Gastrosaccus yuyu, Bamber, Roger N & Morton, Brian, 2012

Gastrosaccus yuyu sp. nov.

Figs. 2–5 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5

Type-material. 1 adult 3, holotype (BMNH 2012.1054). 1 brooding Ƥ, allotype (BMNH 2012.1097). 17 mature 3, 18 mature Ƥ (7 brooding), 5 subadult 3, 9 subadult Ƥ, paratypes (BMNH 2012.1098-1107). 1 brooding Ƥ and 1 adult 3, dissected.

Type-locality. Parangkusomo Beach, Java, Indonesia, 8°1'19.5" S 110°19'29.4" E, littoral, 4 November 2011, coll. BM.

Description of male: Body ( Fig. 2 View FIGURE 2 A) length of holotype 8.4 mm. Carapace elongate, 0.4 times total body length; rostrum rounded, conspicuous ( Fig. 3 View FIGURE 3 A), secondary process ventral to this; posterior border emarginate ( Fig. 2 View FIGURE 2 B, 3A), median margin with 11 or 12 slender spine-like filaments, lateral lobes with six or seven such filaments. Abdominal somites 1 to 4 subequal, fifth somite 1.6 times as long as fourth, and with mid-dorsal posterior apophysis ( Fig. 3 View FIGURE 3 B, C), sixth somite 1.3 times as long as fifth; somites 1 to 5 with posterolateral melanophore on each side, sixth somite with two posterior melanophores on each side.

Antennule ( Fig. 2 View FIGURE 2 C), first article of peduncle 1.6 times as long as wide; second article broader than long, 0.3 times as long as first article, with inner distal seta and two spines on outer margin; third article twice as long as second article, bearing short digitate process at the base of the outer flagellum on the dorsal side (little more than a prolongation of distal margin), and one outer marginal spine. A well-developed hirsute lobe present near base of the outer flagellum, bearing about 30 aesthetascs.

Antennal scale ( Fig. 2 View FIGURE 2 D) just over three times long as wide, distally exceeding tip of second peduncle article; lateral margins of scale straight, outer distal corner terminating in a prominent spine; inner and distal margins armed with about 18 plumose setae; suggestion of incomplete distal suture. Second antennal peduncle article twice as long as wide, with four plumose inner marginal setae; third peduncle article with inner distal plumose seta, penicillate seta and simple setule.

Mouthparts typical for the genus. Labrum ( Fig. 2 View FIGURE 2 E) simple, with single distal spiniform process. Mandibular palp ( Fig. 2 View FIGURE 2 F), first article naked, second and third articles densely covered with barbed setae; third article with a terminal comb of setulose setae and two barbed apical setae.

First thoracic limb ( Fig. 4 View FIGURE 4 A) with well-developed endite on basal article. Endopod short, robust, densely setose along inner margins. Outer distal corner of carpus with one seta. First exopod article expanded, outer distal angle without tooth. Flagellum composed of 11 segments.

Second thoracic limb ( Fig. 4 View FIGURE 4 B) similar in form to first, first exopod article bearing tooth on outer distal angle (as on seventh thoracic limb, Fig. 4 View FIGURE 4 C).

Third to eighth thoracic limbs similar in form (e.g. Fig. 4 View FIGURE 4 C). Carpus and propodus of endopod fused, subdivided into 12 to 14 articles, each with one short spine and longer setae. First exopod article of each limb with tooth on the outer distal angle, including on eighth limb. Exopod flagella composed of 12–14 segments on third to eighth limbs. Penis ( Fig. 3 View FIGURE 3 D) with four outer barbed setae and one distal simple seta.

All pleopods biramous, all their setae plumose. First pleopod ( Fig. 5 View FIGURE 5 A) with swollen sympod, 1.5 times as long as wide, outer margin with nine setae. Endopod four times as long as wide, unsegmented, bearing two distal and one subdistal setae. Exopod 2.5 times as long as endopod, nine-segmented.

