Dipsacaster fisheri, 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 : 662-666

Dipsacaster fisheri , new species

( Figs. 3–5 )

Material examined. One wet specimen, R:r = 58: 26 mm ( ZRC.ECH.1301), designated as the holotype and one small dry specimen (presumed immature), R:r = 27: 13 mm ( ZRC. ECH.1599), deposited in the Zoological Reference Collection of the LKCNHM. Two wet specimens (paratypes), R:r = 61: 25.3 mm ( RCO.ECH.3333) and R:r = 52.8: 22.9 mm ( RCO.ECH.3332), deposited in Reference Collection of the Research Centre for Oceanography. Stn. CP 07, Sunda Strait between Tabuan Island and Sumatra ( Fig. 1 ), S05°44.797′ E104°51.606′ (mid-point of beam trawl), 25 March 2018, depth = 379–409 m. Substrate: coarse sand, gravel, rubble, and wood .

Description. Stellate 5-rayed sea star with short tapering arms that are obovate apically, i.e., rounded distally with a blunt extremity. Interbrachial arcs rounded. Paxillated abactinal area extends all the way to the arm tips and is slightly inflated along the radial axes. Abactinal paxillae arranged in obliquely transverse chevrons on the arms, the series transitioning on the disc to an interradial alignment; towards the disc centre the paxillae tend to diminish in size and are less regularly arranged; the largest paxillae occur on the madreporite, located interradially, slightly closer to the inferomarginals than the disc centre. Paxillae are columnar, widening very slightly from the base to an apical end that bears a dense crown of spinelets, a central group of 5–12 larger ones being surrounded by numerous small spinelets.

Broad superomarginal plates (SMPs), numbering 20–21 (only 15 in the small immature specimen) from interradius to arm tip, form a distinct border to the paxillose abactinal area ( Fig. 3A, B ). SMPs are wider than long but shape and width- length ratio varies with position along the arm. Interradial superomarginals are slightly trapezoid but distally along the arm the shape transitions to oblong. In the larger, mature specimens, width-length ratio approaches 3:1 interradially, decreasing to slightly more than 1.5:1 for the mid part of the arm then, distally, the ratio increases again, approximating 2:1 at plate 17 or 18 before declining towards the arm tip. Changes in plate dimension ratios for the 3 mature specimens are indicated in Fig. 2 . The broadening of distal superomarginals results in a slight curvature to the taper of the paxillose margin terminally. Then, towards the arm tip, a rapid decrease in superomarginal width, together with a decrease in the extent to which inferomarginals protrude, creates a curved, obovate or leaf-like margin of the arm terminally ( Fig. 3A, B ). Superomarginal plate surfaces are studded with short, cylindrical, round-ended granules ( Fig. 5A ).

Inferomarginal plates are broader than the superomarginals and define the edge of the disc, their lateral margins projecting beyond the superomarginal edge by about half the width of the latter for much of the arm, except near the tip ( Fig. 3A, B ). The outer extension of each inferomarginal is narrower than length of the main part of the plate ( Fig. 5C, D ) and is a continuation of a narrow transverse ridge on the actinal side. Ridge spinelets are short, cylindrical, and flat-topped, becoming more rounded and slightly appressed peripherally, then transitioning to progressively longer, pointed, laterally projecting spines up to ca. 2 mm in length ( Figs. 4B , 5A ). Opposing inferomarginal faces are densely covered with narrow fasciolar spinules. Actinal plate spine clusters comprise short, clavate, flat-topped spines with small spinules around and below them ( Figs. 4B , 5B ). Furrow spines number from 6 to 9 in a comb-row backed by other small adambulacral spines plus one or two that are longer and more robust than the comb row series. Mouth-plate spines number 4–5. Tube feet pointed. No pedicellariae. Gonads extend into the arms. Colour in life: abactinal paxillar area orange; marginal plates and underside paler ( Figs. 3A , 4A ). Etymology. This new species is named in honour of Walter K. Fisher for his very extensive contribution to knowledge of sea star biodiversity, particularly for deep-sea material, and in recognition of the fact that in the earlier part of the last century he described a high proportion of the species in this genus.

