Echinocyamus truncatus L. Agassiz 1841
Creators: name: Arachchige, Gayashan M.
name: Jayakody, Sevvandi
name: Mooi, Rich
name: Kroh, Andreas
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Echinoidea
Clypeasteroida
Fibulariidae
Echinocyamus
Echinocyamus truncatus

description: Echinodiscus cf. truncatus L. Agassiz, 1841
Figures 51B, 55–58
1841 Lobophora truncata L. Agassiz: p. 66–67; pl. 11: figs. 11–16.
1948b Echinodiscus bisperforatus truncatus (L. Agassiz).—Mortensen: p. 410–411.
Material studied. Five denuded specimens: WUSL/EI/60, EI/61, and EI/63, from Periyapaduwa and WUSL/EI/62 and EI/64, from Negombo 2, Sri Lanka.
Description. Shape and size —Test small, 22.5–48.1 mm TL; TW 107–114% TL; height c. 14% TL; margins of ambulacra II, III, IV, and interambulacrum 5 indented; oral side flat to slightly concave, infundibulum depth 1–4% TL; food grooves bifurcating, closely following lunules and pressure drainage channels; well-defined fields of smaller tubercles in ambulacra compared to interambulacra on oral surface; plate sutures of aboral surface marked with light coloured margins, slightly depressed suture area gives impression of slight inflation of plate centres.
Apical system —Monobasal, with four circular gonopores; subcentral, 45–51% TL from anterior margin.
Ambulacra —Petals small, closed distally, except for petal III which is slightly open, especially so in small specimens; petaloid length approximately 42% TL; petal III slightly longer than others, length 22% TL, anterior paired petals approximately 20% TL, posterior paired petals shortest, 17% TL; relative width of all petals similar, c. 10% TL; pores distinctly conjugated, with sharply delimited furrows deepening towards outer pore; inner pores small and rounded, outer pores large and elongated; 3–7 primary tubercles on ridges between furrows; pairs of miliary tubercles between consecutive primary tubercles; elongated lunules in posterior ambulacra, mean LL 21% TL; lunules of variable width, 5–11% TL; SI of Stara & Fois (2014) 0.2–0.5; WI 3.7–5.7; LL greater than that of shortest petal; angle between lunules 75°–87°; distance between tips of posterior petals and lunules 7–11% TL; two pairs of plates between petal and lunule; four pairs of basicoronal plates in oral ambulacrum; food grooves bifurcating close to peristome, with more or less well-developed lateral branches encroaching upon interambulacra; oral ambulacra with much more densely packed and smaller tubercles than interambulacra; sharply delimited darker-coloured ambulacra relative to interambulacra; ambulacral width at ambitus 25–28% TL.
Interambulacra —Oral interambulacra with less dense field of larger tubercles than ambulacra; interambulacrum 5 width at ambitus 37–40% TL; oral interambulacra all disjunct, widest gap observed in interambulacrum 5.
Peristome —Round to subpentagonal, small, 4–7% TL in diameter; absolute peristome size not increasing at same rate as test with growth, relative peristome size is smallest in largest specimen; peristome anterior of centre, 44–48% TL from anterior margin.
Periproct —Small, 2–4% TL; lies orally, 4–8% TL from posterior edge; bounded by plates 5.a.2/5.a.3/5.b.3.
Geographic range. Indian Ocean, from Red Sea, East Coast of Africa (Mortensen 1948b) to Singapore (Stara & Sanciu 2014).
Bathymetric range. Littoral to 12 m (Mortensen 1948b).
Observed occurrence in Sri Lanka. Specimens were collected from muddy shorelines along the northwestern coast (Periyapaduwa) at 1–2 m, and near a fish landing site in Negombo 2 Sri Lanka (Fig. 58).
Remarks. Following Stara and Sanciu (2014) the specimens in this study were identified as Echinodiscus because of the “almost vertical” (i.e. almost parallel to the anterior-posterior axis) suture between plates 5.a.2 and 5.b.2 and the low number of plates between the ends of the petals and the lunules (Fig. 51B). The shape and size of the lunules show marked variation among the examined specimens. The largest specimen has a low SI, but high WI, small specimens show the opposite trend.
The second Sri Lankan Echinodiscus species, E. bisperforatus, has long, slit like lunules (length 34% TL) and a wide angle (112°) between them, whereas Echinodiscus cf. truncatus has shorter and wider lunules (length 21% TL), which form an acute angle of about 81°.
The Sri Lankan material differs from E. truncatus described by Stara and Sanciu (2014) by its higher angle between lunules (81° vs. 67°), different periproct position (5.a.2/5.a.3/5.b.3 vs. 5.a.2/5.b.2/5.b.3), higher SI (0.37 vs. 0.22), lower WI (4.7 vs. 8.68), and much simpler food groves.
Echinodiscus cf. truncatus differs from Echinodiscus sp. 1 by its large peristome (5.6% TL vs. <2.5% TL) and by the position of the periproct. E. andamanensis Stara & Sanciu, 2014 differs from the examined specimens by the position of the periproct, which opens between the plates 5.b.2/5.a.2 or between 5.b.2/5.a.2/5.a.3.
Stara and Sanciu (2014: 305) report a fossil species called Echinodiscus sp. 2 from the Pleistocene of Hurgada (Red Sea) that shows many similarities to the present material, notably in periproct position, angle between lunules, and complexity of food groves.
According to Clark (1914) Echinodiscus bisperforatus var. truncatus has “lunules shorter than shortest petal” but in the examined specimens the lunules are usually longer than the shortest petal, sometimes matching petal III in length. According to Stara and Sanciu (2014), Clark’s (1914) E. truncatus may have been a misidentified specimen of Sculpsitechinus tenuissimus.
description: Published as part of Arachchige, Gayashan M., Jayakody, Sevvandi, Mooi, Rich & Kroh, Andreas, 2019, Taxonomy and distribution of irregular echinoids (Echinoidea: Irregularia) from Sri Lanka, pp. 1-100 in Zootaxa 4541 (1) on pages 63-67, DOI: 10.11646/zootaxa.4541.1.1, http://zenodo.org/record/2617460