Sympod of second pleopod ( Fig. 5 View FIGURE 5 B) 1.4 times long as wide. Endopod with nine segments, just shorter than sympod. Pseudobranchial lobe on first endopod segment, bearing three distal setae. Exopod of ten articles, little longer than endopod.

Endopod of third pleopod ( Fig. 5 View FIGURE 5 C) with seven segments, first article considerably enlarged, bearing conspicuous pseudobranchial lobe with three distal setae. Exopod with first article subdivided into four or five “semi-segments”, with three or four semi-fused segments proximally, and longer distal segment; second article 0.8 times as long as first; third article 0.8 times as long as second article; fourth article half as long as second article, with one subdistal spine and two distal barbed spines.

Fourth ( Fig. 5 View FIGURE 5 D) and fifth ( Fig. 5 View FIGURE 5 E) male pleopods similar in form, endopods each reduced to one article and armed with four distal setae. Exopods composed of eight segments.

Uropod ( Fig. 6 View FIGURE 6 A) exopod subequal in length to endopod, outer margin armed with some 15 curved spines that are finely plumose on their concave (posterior) sides; proximal spine shorter, submarginal. Endopod more slender than exopod, with seven or eight irregularly spaced spines confined to proximal two-thirds of inner border (smallest, proximal marginal spine in Fig. 6 View FIGURE 6 A not always present), and submarginal spine adjacent to statocyst. Otherwise, all margins with plumose setae.

Telson ( Fig. 6 View FIGURE 6 B) 2.8 times long as basal width. Lateral margins with ten spines including apical spines, with distinct gap between the apical spine and the subapical spine, subapical spine only just shorter than apical spine. Distal cleft one-eighth of total length of telson and armed with about 16 spinules along each margin.

Comments on female. Brooding females of two sizes: body length of allotype is 9.1 mm (as most adult females) and of dissected specimen is 12.9 mm. First pleopod ( Fig. 4 View FIGURE 4 D) biramous, other pleopods (e.g. Fig. 4 View FIGURE 4 E) uniramous, simple.

Etymology. The local guide in Java referred to the mysid as “ yuyu ”, although this is in fact the Bhasa Javanese word for any small marine or freshwater crustacean.

Remarks. Heard and Price (2006) reviewed and presented an identification key to the “burrowing” genera of the Gastrosaccinae : Gastrosaccus yuyu sp. nov. has the multiarticulated third male pleopod with a simple styliform exopod, no lateral spines on the anterior margin of the labrum, a biramous first female pleopod, and a distinct gap between the terminal and subterminal lateral spines on the telson (although these spines are subequal in length) characteristic of the genus Gastrosaccus .

Gastrosaccus yuyu is characterized by its having a mid-dorsal apophysis on the fifth abdominal somite at all stages of development, and a fringe of slender spine-like filaments along the posterodorsal margin of the carapace, including the posterior lateral lobes. The inner spines on the uropod endopod do not occur on the distal third of the margin, while the outer spines on the uropod exopod extend for its full length. The telson is cleft to one-eighth of its length, and the margins bear ten spines (including the apical spine), with the apical spine only very slightly longer than the sub-apical spine.

Wooldridge and McLachlan (1986) described as new Gastrosaccus sorrentoensis from near Perth, Western Australia, the only previously-known species of the genus to have posterior spine-like filaments on the posterior lateral lobes of the carapace as well as on the posterodorsal margin (in addition to having a mid-dorsal apophysis on the fifth abdominal somite). As the present species is generally similar to G. sorrentoensis , type material (Registration no. C15771) of the Australian species, generously made available by the Western Australian Museum, was examined.

