Description of the supergiant isopod Bathynomus raksasa sp. nov. (Crustacea, Isopoda, Cirolanidae) from southern Java, the first record of the genus from Indonesia

Abstract The giant isopod genus Bathynomus A. Milne-Edwards, 1879, is recorded for the first time in Indonesian waters, from deep waters off southern Java in the Indian Ocean. Bathynomus raksasasp. nov. is described and notes on juvenile specimens of an unidentified species found in the same locality are also provided. Bathynomus raksasasp. nov. is characterized by the large size (averaging at 330 mm), narrowly rounded clypeus apex, prominent longitudinal carina on the clypeus, convex lateral margins of the uropodal exopod and endopod, produced distolateral corners of the uropodal exopod and endopod which have acute ends, an uropodal exopod with a setal fringe of medium length (69%), a pleotelson 1.6 times wider than long with the posterior margin medially concave, and the large number (11–13) of spines on the pleotelson.


Introduction
The genus Bathynomus A. Milne-Edwards, 1879 inhabits the deep sea in the Atlantic, Pacific and Indian Oceans, with some species reaching large sizes in excess of 30 cm length (Lowry and Dempsey 2006). Nineteen extant species are known in the genus (Bruce 1986, Magalhães and Young 2003, Lowry and Dempsey 2006, Boyko et al. 2008, Shipley et al. 2016, Kou et al. 2017. Lowry and Dempsey (2006) revised the Indo-West Pacific taxa and recognized 16 species, of which seven were categorized as "supergiants"; species maturing above 150 mm and reaching 500 mm in length. Five "supergiant" species occur in the Indian and Pacific Oceans: Bathynomus lowryi Bruce & Bussarawit, 2004 (Andaman Sea), B. crosnieri Lowry & Dempsey, 2006 (Madagascar), B. keablei Lowry & Dempsey, 2006 (India, Sri Lanka, Burma), B. kensleyi Lowry & Dempsey, 2006 (Coral Sea, Philippines, South China Sea), and B. richeri Lowry & Dempsey, 2006 (New Caledonia) (Lowry and Dempsey 2006). Two other "supergiant" species are known from the western Atlantic: B. giganteus A. Milne-Edwards, 1879, and B. miyarei Lemos de Castro, 1978(Boyko et al. 2008). The new species described here adds another "supergiant" Bathynomus from the Indian Ocean to this list, and is the first from Indonesia.

Material and methods
The material was collected by the 2018 South Java Deep Sea Survey (SJADES 2018), a joint project between NUS and LIPI, with localities mostly in southern Sumatra and Java (Fig. 1). The terminology used and description format follows Lowry and Dempsey (2006).

