Nacospatangus altus (A. Agassiz, 1864)

Arachchige, Gayashan M., Jayakody, Sevvandi, Mooi, Rich & Kroh, Andreas, 2019, Taxonomy and distribution of irregular echinoids (Echinoidea: Irregularia) from Sri Lanka, Zootaxa 4541 (1), pp. 1-100 : 86-89

Nacospatangus altus (A. Agassiz, 1864)

Figures 74–75

1864 Maretia alta A. Agassiz : p. 360.

1951 Pseudomaretia alta (A. Agassiz) .—Mortensen: p. 58–62; pl. 3: figs. 12–14, 16–20; pl. 46: figs. 1–6, 8–11, 13, 19, 20.

Material studied. Five denuded specimens: WUSL/EI/82, EI/83, EI/116, and EI/117, from Kamburugamuwa, and WUSL/EI/115, from Mount Lavinia, Sri Lanka.

Description.

Shape and size —Test thin and fragile; medium-sized, 32.7–46.95 mm TL; width 73–80% TL, greatest width about halfway along length; outline of anterior end gently curved with very inconspicuous anterior notch; TH 42–45% TL, highest point along posterior interambulacrum 5 which forms a sharp keel; posterior end obliquely truncated in lateral view; oral side flat but sternum rises into a conspicuous keel at its posterior end; ambitus sharply curved from aboral to oral surface.

Apical system —Ethmolytic, with three gonopores (no gonopore in genital plate 2); posterior pore pair larger than anterior gonopore; madreporite not conspicuously extending beyond posterior oculars; situated anterior of centre, c. 33% TL (SD=1, N=6) from anterior margin.

Ambulacra —Ambulacrum III narrow, very slightly sunken; paired petals with unequal poriferous zones, nearly closed, pores in single pore pair equal in size, slightly pointed at inner ends, pore series converging distally; adapical pore pairs rudimentary in both anterior and posterior paired petals; poriferous zones of anterior paired petals highly unequal, in anterior paired petal, posterior poriferous zone (petal IIa, IVb) usually 7–9 pore pairs longer than anterior poriferous zone (petal IIb, IVa); in posterior paired petals (petals I, V), length difference not as pronounced and posterior poriferous zones (petal Ia, Vb) usually a single pore pair shorter than anterior ones (petal Ib, Va) (however, largest examined specimen [WUSL/EI/82] shows equal pore pair numbers in both anterior and posterior poriferous zones); length of rudimentary area of anterior poriferous zones (petal IIb, IVa) greater than posterior poriferous zones (petal IIa, IVb) of anterior paired petal, c. 17% and 3% TL respectively; length of rudimentary area of anterior (Ib, Va) and posterior poriferous zones (petal Ia, Vb) of posterior paired petal nearly equal, c. 8% TL respectively.

Tuberculation —Aboral interambulacra uniformly and densely covered by small secondary tubercles, except for a few enlarged primary tubercles in interambulacra 1, 2, 3, and 4; aboral interambulacrum 5 without enlarged primary tubercles; in aboral interambulacra 2 and 3, many enlarged primary tubercles present along adjoining anterior ambulacrum; these tubercles without sunken areoles; two series (or exceptionally three) of enlarged tubercles along interambulacra 1 and 4 on each side starting anteriorly from very close to posterior poriferous zones of anterior paired petals, first series consists of 3–4 and rarely 5 enlarged primary tubercles just aboral to ambitus ( Fig. 74C, D ); second series 1–2 tubercles just above first series; rarely third series with a single enlarged primary tubercle; these tubercles non-crenulate with deeply sunken areoles; oral interambulacra with non-crenulate primary tubercles, mamelon of tubercles eccentric; these primary tubercles smaller than tubercles on aboral interambulacra 1 and 4, and larger than tubercles on aboral interambulacra 2 and 3; tubercles on adoral interambulacrum 5 smaller than other tubercles on oral interambulacra; larger tubercles present along adjoining ambulacra.

Plastron —Narrow, width c. 16% TL (SD=0.43, N=6); sternal plates tuberculate except for small part along anterior end; sternal plates forming conspicuous median keel towards posterior end; labrum arrow, slightly elevated at peristome with narrow posterior prolongation.

Fasciole —Subanal fasciole well-developed, heart-shaped, adoral end pointed, adapical fasciole following lower edge of periproctal cavity; height of subanal fasciole approximately 21% TL (SD=1.1, N=6) and width approximately 28% TL (SD=0.7, N=6); usually encloses three pore pairs on each side, exceptionally, four pore pairs in each side of largest specimens examined (WUSL/EI/82); no anal branches.

Peristome —D-shaped; oblique; moderately large, length 9–12% TL, width 15–17% TL; 24–25% TL from anterior margin.

Periproct —Large, length 12–19% TL, width 14–19% TL; fully visible in oral view..

Geographic range. Indo-West Pacific, from Islands of West Indian Ocean ( Clark 1925a), Mauritius ( De Loriol 1883), Sri Lanka ( Clark 1915), Bay of Bengal ( Koehler 1914), East Indies (de Meijere 1904) to Philippine Islands ( Mortensen 1948e; Mortensen 1951) and China & South Japan ( Agassiz 1872).

Bathymetric range. 1.5–204 m ( Mortensen 1951).

Observed occurrence in Sri Lanka. Specimens were collected from sand patches between patch reefs on the western and southern coasts of Sri Lanka, at a depth of 20–30 m ( Fig. 75 ).

Remarks. The number of the distal pore pairs in the anterior series of the anterior paired petals is variable. Out of six specimens, only one (WUSL/EI/127, 38.0 mm TL) has 5 distal pore pairs, while four specimens have 8 distal pore pairs and the largest examined specimen has 9. Mortensen (1951: 59) observed 5–6 distal pore pairs in the anterior series of the anterior petals in his specimens, even though he had examined specimens up to a size of 54 mm. Likewise, number of enlarge primary tubercles is variable: some of the examined materials (WUSL/EI/116, 34.2 mm TL; WUSL/EI/83, 37.3 mm TL; WUSL/EI/127, 37.95 mm TL) and (WUSL/EI/82, 46.95 mm TL) have seven and eight large tubercles in interambulacrum 4 respectively. This differs from Mortensen’s (1951) observations. These features suggest intraspecific variation and can thus only be used in conjunction with the additional characters listed above.

Figures

FIGURE 74. Nacospatangus altus: A, aboral view (WUSL/EI/83); B, oral view (WUSL/EI/83); C, left side (WUSL/EI/82), D, right side (WUSL/EI/82), lateral views; E, anterior (WUSL/EI/83); F, posterior (WUSL/EI/83); G, sternal view (WUSL/EI/83).

FIGURE 75. Distribution map of Nacospatangus altus recorded in this study.