New records of Benthesicymus Bate, 1881 (Dendrobranchiata, Penaeoidea, Benthesicymidae) from the abyssal depths of Taiwan

Abstract The deep-sea Benthesicymus shrimps generally inhabit waters deeper than 1000 m deep. Recent deep-sea cruises off Taiwan collected two species of Benthesicymus Bate, 1881 from the abyssal depths greater than 3,000 m. They are B.crenatus Bate, 1881 and B.laciniatus Rathbun, 1906. Both of them are new records for Taiwan, with B.crenatus also representing the deepest (5,314 m) marine animal so far known for the island. The major distinguishing characters of these two species are described and illustrated.


Introduction
The eastern and southern coasts of Taiwan are deep-sea areas including the abyssal zone. Knowledge on the deep-sea fauna of Taiwan, however, was rather limited only until recently. The ongoing Taiwan deep-sea cruises begun in 2000 have successfully sampled the deep-sea benthic fauna off the island and reported on several abyssal decapod crustaceans (Osawa et al. 2006, 2008a, b, Baba et al. 2009, Komai and Chan 2009).
Shrimps of the genus Benthesicymus Bate, 1881 are generally distributed in waters between 1000 and 2000 m depth (Crosnier 1986, Kikuchi andNemoto 1991). Like other members of the family Benthesicymidae Wood-Mason & Alcock, 1891, they have a thin integument and short rostrum. The characteristics of this genus are the presence of podobranchia on the second maxilliped to third pereiopod, a telson with an acute tip and bearing at least three pairs of movable lateral spines, and dactyli of the fourth and fifth pereiopods not being subdivided (Kikuchi andNemoto 1991, Pérez Farfante andKensley 1997). Only one species of Benthesicymus, B. investigatoris Alcock & Anderson, 1899, has been listed from Taiwan but without further information (Lee et al. 1999). During recent deep-sea cruises off Taiwan, many specimens of Benthesicymus were collected and some of them were obtained from abyssal depths of more than 3000 m, including the deepest trawling station down to 5,314 m. Careful examination of the abyssal material of Benthesicymus revealed two species, namely B. crenatus Bate, 1881 andB. laciniatus Rathbun, 1906. Both are new records for Taiwan and with specimens of B. crenatus representing the deepest marine animals currently known in Taiwan. The present work reports upon these findings.
The specimens are deposited at the National Taiwan Ocean University (NTOU). The measurement given is carapace length (cl) measured dorsally from the postorbital margin to the posterior margin of the carapace. The synonymy provided is restricted to important works on the species, and the description given is based on the material from Taiwan.  Description. Integument membranous and soft. Rostrum dorsally compressed and slightly elevated into a low crest, dorsal margin with three or rarely four (only in one specimen of the present material) teeth, ventral margin without teeth. Antennal spine minute but distinct. Hepatic spine present with deep cervical groove behind it. Hepatic and branchiocardiac carinae elevated ( Fig. 2A). Fourth to sixth abdominal somites with posteromedian spines. Posterior margin of fourth abdominal tergite distinctly crenate; with 19-33 (usually 23-28) teeth, medial teeth larger but obtuse (except for median tooth which is largest and acute) while lateral teeth sometimes sharper but smaller (Fig. 2B). Telson with three pairs of movable lateral spines and one pair of terminal spines, distal pair of lateral spine adjacent to terminal spine (Fig. 2C). Third maxilliped and first pereiopod both with merus and ischium bearing a sharp distomesial spine (Figs. 2D-E). Distal segment of third maxilliped with sharp spine at tip (Fig.  2F). Thelycum lacking seminal receptacles but bearing median longitudinal carinae on fifth and sixth thoracic sternites. Seventh and eight thoracic sternites with large tubercle and sharp median spine, respectively (Fig. 2G). Petasma with lateral lobe generally wide and flat, except for a long strong and narrow submarginal fold at ventrolateral lobule, another shorter subdistal fold also present on dorsolateral lobule; median lobe with distal margin minutely serrated (Fig. 2I), dorsomedian lobule strongly folded and densely covered with small hooked spines (Fig. 2H).
Distribution. Northwest and Central Pacific, at depths of 3,530 to 6,350 m. There is one record of 0-5,700 m for this species (Kikuchi and Nemoto 1991) but it is very likely that the material was collected from more than 3500 m deep.
Remarks. The 27 specimens examined were collected from 4,412 to 5,134 m deep and most of them are damaged due to their fragile bodies even though their sizes are quite large. Nevertheless, they can be positively identified as B. crenatus by the characteristic comb-like crenation on the posterior margin of the fourth abdominal tergite. The Taiwanese material also generally fits well with previous descriptions of B. crenatus (Bate 1881, 1888, Crosnier 1986, Kikuchi and Nemoto 1991, Kim et al. 2000. The present report of B. crenatus from 5,324 m represents the deepest marine animal recorded from Taiwan. The previous deepest records from Taiwanese waters were the squat lobsters (Galatheidae Samouelle, 1819), Munidopsis profunda Baba, 2005 and M. taiwanica Osawa, Lin & Chan, 2008b from 5,011 m deep (Osawa et al. 2008b, Baba et al. 2009).
The 15 species known in Benthesicymus (De Grave and Fransen 2011) are often separated into two groups by the following characteristics: (1) branchiostegal spine not sharp and located at margin of carapace in group I vs. very sharp but situated behind the carapace margin in group II; (2) exopod of first maxilliped abruptly narrow to tip in group I vs. tapering to tip in group II; (3) merus of second maxilliped expanded in group I vs. unexpanded in group II; (4) dactylus of third maxilliped triangular with only one terminal spine in group I vs. subrectangular with more than one spine in group II; (5) exopods of all pereiopods small but visible in group I vs. very tiny in group II (Burkenroad 1936, Kikuchi andNemoto 1991). Groups I and II consist of ten and five species respectively, and B. crenatus belongs in group I.

Benthesicymus laciniatus
Coloration. Unknown. Distribution. Worldwide distribution and reported from eastern Atlantic, eastern Pacific and Indo-West Pacific, at depths of approximately 1,325-4,000 m but generally from 1,500-3,000 m (Crosnier 1986).
Remarks. Benthesicymus laciniatus is recorded from Taiwan for the first time. As mentioned by Kim et al. (2000), B. laciniatus generally inhabits shallower waters than B. crenatus and only sometimes occurs in the abyssal zone. The two Taiwanese specimens have been collected from depths of 3,030-3,070 m and 1,301-1,302 m respectively. In Benthesicymus only two species have the posterior margin of the fourth abdominal tergite crenate (see Burkenroad 1936, Crosnier 1986). The distinguishing characters of these two species are given in the "Remarks" of B. crenatus.
The present two males collected from Taiwan generally agree with previous descriptions of the species except for the petasma with the distal margin of the median lobe not distinctly serrated (see Burkenroad 1936: fig. 1, Crosnier 1986: fig. 8a, b, e). Such difference may be due to the present males are much smaller (cl 14.3-25.0 mm vs. cl 33-36 mm for Burkenroad 1936: fig. 1; Crosnier 1986: fig. 8a, b, e) and probably juveniles.