Plesiodiadema antillarum ( A. Agassiz, 1880 )

Gondim, Anne Isabelley, Moura, Rafael Bendayan De, Christoffersen, Martin Lindsey & Dias, Thelma Lúcia Pereira, 2018, Taxonomic guide and historical review of echinoids (Echinodermata: Echinoidea) from northeastern Brazil, Zootaxa 4529 (1), pp. 1-72 : 18-19

Plesiodiadema antillarum ( A. Agassiz, 1880)

Figure 5 A–G

Aspidodiadema antillarum A. Agassiz, 1880: 73 –74.

Dermatodiadema antillarum A. Agassiz, 1898: 76 .

Dermatodiadema (Plesiodiadema) antillarum Mortensen, 1904: 37 , pl. 3, fig. 18, pl. 4, figs 8, 16, 29, 33, 35, pl. 5, figs 4, 32.

Plesiodiadema antillarum H.L. Clark, 1925: 39 .– Tommasi, 1966a: 12, figs 64–65.– Campos & Moura, 2008: 136 (table), 139.

Plesiodiadema microtuberculatum Mironov, 1981: 175 –177, figs 1a, b, d, g, and 5, pl. 1, figs 5–6, pl. 2, figs 4, 6, 7 [not Plesiodiadema microtuberculatum A. Agassiz, 1879 ].

Material examined. Rio Grande do Norte: 2 spms, Potiguar Basin , V.2011, 2094 m [UFPB/ECH.2293] .

Description. Test small, high, spherical or ovoid, and fragile ( Fig. 5 A– C). Aboral surface slightly flattened. Apical system monocyclic, genital and ocular plates nearly uniform in size. Genital plate almost pentagonal, with large number of small tubercles. Gonopore large and located almost in centre of genital plate. Ocular plate slightly rectangular (with rounded corners), broader than genital plate, with large and some small tubercles. Periproct circular, with circle of periproctal plates of irregular sizes and densely covered by long spines surrounding anus ( Fig. 5A ). Ambulacra narrow, with tubercles and spines much smaller than in interambulacra ( Fig. 5C ). Interambulacra with two series of large, strongly crenulate primary tubercles, secondary tubercles small and uniform ( Fig. 5C ). Ambulacral plates simple (unigeminate). Primary spines very long, slender, glassy, verticillate, curved downwards and extremely fragile ( Fig. 5A ). Secondary spines similar to primary spines, but shorter, thinner, and straighter ( Fig. 5A, B ). Tubercles perforate and crenulate ( Fig. 5C ). Peristome with shallow, rounded buccal notches ( Fig. 5B ). Ten large and naked buccal plates, covering nearly whole peristome, each with single buccal tube foot ( Fig. 5B ).

Pedicellariae (description based on the figures in Mortensen, 1904). There are small and large forms of ophicephalous pedicellariae ( Fig. 5D, E ). The large form has pyramidal valves ( Fig. 5D ), while the small form has short valves, with the distal portion diamond-shaped (with rounded corners) with serrate margin ( Fig. 5E ). Some small ophicephalous pedicellariae have a large, thin purse of tissue covering the stalk. Triphyllous pedicellariae elongate, with ovoid distal portions ( Fig. 5F ). Tridentate pedicellariae long, with stalk and neck long. Valve narrow and serrate ( Fig. 5G ).

Colour. When alive this species is light violet or greyish pink, with spines of the same tint ( A. Agassiz 1880). It may have a dark purplish red test and spines ( Schultz 2011).

Distribution. Amphiatlantic ( Turner & Graham 2003). In the West Atlantic, from Florida, Gulf of Mexico, Mexico, Bahamas, Cuba, Haiti, Jamaica, Trinidad and Tobago, Puerto Rico, Venezuela, Guyana, and Brazil ( Mironov 2006; Laguarda-Figueras et al. 2005b; Alvarado 2011; Solís-Marín et al. 2013). In Brazil, only known from off Maceió (AL) ( H.L. Clark 1925). We present herein the first record for the State of Rio Grande do Norte (Potiguar Basin). It occurs from 289 to 5278 m ( Mironov 2006; Smithsonian Database). Specimens from Potiguar Basin were collected at 2094 m.

Remarks. The genus Plesiodiadema contains seven extant species, of which only P. antillarum occurs in the Atlantic Ocean. Mortensen (1940) stressed that P. antillarum and P. microtuberculatum are very similar, and that the differences between them are not clear. It is possible that they represent a single species ( Schultz 2011). However, no study has been conducted to evaluate the taxonomic status of these taxa. According to Schultz (2011), P. antillarum differs from its congeners by having only a few small periproctal plates lying in the centre of the membrane. Moreover, the coronal plates are less numerous than in other species of the genus and the shape of the tridentate pedicellariae is constant in P. antillarum ( Mironov 2006) . According to H.L. Clark (1925), the specimen of P. antillarum from H.M.S. “Challenger” St. 122 (off eastern Brazil) is very young, and its identity therefore not certain. Clark (1925) remarks that all specimens collected in Brazil were initially erroneously identified as P. microtuberculatum . On the basis of the collected material, we confirm the presence of P. antillarum for the Brazilian continental shelf. We note that Brazilian material has a slightly lower test than the specimens from the Caribbean. A. Agassiz (1883) did not find noteworthy differences among young and adult individuals, and small differences were considered as related to ontogenetic development.

Ecological notes. There is little available information on the biology and ecology of P. antillarum . This species feeds on mud and detritus, and does not appear to ingest plant material ( Serafy & Fell 1985; Pawson 1982). Emlet (1995) reported that P. antillarum has planktotrophic larvae.

Figures

FIGURE 5. Morphological characters of Plesiodiadema antillarum (UFPB/ECH.2293). (A–G) and Cassidulus infidus (H–I). (A) Aboral view; (B) Oral view, arrow showing buccal plate; (C) Lateral view; (D–G) Illustrations of pedicellariae of P. antillarum from Mortensen (1904); (D) Valve of large form ophicephalous pedicellariae; (E) Valve of small form ophicephalous pedicellariae; (F) Valve of triphyllous pedicellariae; (G) Valve of tridentate pedicellariae; (H) Aboral view; (I) Oral view.