Clypeaster reticulatus (Linnaeus, 1758)

Figures 22–24

1758 Echinus reticulatus Linnaeus: p. 666. 1948b Clypeaster (Rhaphidoclypus) reticulatus (Linnaeus) .—Mortensen: p. 71–76; pl. 18: figs. 1–21; pl. 26: fig. 3; pl. 65: figs. 2, 13, 16.

Material studied. Two denuded specimens: WUSL/EI/19 from Hiriketiya and WUSL/EI/20 from Nilwella, Sri Lanka.

Description. Shape and size —Small, 35 and 41 mm TL; test elongate ovoid to pentagonal, distinctly longer than broad, TW 75% and 76% TL; test low, 26% and 31% TL in height; test margin greatly thickened, forming a distinct ring-shaped depression between margin and petalodium; aboral side slightly raised toward apex, concave on oral side, infundibulum deep, bowl-shaped, c. 14% TL and 17% TL.

Apical system —Monobasal; situated slightly anterior of centre on aboral surface, c. 45% and 48% TL from anterior margin; circular gonopore in each interambulacrum near madreporic plate; ocular pores small and indistinct.

Ambulacra —Petaloid area 63% and 65% TL; petals inflated, closed distally; petal III longest, c. 32% TL; anterior paired petals c. 23% TL, considerably shorter than other petals; posterior paired petals c. 30% TL; paired petals broader distally than other petals; interporiferous zone conspicuously elevated, consisting of 5–6 primary tubercles along widest part of petals; furrow connecting pores in each pore pair deep and sharply delimited; 2–5 primary tubercles between furrows; oral ambulacra simple, inconspicuous food groves present along axis of each ambulacrum.

Interambulacra —All interambulacra on oral surface disjunct; on aboral surface, small, shallow pits present in corners of sutures (Fig. 23, A); primary tubercles less dense on oral surface than aborally.

Tuberculation —Primary tubercles perforate, areoles sunken, dense miliary tuberculation in between primary tubercles; oral side with adoral region of much smaller tubercles; largest tubercles present toward test edge, intermediate sized tubercles present very close to peristome, smallest tubercles present in between zone of largest tubercles and zone of intermediate-sized tubercles (Fig. 23, B).

Peristome —Round to subpentagonal, small, width c. 7% TL, located close to centre, c. 46% and 51% TL from anterior margin.

Periproct —Round to transverse oval, length 4% and 5% TL, width 4% and 6% TL; significantly smaller than peristome, c. 64% of peristome length; situated close to posterior edge of test, c. 4% TL away from posterior margin.

Internal buttressing —Pillars inside test more abundant along midlines of interambulacra; marginal buttressing largely absent (Fig. 23, C).

Geographic range. Indo-West Pacific, from Islands of West Indian Ocean (Clark 1925a), Mauritius (de Loriol 1883), East Africa & Madagascar (Brown 1910a), Red Sea (Agassiz 1872), South East Arabia (Mortensen 1948c), Persian Gulf (Mortensen 1940), West Indian & Pakistan (Koehler 1922), Maldives area (Koehler 1922), Sri Lanka (Herdman et al. 1904; Koehler 1922), Bay of Bengal (Koehler 1922), North Australia (Clark 1925a) and East Indies (de Meijere 1904; Mortensen 1948a) to Philippine Islands (Mortensen 1948e; Mooi & Munguia 2014), China & South Japan (Clark 1925a), South Pacific Islands (Clark 1954) and Hawaiian Islands (Clark 1925b).

Bathymetric range. Littoral zone to 125 m (Mooi & Munguia 2014).

Observed occurrence in Sri Lanka. On the beach of Hiriketiya Bay and Nilwella along the southern coast of Sri Lanka (Fig. 24), in biogenic sand; first recorded in Sri Lanka by Herdman et al. (1904). Bathymetric range in Sri Lanka 0–100 m.

Remarks. C. reticulatus can be distinguished easily from C. humilis by having a greatly thickened test margin that forms a distinct ring-shaped depression between the margin and petaloid area. The oral surface of C. reticulatus is strongly concave, making a bowl-shaped infundibulum. Primary spine tubercles decrease in size towards the peristome, then increase again adjacent to the peristome.