Tromikosoma australe (Koehler 1926)

Anderson, Owen F., 2016, A review of New Zealand and southeast Australian echinothurioids (Echinodermata: Echinothurioida) — excluding the subfamily Echinothuriinae — with a description of a new species of Tromikosoma, Zootaxa 4092 (4), pp. 451-488 : 464-466

Tromikosoma australe (Koehler 1926)

( Figures 10–13 )

Material examined. 10 specimens from 3 records (all NIWA).

Northeast Tasman Sea: 1 specimen (160 mm TD), 34° 31.5´S, 166° 21´E, 2930 m, NIWA45040; 8 specimens (180, 160, 98 (dry), 88, 82, 74, 61, 51 mm TD), 35° 43.7´S, 160° 16.2´E, 3480 m, NIWA45092; 1 specimen (29 mm TD), 38° 37.3´S, 165° 36´E, 2417–2421 m, NIWA45121. Unless stated all stored in 80% ethanol.

Size range. The 10 specimens range in size from 29 mm to 180 mm TD.

Occurrence. All three records are from deep water in the Tasman Sea, near the Lord Howe Rise ( Figure 11 ). The potential depth range for the species is 2417–3480 m.

Remarks. These specimens represent the first records of this species since it was discovered off South Australia by the Australasian Antarctic Expedition of 1911–14 and subsequently described by Koehler (1926). Recent examination of the holotype in the Australian Museum revealed the (dry) specimen to be in very poor condition and the test could not be examined in any detail without causing further damage. However the Aristotle’s lantern had disarticulated and the components could be examined. Comparison of the hemipyramids and rotulae with those of the Tasman Sea specimens showed a perfect match. Although the Aristotle’s lantern is not typically used for distinguishing species, personal observation of Araeosoma species has revealed considerable variation in that genus, and features of the lantern should be assessed for their taxonomic value.

The pedicellariae figured by Koehler also matched well to the new specimens: two forms of tridentate pedicellariae are present ( Figure 12 ); a large form of a design typical for the genus (see Mortensen 1935, Mironov 1971, Campbell 1993) with an elongated, open blade edged with very fine serrations, about 0.7–1.6 mm long; and a small form (0.4–0.5 mm long) not much longer than wide with a rounded, spoon-shaped blade. As Koehler stated, although it was not well shown by his figures, the narrowing of the valve between the base and blade is more pronounced in the smaller form than in the larger. The triphyllous pedicellariae (also about 0.4 mm long) are wide for their length and typically have a strongly truncated blade tip and the cover-plate not well developed. Koehler (1926) found no ophicephalous pedicellariae on the type specimens, but a single example was taken from the smallest of the new specimens ( Figure 12 ). The shape of this pedicellaria is typical for the genus, and most similar to that of T. tenue or T. koehleri .

The preserved tests are all a dull grey/brown colour, some with small darker brown or purple patches around the peristome and other parts of the oral surface. Details of the plate structure are not given in great detail by Koehler as both his specimens were already in poor condition when first examined; the test of one was completely broken up and unusable and although the other (the holotype) was better preserved, no spines remain on either specimen. Much of the new material is also not in very good condition, having not been very well preserved. However, the presence of large primary tubercles outside the pore-series in a few of the distal oral ambulacral plates in Koehler’s (1926) photograph (Plate 107) can also be seen in the larger of the new specimens examined (this feature is otherwise reported in Tromikosoma only for the Atlantic species T. uranus ), along with an inner series of primary tubercles running down the distal 5–6 ambulacral plates ( Figure 13 ). The somewhat clearer plating and tuberculation patterns in the new material also reveal the presence of a large, adradially placed primary tubercle on each of the outer 3–7 plates of the oral interambulacra, as well as an interradially placed tubercle in the outer 1–2 plates. The primary tubercles are seen to be much smaller on the aboral surface (except for a few near the ambitus) but numerous, with two distinct series running down each end of the main plates in the ambulacra, and one or two tubercles fairly regularly on each plate in the interambulacra but forming no distinct series.

Koehler’s photograph shows that the interambulacral and ambulacral columns are of similar width and this is true also of the new specimens of similar size; in smaller specimens the ambulacra are narrower, barely half the width of the interambulacra in a specimen of 29 mm TD. The two secondary plates abut laterally, and aborally they occasionally completely separate the primary plates; this is the typical arrangement in T. tenue , but is otherwise rare in this genus. Importantly, there is no evidence of any non-poriferous plates in the specimens examined, a feature occasionally observed in the oral ambulacra of T. uranus and T. tenue (Mortensen 1935) and diagnostic for Sperosoma .

Spines were usually broken or missing in the specimens examined, but one of the smaller specimens in NIWA 45092 has several oral spines up to about 20 mm long with hoofs intact ( Figure 13 ); these are slightly narrowed and flattened at the tip, more like T. uranus than the conical shape seen in the other Pacific species T. hispidum and T. tenue . The spines are otherwise unremarkable—larger examples (up to 1.2 mm diameter) bear about 40 longitudinal striations.

Minute buccal sacs were discernable in some specimens. These are rudimentary finger-like projections typical of Tromikosoma species in which they have been seen.

The new material of T. australe shows strong similarities to T. uranus , differing mainly in the lack of any observable non-poriferous ambulacral plates in T. australe and in the pedicellariae (of which only the large tridentate has ever been figured for T. uranus ), while Mortensen (1935) considered T. australe to be most closely related to T. tenue , based on the small tridentate pedicellariae and the tuberculation of the oral surface. For the present, with little material available for most of the known species, it seems appropriate to maintain T. australe as a separate species.

Figures

FIGURE 10. Tromikosoma australe, NIWA 45092. (82 mm TD). Left, aboral view; right, oral view.

FIGURE 11. Tromikosoma australe. Distribution of confirmed records in New Zealand and southeast Australia, from material examined. Number of records = 3.

FIGURE 12. Tromikosoma australe, NIWA 45092 (160 mm TD). A – D, head and valves of larger tridentate pedicellariae; E – F, valves of small tridentate pedicellariae; G – H, valves of triphyllous pedicellariae; I, valve of ophicephalous pedicellaria (NIWA 45121, 29 mm TD).

FIGURE 13. Tromikosoma australe, NIWA 45092 (97 mm TD). Details of interambulacral (top) and ambulacral (bottom) plates: left, aboral; right, oral. Also shown is an oral primary spine with terminal hoof (NIWA 45092, 80 mm TD).