The Echinoid Directory

• Apical disc monocyclic
• Ambulacra trigeminate; on the aboral surface with the large element subdivided into two, an adradial part with a pore-pair and a perradial part without a pore-pair. Two accessory plates occluded from the adradial suture bearing pore-pairs occur medially.
• Pore-pairs forming a single irregular median band; with pore-pairs reduced on some elements in the type species.
• Ambulacra about the same width as interambulacra at the ambitus and wider adorally.
• Interambulacral plates without membranous gaps.
• Primary tubercles perforate and non-crenulate.
• Shallow but well developed buccal notches.
• Only ambulacral plates extending over the peristome, with pore-pairs arranged uniserially.
• Sphaeridia on inner accessory element only in each triad.
• Spines with expanded cone-shaped hyaline hoof.

Recent, Indo-Pacific and Atlantic.

Sperosoma grimaldii Koehler, 1897, by original designation.

• S. grimaldii Koehler, 1879; Recent, East Atlantic.
• S. giganteum Agassiz & Clark, 1907; Recent, Japan.
• S. armatum Koehler, 1927; Recent, Arabian Gulf.
• S. durum Doderlein, 1905; Recent, Indian Ocean.
• S. quincunciale de Meijere, 1904; Recent, Indo-Pacific.
• S. biseriatum Doderlein, 1901; Recent, Bering Sea.
• S. tristichum Mortensen, 1934; Recent, Borneo.
• S. obscurum Agassiz & Clark, 1907; Recent, Hawaii.
• S. crassispinum Mortensen, 1934; Recent, Indo West Pacific.
• S. antillense Mortensen, 1934; Recent, Barbados.
• S. koehleri (Mortensen, 1903); Recent, Davis Straight.
• S. uranus (Wyville Thomson, 1877); Recent, East Atlantic.
• S. tenue (Agassiz, 1879); Recent, North Pacific.
• S. australe (Koehler, 1922); Recent, New Zealand.
• S. hispidum (Agassiz); Recent, West Pacific.

Euechinoidea; Echinothurioida; Echinothuriidae.

Inclusion of Tromikosoma changes Sperosoma from a paraphyletic grade  to a monophyletic clade (Mooi et al 2004).

Mortensen distinguished this taxon from other Echinothuriidae (except for some species of Tromikosoma) in having the primary ambulacral plate effectively split into two parts, the inner part having no pore-pair. In Tromikosoma there are no buccal notches or 'gills' whereas in Sperosoma the buccal notches are always evident. The two are however, undoubtedly closely related.

Mooi et al. (2004) have argued that the divided ambulacral plate is an artefact caused by fracturing of thin plates during collection and subsequent handling.  In Mooi et al's (2004) cladistic analysis Spermosoma is shown to be a paraphyletic grade  of species below a clade of Tromikosoma species.  Given the paraphyletic nature of Sperosoma, it seems best to unite Sperosoma and  Tromikosoma to form a single large monophyletic genus.

Koehler, R. 1897. Sperosoma grimaldii, nouveau genre d'Echinothurides. Zool. Anzeig. 20, 302-307.

Mooi, R., Constable, H., Lockhart, S. & Pearse, J. 2004. Echinothurioid phylogeny and the phylogenetic significance of Kamptosoma (Echinoidea: Echinodermata). Deep Sea Research II 51, 1903-1919.

Mortensen, T. 1935. A monograph of the Echinoidea. Volume 2, Bothriocidaroida, Melonechinoida, Lepidocentroida and Stirodonta. C.A. Reitzel, Copenhagen.