Pteraster sjadesensis, Lane & Vimono, 2020

Lane, David J. W. & Vimono, Indra B., 2020, Two new species of sea star (Asteroidea, Echinodermata) from mesopelagic depths in the Sunda Strait, Indonesia, Raffles Bulletin of Zoology 68, pp. 662-669 : 667-668

Pteraster sjadesensis , new species

( Fig. 6 )

Material examined. 1 wet specimen, R:r = 15: 6 mm ( R:r = 2.5) ( ZRC.ECH.1382), Stn. DW 16, Sunda Strait, (seamount reef), S06°09.803′ E104°57.976′ (drop point of dredge), 26 March 2018, depth = 92–103 m. Substrate: gravel, sand, and some mud .

Description. 5-rayed, stellate sea star with markedly tapering, triangular arms ( Fig. 6A, B, C ). Arm extremities recurved almost 180° in living sea star such that the terminal end of the ambulacral groove is visible from above. Supradorsal canopy supported by paxillae, single spines, and spine clusters. The single spines protrude only slightly though the membrane whereas the spine clusters, restricted to the dorsal surface, often cross apically together wigwam fashion and protrude further. Membrane muscle bands connect adjacent spines and spine clusters. The chamber beneath the supradorsal membrane connects to the exterior via a central osculum whose opening is regulated by an encircling membranous fringe embedded with at least 20 vertically oriented spines. Spiracle number is variable and difficult to count due to multiple membrane ruptures. Adambulacral plates similar in size along the furrow; each plate bears a transverse row of 4 small webbed spines linked by membrane to a large actinolateral spine furthest from the furrow; larger spines collectively are embedded in an actinolateral membrane that extends almost to the margins of the arms. Mouth plates each bear 10 peripheral spines, the second being slightly longer than the first; the series then diminishing in size distally along the plate edge ( Fig. 6D ). Oral spines are webbed together, to their tips, but the webbing is independent for each oral plate ( Fig. 6D ). A large, lanceolate, suboral spine ( Fig. 6D , arrow) on each oral plate is free from membrane and articulates on a prominent boss; the hyaline outer portion is pointed or round-ended and tricarinate in cross section with concave faces. Tube feet aligned biserially ( Fig. 6C ). Colour in life: supradorsal membrane with irregular light and dark orange pigmentation; membrane pigmentation tends to be reduced on some of the protruding spine clusters and also tends to be lost then regained towards the arm apices.

Etymology. Species named after the SJADES 2018 Expedition during which it was collected.

Distribution. Recorded at a single location, a seamount ridge/reef at the southwest opening of the Sunda Strait to the Indian Ocean ( Fig. 1 ). This location, just 50 km from and possibly impacted by the cataclysmic 1883 Krakatau eruption, may have been subsequently re-colonised from more widely distributed populations outside the Strait. The depth at which this new Pteraster species was found (ca. 100 m) in the Sunda Strait corresponds approximately to the peak species diversity-depth interval globally for the genus ( Villier et al., 2004).

Remarks. Pteraster , currently with 56 described species ( Mah, 2020) is one of two highly speciose genera in the Pterasteridae —a monophyletic family ( Blake, 1987; Gale, 1987) of predominantly deep-sea stars—the other being Hymenaster , a genus with similar body form but distinguishable from Pteraster , including the present specimen, by the adambulacral spine rows not being transverse and by the absence of webbing between the oral spines ( Fisher, 1911; Villier et al., 2004). There is resemblance also to sea stars of the genus Euretaster but, although small in size and possibly a juvenile, the present specimen can be distinguished from Euretaster and assigned to the genus Pteraster based on a number of morphological characters featured in a preliminary cladistic analysis of the family ( Villier et al., 2004), namely: (a) all adambulacral plates being of similar form, (b) the adambulacral comb row of spines being transverse rather than oblique or aligned with the arm furrow, and (c) the arm being semicircular in cross section. Euretaster in contrast has alternating adambulacrals of two types: oblique adambulacral comb rows and, except for Euretaster attenuatus , arms that are circular in cross section ( Villier et al., 2004). Euretaster attenuatus , described from New Caledonia ( Jangoux, 1984) does have features in common with Pteraster sjadesensis , new species (semicircular arm section; stellate appearance) but, apart from generic differences in adambulacral spine row direction, the latter lacks the longer-spined, spikey appearance of E. attenuatus .

Pteraster sjadesensis , new species, resembles Pteraster uragensis Hayashi, 1940 and Pteraster trigonodon Fisher, 1910 ( Fisher, 1910b) in the independent webbing of oral plate spines on each side and in the three-edged form of the suboral spines. It differs, however, in that the arms are longer, giving a higher R:r ratio, and oral plates have double (10) the number of webbed oral plate spines compared with P. uragensis and P. trigonodon .

Figures

Fig. 6. Pteraster sjadesensis, new species, holotype. A, abactinal view (live), osculum is open; B, abactinal view of arm and disc (live), osculum closed; C, actinal view (live), open ambulacra reveal biserial tube feet rows; D, oral region (specimen alcohol preserved and dried) showing oral spine webbing that is independent for each oral plate. Arrow indicates a tricarinate suboral spine with a dense basal boss and a hyaline apical region. Scale bars: A = 5 mm; B = 2 mm; C = 2 mm; D = 1 mm.

Fig. 1. The Sunda Strait between the islands of Java and Sumatera (Sumatra). Inset shows the location of the Strait within the Indonesian Archipelago. This map indicates the proximity of sites CP07 (at which Dipsacaster fisheri, new species, was found) and DW16 (at which Pteraster sjadesensis, new species, was found) to the 1883 VEI 6 eruption series and remnant of Krakatau. Scale bar = 20 km.