Taxonomy
Pleotelson (Fig. 2D) broader than long, 1.6 times as wide as length, posterior margin medially concave, smooth (minute pores), conspicuous longitudinal carina on dorsal surface, with 11 distal and 2 lateral straight acute prominent spines along distal margin, without setae between spines, central distal spine simple.
Female. Similar to male. Variation. Robust setae count on female as follows: exopodal lateral margin with 7-10 robust setae, distal margin with 4 or 5, endopodal lateral margin with 3-5 and distal margin with 8-10; pleotelson with 9 distal and 2 lateral straight acute prominent spines along distal margin.
Etymology. The epithet is the Indonesian word "raksasa" for giant, alluding to its enormous size and the significance of the find. The name is used as a noun in apposition. Remarks. Bathynomus raksasa sp. nov. can be readily identified by its large size (330 mm on average), narrowly rounded clypeus apex, produced and acute distolateral corners of uropodal rami, wider rather than long pleotelson with medially concave posterior margin and the presence of 11-13 pleotelson spines. Bathynomus raksasa sp. nov. is the sixth "supergiant" species from the Indo-West Pacific and is one of the largest known members of the genus.
In general appearance, B. raksasa sp. nov. is most similar to B. giganteus and B. lowryi. All three are large, averaging 300 mm in length, possess a prominent longitudinal carina on the dorsal surface of the pleotelson and have acute spines on the distal margin of the pleotelson. The new species is closest to B. giganteus, sharing the relatively medium length of antenna 2 (reaching to between the posterior of pereonite 2 and anterior of pereonite 3), lateral and posterior shape of the uropodal exopod and endopod, and the pleotelson spine count. Bathynomus raksasa sp. nov., however, differs markedly from B. giganteus by its more conspicuous longitudinal carina on the clypeus ventral surface ( Bathynomus raksasa sp. nov. shares the same general uropodal exopod and endopod shape as B. crosnieri, B. kensleyi and B. richeri but can easily be distinguished from them in its possession of a conspicuous longitudinal carina on the dorsal surface of the pleotelson (Fig. 5A). Although the number of spines on the margin of the pleotelson (at least 11) is similar to those of B. crosnieri and B. richeri, the presence of the longitudinal ridge on the pleotelson easily separates B. raksasa sp. nov. from these species. Bathynomus raksasa sp. nov. also has the same number of spines on the margin of the pleotelson but can easily be distinguished from B. keablei in having the distolateral corners of the uropodal exopod and endopod distinctly produced (Fig. 3D, E) (vs. rounded and not produced in B. keablei; see Lowry and Dempsey 2006: fig. 17).
The appendix masculina is absent on the holotype male of B. raksasa sp. nov. (Fig.  5G) but this is almost certainly not a species-character. It is known to be sometimes absent in B. doederleini from Taiwan (present study); with five out of seven males below the size of 130 mm lacking it. The largest males of B. doederleini (136-145 mm) possess appendix masculina. The absence or presence of appendix masculina has been previously used by Soong and Mok (1994) to determine the maturity of males of Bathynomus doederleini; "mature males" were males with appendix masculina and "maturing males" were those without appendix masculina and testes. Barradas-Ortiz et al. (2003) reported that some adult males of B. giganteus from Brazil (mostly smaller specimens below 290 mm) lacked appendix masculina, especially in summer. They suggested that these smaller males might have been less reproductively active in summer and/or the appendix masculina may be a non-permanent organ which is lost or regrown when the animals moult (Barradas-Ortiz et al. 2003). Barradas-Ortiz et al. (2003) also noted that larger male specimens of B. giganteus tend to keep the organ for longer periods than smaller ones, although even large individuals (310 mm) sometimes do not possess the structure. We cannot be certain that either of the patterns above apply to B. raksasa sp. nov. as only one male was collected. The appendix masculina (Fig. 5H) is present on the large male American specimen of B. giganteus (ZRC 2014.0837) examined here.
The SJADES cruise also obtained four juvenile and subadult specimens from southern Java (here identified as Bathynomus sp.) (Fig. 6) which we are unable to identify to the species level, especially as the diagnostic characters may not be developed. They are clearly not Bathynomus raksasa sp. nov. with a different pleotelson spination, shapes of pleotelson and uropodal rami. The largest specimen in the lot (107 mm) has an almost fully-developed pereopod 7 which indicates that the adult would not be too much larger in size. This, along with the presence of setae between the pleotelson spines, suggest that this species belongs to the "giant" group. The number of spines on the posterior margin of the pleotelson ranges between 5+2, 7+2 and 9+2. Soong and Mok (1994) used the development of pereopod 7 as one of the characters to classify the development stages of Bathynomus doederleini. According to Soong and Mok (1994), individuals with "small, white" pereopod 7 and lacking either oostegites or penes and/ or appendix masculina were categorised as "subadult I" which equals to stage 2 of five development stages they proposed. However, we will not apply this approach to Bathynomus sp. because of the limited specimen number.
Bathynomus sp. superficially resembles the poorly known Bathynomus affinis Richardson, 1910, described from the Philippines from one specimen. There is, however, a problem with what has been identified as "Bathynomus affinis" by Lowry and Dempsey (2006: 169, figs 2, 3), who listed among their material, the type from the Philippines as well as two females from the Arafura Sea, providing figures of the latter. Bruce (1986: fig. 87A-E) had earlier figured the uropods, and pereopods 1 and 3 of the type specimen (sex not specified). The problem is that the distolateral corners of uropodal rami of the holotype from the Philippines is distinctly acute and curved (Bruce 1986: fig. 87A-C) whereas that of Lowry and Dempsey (2006: fig. 3D, E) from the Arafura Sea is distinctly wider and not produced. Significantly, Richardson's (1910: fig. 1) figures of the uropods are the same as those by Bruce (1986). The material from Arafura Sea are thus unlikely to be B. affinis s. str. Our material of Bathynomus sp. from Java resembles the "B. affinis" of Lowry and Dempsey (2006) in possessing the same relative length of antenna 2 (reaching between pereonites 3 and 4), straight clypeus distal margins, the setal fringe on the uropodal exopod is long and continuous (± 90%), and similar pleotelson spine count (5+2, 7+2 and 9+2). The marked difference in the form of the uropodal endopod distolateral corner, however, indicates they are not conspecific. In addition, the uropod of Bathynomus sp. reaches to the end of the pleotelson (Fig. 6D) (vs. slightly extended beyond the pleotelson; Lowry and Dempsey 2006: fig. 2F) and the pleotelson central spine is weakly bifid (Fig. 6C) (vs. simple; Lowry and Dempsey 2006: fig. 2F). The uropods of our material from Java agree very well with the figures by Richardson (1910) and Bruce (1986), but until a complete redescription of the holotype of B. affinis is done and more character states are known, we are not certain if they are actually conspecific.
Distribution. Sunda Strait and Indian Ocean, South Java, Indonesia; at depths of 957-1259 m.