Mysis spinifera Goës, 1864; by monotypy.

Norman (1868) instituted the genus Gastrosaccus to accommodate mysid specimens having a characteristic formation of marsupium, for which the pleura of the female first abdominal somite are greatly expanding as to support the oostegites. When establishing Gastrosaccus, Norman actually intended to place Mysis spinifera Goës, 1864, in the genus but he mistakenly referred his specimens to Mysis sancta van Beneden, 1861 because he first thought that Mysis spinifera could be a synonym of Mysis sancta. This was later corrected by replacing it with Gastrosaccus spinifer (see Norman 1892, Tattersall and Tattersall 1951). The genus currently contains 24 species (Mees and Meland 2013).

Male (TL ca. 7.5 mm) (NSMT−Cr 22940), sandy beach, Padak Guar (08°25.665'S, 116°42.561'E), Lombok Island, push net, evening time (17:00), 6 June 2010, coll. R. Mantiri.

11 males (TL 4.0−ca. 7.5 mm), 3 females (TL 4.0−ca.7.5 mm), 3 juvs (TL 2.8–3.5 mm) (NSMT−Cr 22941), data same as for holotype.

Rostrum produced into sub-triangular fig. Posterodorsal edge of carapace bearing 7−12, commonly 9−11, spine-like filaments between dorsolateral slits and normally further 5−10 feeble denticulations/undulations on each side of posterolateral lobe posterior to slit. Fifth somite of abdomen with articulated triangular process at posteromedian end; pleuron of female first somite greatly expanded, fully covering marsupium. Telson typical of Gastrosaccinae form, with apical cleft occupying 1/8−1/6 length of telson, armed laterally with 8 or 9 robust spine-like setae, including posteriormost one. Uropod with exopod shorter than endopod, armed laterally with 13 or 14 spine-like setae; endopod with 6 or 7 spine-like setae on mesial margin. Labrum with single anteromedian tooth and additional smaller spines absent. Third pleopod of male with endopod multi-articulated; exopod greatly elongated, extending well beyond sixth abdominal somite, comprising 4 major segments, and basal segment without distinct sub-articles, distal segment 0.65–0.7 times length of penultimate one, armed with short sub-terminal seta and moderately long 2 terminal setae. First pleopod of female bi-lobed, second to fifth pleopods rudimentary, unsegmented lobe.

Male. Body (Fig. 1) moderately robust.

Carapace (Figs 1, 2a, b) produced into sub-triangular rostrum with sub-acute apex; posterior dorsal margin deeply excavate, leaving exposed last thoracic somite, dorsolateral part of emargination forming slit with posterior lobe overlapping onto anterior one; lateral wing of carapace well developed, extending to anterior 1/3 of first abdominal pleuron; anterolateral part rounded; cervical sulcus marked dorsally anterior to anterior 1/3; posterior dorsal edge between dorsal slits bearing 7−12, most commonly 9−11, spine-like filaments, and further 5−10 feeble denticles/undulations on each margin posterior to slit (often hard to define exact numbers due to its poor development particularly in posterior part).

Abdomen (Figs 1, 2c) with anterior 4 somites rounded dorsally, sub-equal in length; fifth somite rounded dorsally, 1.3−1.4 times longer than precedents, with remarkable articulated sub-triangular process on posterodorsal margin; sixth somite very slightly longer than fifth, with transverse carinate ridge on anterodorsal part; brown to dark-brown melanophores on lateral surface of first to fifth somites and also in posterolateral part of sixth somite in ethanol preserved specimens.

Antennule (Fig. 2d) with basal segment longest, very slightly shorter than length of 2 anterior segments combined; second segment shortest, about 0.55 times longer than wide, with 2 spine-like setae, one at mid-length and another one at anterior end of lateral margin; third segment about twice the length of second, with short spine-like seta at distal 1/3 of lateral margin; lateral flagellum slightly widened at mesiobasal part, forming somewhat small male lobe bearing moderately long sensory setae; mesial flagellum more slender than lateral one. Antenna (Fig. 2e) with scale proportionately short, barely reaching or falling slightly short of anterior end of second antennular segment, slightly more than 3 times as long as wide, lateral margin smooth, distolateral spine not extending beyond anterior margin of lamella; antennal peduncle long, extending slightly beyond scale, penultimate segment elongated, approximately 3 times as long as distal one.

Labrum (Fig. 2f) with single sharp anteromedian tooth, additional teeth absent.

Mandibular palp (Fig. 2g, h) composed of 3 segments, distal segment about 0.8 times length of penultimate, with 2 long terminal setae, of which proximal one curved around its mid-length, often forming sub-chelate structure with terminal segment, and another seta moderately long at distal end, and also about 3 short spine-like setae as well as several short obtuse setae, all confined to distal part, forming comb-like structure. Mouthparts (maxillule, maxilla and second thoracopod) general form of genus (Fig. 3a, b, e). First thoracopod (Fig. 3c, d) with basal fig of exopod about 1.6 times length of its width.

Third to eighth thoracopods (Figs 3f, 4a, b) similar in basic structure; exopods with basal fig smooth on lateral margin, and small process at distolateral corner except for eighth limb, and also with 11–14 segmented flagellar part; third to eighth endopods with carpopropodus composed of 5−9 segments, progressively increasing in numbers posteriorly, in which eighth endopod is divided into 8 or 9 segments.

Penis (Fig. 4b) about 1.7 times as long as wide, with short terminal seta directed posteriorly and 4 or 5 long setae along lateral surface.

