than spines on the lateral margins of the telson; this feature, however, is shared with M. iliffei.The lateral telson margins in M. pumae bear more laminae than the laminae or spines in the remaining species The sympod of the fourth male pleopod in M. columbiae differs from that in all remaining species of the genus by a strong endite at 2/ 5 distance from the basis (reservation: this feature is not clear in M. iliffei).
In M. antillarum sp.nov.this endite is reduced to a low, sometimes indistinct projection.A very short endite was figured by Brattegard (1969: fig. 27E) for material from the Bahamas, reported by him as M. columbiae, now considered as potentially indicative of M. antillarum sp.nov.Mysidium columbiae shares this series of setae on the rostral face of this sympod only with M. antillarum sp.nov.Zimmer (1915a: fig. 28) did not illustrate most of these setae for M. columbiae because he drew the opposite face of the sympod.It remains unclear where the most proximal seta in his figure inserts.Series of such rostrally inserting setae were identified by us in the types and in all additional specimens examined in this respect.We therefore included these setae in the revised definition of M. columbiae.In addition to the different size of the endite of the sympod of male pleopod 4, both sexes of M. antillarum sp.nov.differ from M. columbiae by more acute laminae on the terminal margin of the telson including the cleft.
Further differences the new species shows are typically a longer, more slender appendix masculina, more spines on the lateral margins of the telson, and fewer laminae on the terminal margin.

Definition and availability of Mysidium iliffei Băcescu, 1991
Among the characters described by Băcescu (1991), two striking ones are clearly out of the ranges of variation found in M. columbiae and M. antillarum sp.nov.: (1) antennal scale stouter and not extending beyond antennular trunk, and ( 2) lateral margins of telson with laminae rather than spines.
A potential third point is not clear for M. iliffei: Băcescu (1991) wrote "l'article basal du IV e pléopode ♂ n'a pas la dilatation si caractéristique de M. columbiae".However, only "3 ♂ juv." inspected together with "le IV e pléopode ♂ dépasse à peine le milieu du V e pléonite" suggest that he had treated 'subadult' males in the present terminology.Features of this pleopod given by Ortiz et al. (2017b) in the diagnosis of M. iliffei are based (Manuel Ortiz, pers. comm.)only on the unclear data given by Băcescu (1991).As shown in the above description of M. columbiae, the widening (endite) of the sympod of the fourth male pleopod is missing or unapparent in early subadults.Accordingly, the absence of an endite in subadults cannot be employed as being diagnostic of M. iliffei versus M. columbiae.
Attempts to obtain material of M. iliffei for study failed.Băcescu (1991) had indicated the Grigore Antipa Museum, Bucharest, as the deposition site of the types.However, the types are not in this collection according to the holdings listed by Petrescu & Wittmann (2009).An additional check in 2018 confirmed WITTMANN K.J. & WIRTZ P., Revision of the genus Mysidium that there is neither any entry in the inventory nor any corresponding material in the collection (Melanya Stan and Iorgu Petrescu, pers. comm.).An online request (platform 'CRUST-L', 8 Mar.2018) to the crustaceology community yielded no conclusive information about a potential alternative deposition of the types and any other potential material of this taxon.No additional samples were published after the first description by Băcescu (1991).Accordingly, M. iliffei is currently not assigned to any subgenus due to insufficient knowledge and inaccessibility of material.
segment occupying 58% total length.Endopod reduced to lobe with ≈ 11% sympod length; apically with one long seta and more proximally six additional, smaller setae.Uropodal endopod 0.6-0.7 times as long as exopod.Telson linguiform, tapering posteriorly, length 1.9 times maximum width near basis; lateral margins slightly convex, almost straight; terminal margin clearly convex, well rounded.Proximal half of lateral margins smooth, distal half with dense series of 30-37 short, blunt or pointed spines on each side; lateral margins each with about 19 spines, terminal margin with about 24 spines.

Table 3 .
Number of documented morphological differences between and within the proposed subgenera ofMysidium Dana, 1852.Bold print marks differences between species pertaining to different subgenera.European Journal of Taxonomy 495:1-48 (2019)(exception: number of spines in M. integrum).The basal segment of the carpopropodus of the third thoracic endopod is longer in M. pumae than in the remaining species of the genus.

within the new subgenus Orientomysidium Only
M. columbiae, M. antillarum sp.nov.andM.iliffeishow a telson with a distinct, subtriangular cleft, separating two rounded latero-apical lobes; cleft and terminal portions of the lobes are densely furnished with acute or blunt laminae, while the armature of the lateral margins differs more strongly between species.Additional exclusive features are shared by M. columbiae and M. antillarum sp.nov., but are unknown in M. iliffei: third and fifth male pleopods longer and more slender compared to that in the remaining species of the genus.Serrated setae (Figs9D, 12B) are present on the merus of the third thoracic endopod in M. columbiae and M. antillarum sp.nov., but this detail is unknown in M. iliffei and M. rickettsi, and such modified setae are absent in the remaining six species of the genus.In addition, M. columbiae and M. antillarum sp.nov.share a strong endite (Figs 9G, 12J) on the third male pleopod with M. pumae, as opposed to the medial widening (Figs1E, 2E, 3E, 4E, 7K) visible in M. gracile, M. integrum, M. cubanense, M. rubroculatum, and M. triangulare sp.nov.They also share on average fewer setae on the mediodistal extension of the antennular trunk compared to the latter five species (Figs 9B, 10B versus Figs 1B, 2B, 3B, 4B, 5B).