Distribution. Dipsacaster fisheri , new species, was found at just a single location, namely at the Indian Ocean end of the Sunda Strait within a mesopelagic depth zone that is characteristic for most members of the genus ( Table 1). This site was one of 41 mesopelagic stations sampled in the Sunda Strait and nearby Indian Ocean, so this new species is presumably quite rare. However, the range of the species probably extends beyond the Strait into the Indian Ocean mesopelagic realm, a supposition supported by the region’s recent volcanic history. The location recorded for D. fisheri is just 75 km from the 1883 eruptions of Krakatau (see Fig. 1 ) the largest of which, at an estimated Volcanic Eruption Index of 6, represents one of the most violent volcanic eruptions in recorded history ( Siebert et al., 2011). The scientific consensus is that all life within a 15 km radius of this eruption was extirpated ( Simkin & Fiske, 1983), including marine life that would have been impacted by submarine pyroclastic flow deposition ( Mandeville et al., 1996). Furthermore, subaerial pyroclastic flows extended over the sea for distances up to 80 km and at the periphery resulted in the deposition (on land) of several tens of centimetres of tephra material ( Carey et al., 1996). If, as is likely, seabed sedimentation of pyroclastic and ash material adversely impacted the benthos tens of kilometres distant from the eruption, then the present benthic fauna within the Sunda Strait, including D. fisheri , may represent re-colonisation from outside the Strait within the last 130 plus years.

Remarks. Several key features—namely: inferomarginal plates projecting beyond superomarginals, spinose edge to inferomarginal series, abactinal arm paxillae aligned in chevrons and gonads extending into the arms—clearly mark this specimen as belonging to the genus Dipsacaster ( Table 1). Other distinctive characteristics necessitate designation of the present specimens as a new member of the genus ( Dipsacaster fisheri , new species). Unlike other members of the genus where the superomarginal width gradually decreases peripherally for the entire arm, the width, and more specifically the width to length ratio, in mature D. fisheri initially decreases but then increases peripherally before declining again at the arm apex. This produces a curvature in arm taper similar to the distal arm profiles of D. borealis Fisher, 1910a and D. eximius Fisher, 1905 . However, the curvature is more pronounced for D. fisheri , resulting in both a curved paxillar border and a characteristic obovate, leaflike shape for the arm apex. The spine count for each oral plate in D. fisheri is notably fewer than for other species, where this is recorded. Missing character data for some taxa precludes a full cluster or cladistic analysis but some species relationships are apparent; the general arm shape and low R:r ratio of 2.2–2.3 indicate a similarity to the North and East Pacific deep stars, D. borealis Fisher, 1910a and D. eximius Fisher, 1905 . However, in addition to distinctly different superomarginal profiles, the number of superomarginals in D. fisheri is far fewer than for the other two.

Figures

Fig. 3. Dipsacaster fisheri, new species, holotype. A, abactinal view of whole specimen (live); B, abactinal view of arm (specimen ethanol preserved and dried). Scale bars: A = 10 mm; B = 5 mm.

Fig. 4. Dipsacaster fisheri, new species, holotype. A, actinal view of whole specimen (live); B, actinal view of arm (specimen ethanol preserved and dried). Scale bars: A = 10 mm; B = 5 mm.

Fig. 5. Dipsacaster fisheri, new species, holotype (specimen ethanol preserved and dried). A, interradial superomarginal plates bordering paxillose abactinal area and fringed by protruding spinose extensions of inferomarginal plates; B, actinal plates bearing clavate (club-shaped) spines and finer spinules. The ambulacral groove with its furrow spines is aligned across the top right hand corner; C, X-ray negative radiograph of whole specimen; D, X-ray negative radiograph of arm base; inferomarginal plate extensions bearing the laterally projecting spines become narrower than the plates themselves, particularly interradially. Scale bars: A = 2 mm; B = 1 mm; C = 10 mm; D = 2 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.

Fig. 2. Dipsacaster fisheri, new species. Average superomarginal width-length ratios for the 1st interradial plate (I); 11th mid-arm plate (M); and distal 18th or 19th plate (D) in the holotype (ZRC.ECH.1301) and two paratypes (RCO.ECH.3332 & RCO.ECH.3333). n values in parentheses.