All pleopods of male biramous (Figs 1, 5a–e). First pleopod with sympod bearing several long setae along anterior margin; endopod rudimentary, unsegmented lobe possessing several marginal setae distally; exopod articulated to about 8 segments. Second pleopod with endopod articulated to about 7 segments; exopod articulated to about 8 segments, slightly longer than endopod. Third pleopod with endopod articulated to 6 or 7 segments; exopod greatly elongated, extending beyond posterior end of abdomen fully by distal segment, composed of 4 major segments, basal segment markedly compressed and broader than distal series of segments, sub-equal length or very slightly longer than second one, without distinct sub-segments but with slight indication of 2 incipient articulations, second segment tubular and unarmed, penultimate segment bearing short spine-like sub-distal seta, distal segment shortest, 0.65−0.70 times length of penultimate, armed distally with 2 moderately long spine-like setae with subsidiary setules on its distal half. Fourth and fifth pleopods similar in basic form, endopod unsegmented, bearing a few setae on distal margin; exopod articulated to about 8 segments.

Uropod (Fig. 4d) with exopod shorter than endopod, lateral margin armed with 13 or 14 robust spine-like setae, length increasing posteriorly, each spine-like seta with subsidiary setules on posterior margin and also short terminal protrusion; endopod extending beyond telson and reaching as far as end of distal telson setae, commonly bearing 7, occasionally 6, spine-like setae along entire mesial margin, showing general tendency towards increased setae length distally.

Telson (Fig. 4c) 1.1–1.2 times as long as sixth abdominal somite and 2.6−2.8 times as long as wide, apical cleft occupying about 1/7 length of telson, bearing approximately 15 dentations along each mesial margin; lateral margin with 8 or 9 spine-like setae of varying length, including longest apical seta, penultimate seta distinctly shorter than apical and located noticeably posterior to level of anterior edge of apical cleft, proximal lateral seta sub-equal in length to penultimate one, posterior series of lateral setae spaced equidistantly and unarmed gap between apical and penultimate setae subequal in distance with those of others; procurved sharp process (Fig. 4c’) present at proximal part of ventral side. Melanophores widely distiributed on dorsal surface of telson but not along mesiodorsal axis and often distal part of apical lobes in both sexes.

Female. Sexual dimorphism as common for genus.

Pleuron of first abdominal somite greatly expanded, fully covering marsupium.

Antennule (Fig. 2b) might differ slightly from that of male but is damaged in the present material and not dissected.

First pleopod (Fig. 5f) biramous, with unsegmented endopod and exopod, sub-equal in length and markedly shorter than sympod, exopod and endopod each with several long plumose setae on distal part; sympod with a few long setae near anterobasal part and further several long setae along anterior distal margin. Second to fifth pleopods (Fig. 5g) similar in shape and size, all rudimentary, unarticulated rod-shaped lobe, bearing several long setae on lateral surface and also on distal margin.

The name “lombokiensis” refers to the type locality, Lombok Island.

This species is remarkable among the members of Gastrosaccus in having an articulated process at the posterodorsal end of the fifth abdominal somite throughout its life stages, coupled with a fringe of spine-like filaments on the posterodorsal margin of the carapace. The combination of these intriguing characters is shared by Gastrosaccus sorrentoensis Wooldridge & McLachlan, 1986 from Western Australia (see also Hanamura 1998) and Gastrosaccus yuyu Bamber & Morton, 2012 from Java Island, Indonesia.

Specimens from Lombok Island have the carpopropodus of the thoracic endopods comprising fewer numbers of segments, and at most 8 or 9 segments in the eighth limb compared to 13 or more segments in the previously known species (Table 1). There is a slight possibility that the paucity in the segment counts in Lombok specimens is due to comparatively smaller body sizes, thus showing a condition of younger stages. Although ovigerous females were not available in our collection, the largest female has a well-developed first abdominal pleuron, which is fully covering the marsupium lamellae. Also larger males (> ca. 6.5 mm) were considered as adult because there third pleopods are fully developed, extending well beyond the posterior margin of the abdomen at least by length of distal article. Furthermore, the posterior part of the vas deferens of several males exhibited whitish colour just above the ejaculatory duct, common in mature males.

The male third pleopodal exopod of this group basically has four major segments. In Gastrosaccus sorrentoensis and Gastrosaccus yuyu, the basal article is divided further into two or three short sub-segments instead of an entire basal article, without distinct short sub-segments. In the Lombok population, we do not find distinct short sub-articulations. However, a slight indication of incipient sub-segmentations was observed in the internal muscular part; but an associated segmented exoskeleton could not be found. Also the shape of the telson is noticeably different among the three populations and these features are regarded as size independent.

Compared to those of Gastrosaccus sorrentoensis, the Lombok population tended to have larger number of spine-like setae in the uropodal endopod and telson despite their smaller body size. The female telson of Gastrosaccus sorrentoensis bears a pair of pigmented spots at the inner side of the fifth lateral setae (Wooldridge and McLachlan 1986) whereas in Lombok specimens the telson is tinted by melanophores widely along the margins in both sexes.

According to Bamber and Morton (2012), the Javanese Gastrosaccus yuyu is devoid of an anteroventral process on the telson, and this feature is unique among the three populations.

Our observation suggested that Gastrosaccus lombokensis starts to develop external secondary sexual characteristics at a size around TL 3.5−4 mm in both sexes.

This species is known only from the sandy shore of Lombok Island, Indonesia.