Although the species are of a similar adult size, G. yuyu has only about half-as-many filaments (six to seven) on the inner-posterior margins of the lateral lobes of the carapace as does G. sorrentoensis (11 to 12 on each side); the number of filaments on the postero-dorsal margin of the carapace, while generally fewer in G. yuyu (11 to 12), is more variable in G. sorrentoensis , ranging from 14 to 25. In addition, the conformation of the distal tip of the mandibular palp is quite distinct (see Wooldridge and McLachlan 1986: fig. 1E) and obvious in the whole animal, the rostrum is rounded and protruding in G. yuyu (less protruding and simply triangular in G. sorrentoensis ), the first exopod article of the pereopods on the present species are proportionately broader, and there are differences in the setation of the male and female pleopods as well as the relative proportions of the distal three articles of the third male pleopod (e.g. third article only 0.6 times as long as second article in G. sorrentoensis , compared with 0.8 times in G. yuyu ). Finally, the apical telson spines in G. yuyu are hardly longer than the sub-apical lateral spines and are separated from those spines by a gap distinctly larger than those between the previous spines, while in G. sorrentoensis the apical telson spines are nearly twice as long as the lateral spines but are not separated from those by a distinctly larger gap; the median anterovental spine on the telson of G. sorrentoensis was not found on the present species.

These two species would appear to be siblings, but, with a wide zoogeographic separation (some 24° of latitude), these littoral taxa have presumably undergone allopatric speciation.

Wooldridge and McLachlan (1987), in describing their new species Gastrosaccus namibensis as the “fourth” known member of the genus to possess a fringe of slender spine-like filaments along the posterodorsal margin of the carapace (they did not include G. sorrentoensis ), compared it (with a useful summary table) with the other three, viz. G. spinifer ( Goes, 1864) , G. m u t i c u s W. Tattersall, 1915 and G. simulans, W. Tattersall, 1915 (the last two now moved to Eurobowmaniella Murano, 1995 ). Since that publication, five further species of Gastrosaccus have been described, all of which have either very few ( G. trilobatus Murano & McLachlan, 1998 ) or no ( G. madagascariensis Wooldridge et al., 1997 ; G. sarae Panampunnayil, 1999 ; G. wittmanni Deprez et al., 2000 ; G. mozambicus Wooldridge & Mees, 2003 ) posterior carapace spine-like filaments.

Of these five species with filaments, only Gastrosaccus spinifer (the type-species of the genus) has a middorsal apophysis on the fifth abdominal somite in the adult, as in the present species, although in G. simulans this process is present in immature and newly-hatched young. The morphology of the third male pleopod in G. yuyu is also almost identical to that of G. spinifer .

Recent material of Gastrosaccus spinifer from the Severn Estuary, UK, has been examined to confirm distinctions. G. yuyu differs from G. spinifer most obviously in having marginal carapace filaments on the lateral lobes, while in G. spinifer they are confined to the median margin (cf. Tattersall & Tattersall 1951, Fig. 36H) (as they are also in the other species listed above other than G. sorrentoensis , e.g. Wooldridge & McLachlan 1987: fig. 1A). In addition, the telson has a proportionately shorter posterior cleft (to one-eighth of its length, compared with over one-quarter of its length in G. spinifer ) and more lateral spines (10, compared with 6 to 8 in G. spinifer ); the uropod endopod has spines confined to the proximal two-thirds of the inner margin (along the whole inner margin in G. spinifer ) and the outer exopod spines extend almost to the proximal end (absent from the proximal third in G. spinifer ).

In the conformation of the uropods and telson, and the presence of the apophysis on the fifth abdominal somite, the present species shows some similarity to Iiella kojimaensis ( Nakazawa, 1910) (see Ii, 1964, as Gastrosaccus kojimaensis ), but that species has the uniramous first pleopod characteristic of the genus Iiella Bäcescu, 1968 , and is without posterior marginal filaments on the carapace.

Ecological notes. Gastrosaccus yuyu sp. nov. was living high up on the sandy shore of Parangkusomo Beach in large groups. At Pangandaram, ~ 150 km west of Parangkusomo, Anon. (2009?) records that the tidal range along this stretch of the Javanese coastline is between 93–199 cm. Conversely, 50% of waves are of between 2–3 m with 1%>5 metres. The habitat is, therefore, a wave-dominated rather than tide-dominated beach. At Pangandaram, annual average air temperature is 29.5°C, seawater temperature varies between 27–32°C and salinity between 31–32‰. Hence, Gastrosaccus yuyu is an inhabitant of tropical East Indian Ocean surf beaches occupying the swash region at the apex of the surf zone.