Palaumysis philippinensis, Hanamura & Kase, 2002

Hanamura, Yukio & Kase, Tomoki, 2002, Marine cave mysids of the genus Palaumysis (Crustacea: Mysidacea), with a description of a new species from the Philippines, Journal of Natural History 36 (3), pp. 253-263 : 257-262

Palaumysis philippinensis sp. nov.

(®gures 3± 5)

Material examined

HOLOTYPE ( NSMT Cr 13080): adult male (1.8 mm),` Mapating Cave’ diving site, St. AN-3, SW Maricaban Island, Batangas, Philippines, 13ss40¾09.0 ² N, 120ss48¾58.2 ² E, 46 m, submarine cave, total darkness inside, calcareous sand, 20 November 1998, leg. Ohashi, Kinjo, Tabuki, Cabrera and Kase.

PARATYPES ( NSMT Cr 13081): 52 males (1.6±1.9 mm), 29 females (1.8±2.3 mm) including at least four ovigerous females (2.0 ± 2.3 mm), three juveniles (1.1 ±1.3 mm), one ind. (damaged), data same as for the holotype .

Description

Small-sized mysid, with external surface smooth and no folds or spines (®gure 3a, b).

Carapace reduced, fused with anterior three thoracic somites, with small unfused lobe posteroventrally; posterior margin nearly straight, leaving last ®ve thoracic somites uncovered; anterior margin produced into triangular rostral plate with somewhat rounded apex; anteroventral corner forming acute angle; cervical groove marked (®gure 3a±c).

Abdomen with ®rst ®ve somites subequal in length, somite 6 about twice length of somite 5 and stouter than anterior somites (®gure 3a, b). Telson entire, subtriangular, nearly as long as maximum width, armed with pair of distal spines, no plumose setae on all margins (®gure 3d). Exopod of uropod narrow, somewhat truncate distally, extending distinctly beyond end of telson, armed laterally with three short setae on distal half, increasing in length distally, distal and mesial margins armed with numerous long setae (®gure 3d). Endopod of uropod narrow, longer than exopod, armed with numerous long setae on lateral and mesial margins; statocyst present (®gure 3d).

Eye developed, cornea well pigmented, globular, distinctly larger than eye stalk (®gure 3b, c).

Antennule with peduncular segment 3 robust and longest, with single seta on distomesial margin and about four sensory setae at base of outer ¯agellum, male lobe undeveloped, inner ¯agellum very short, consisting of ®ve segments, armed with terminal seta; peduncular segment 2 shortest; peduncular segment 1 slightly longer than 2 (®gure 3e).

Antenna with rudimentary scale, sub-lanceolate, slightly curving inwards, with somewhat obtuse distal margin, armed with long terminal seta and few short setae on lateral margin (®gure 3f).

Labrum with anterior margin somewhat evenly convex, without frontal process or spine (®gure 3g).

Mouthpart s as illustrated (®gure 4a±e).

Thoracic limbs 3±8 rather stout, all similar in shape; exopods with distal segment composed of six or seven subsegments; endopods with carpo-propodi unsegmented, shorter than merus, dactylus short, with stout terminal seta (®gure 4f).

Lamellar epipod on thoracic limb 1 (®gure 1a).

Penis rod-shaped, with single seta distally.

Marsupium consisting of two pairs of lamellae, anterior one smaller than posterior.

Pleopods in males rudimentary, unsegmented (®gure 4g ±k); pleopod 1 with long, anteriorly curving plumose seta and a few short setae distally; pleopods 2, 3 and 5 similar in shape, posteriormost slightly longer than others, armed with about three setae distally; pleopod 4 laterally armed with two or three setae, distally ending in strong spine, reaching anterior one-third to one-half of abdominal somite 6, articulation between pleopodal plate and terminal spine indistinct.

Pleopods in females tiny, armed with three to ®ve setae distally; pleopod 1 in immature females, like males, with long, anteriorly curving plumose terminal seta, but no plumose seta in mature females; pleopods 2±5 similar in shape (®gure 4l, m).

Colour

The entire body in live specimens is assumed to be semi-transparent, since the foregut can be seen through exoskeleton in preserved specimens (about 6 months after preservation in formalin seawater).

Body size

Largest male is 1.9 mm in total length, largest female is 2.3 mm. Females at a size of 2.0 mm or greater could carry embryos.

Embryo

Larval development of Palaumysis philippinensis is illustrated in ®gure 5. The Stage I embryo (egg) is horse bean-shaped, 0.38 mm along the longest axis (®gure 5a), and 1.00 mm in Stage III larvae (®gure 5f). Although the general form of embryos is not so diOEerent from other members of the Mysidae , the Stage II larvae (®gure 5c±e) were noted to have proportionately long antennule and antenna: e.g. Archaeomysis in Hanamura (unpublished); Gastrosaccus in Brown and Talbot (1972); Leptomysis in Wittmann (1981); Misidium in Davis (1966); and Neomysis in Murano (1964).

The observed maximum clutch size of Palaumysis philippinensis was two. Presumably, P. philippinensis does not carry more than two eggs as the size of female marsupium is not enough to cover all the embryos even in early stage eggs, and in Stage III larvae only a posterior part of the abdomen is covered by both sides of the marsupial lamellae.

Larvae of Palaumysis philippinensis might be released from the marsupium at a size of 1.0 ± 1.1 mm, since the smallest free-living juvenile was 1.1 mm.

Etymology

The speci®c name is derived from the type locality; it is used as an adjective, in agreement with the gender (feminine) of the generic name.

Remarks

Palaumysis philippinensis is distinguished immediately from P. simonae by the carapace with a small unfused lobe posteroventrally, as opposed to having no such free lobe. Palaumysis philippinensis also diOEers from P. simonae in the following minor particulars: the antennal scale in the new species possesses a long terminal seta in contrast to bearing a minute seta; the inner ¯agellum of the antennule is consistently composed of ®ve articles as against four, and the male fourth pleopod is proportionately longer, reaching to anterior one-third to one-half of the sixth abdominal somite, while it slightly extends beyond the posterior end of the ®fth abdominal somite in P. simonae . In general appearance, P. philippinensis appears to have relatively more primitive features than P. simonae .

Distribution Known only from the type locality in the Philippines; 46 m.