The Echinoid Directory

• Test low and delicate. No membranous gaps between plates.
• Apical disc monocyclic. Gonopores opening through membranous gap.
• Ambulacra trigeminate, with a large primary element and two demiplates, the outer demiplate reaching the adradial suture. Sometimes the primary element may be divided.
• Pore-pairs forming a medial band in each column; some elements with rudimentary pore-pairs on aboral surface.
• A primary ambulacral tubercle on every principle plate adorally.
• Ambulacra almost as broad or broader than interambulacra at the ambitus.
• Primary tubercles much larger on the oral surface than aborally.
• Relatively few ambulacral plates extending over the peristome, with pore-pairs arranged uniserially.
• Buccal notches and 'gills' effectively absent.
• Oral spines with cone-shaped hyaline hoof.
• Sphaeridia on inner of the two demiplates only.

Recent, Indo-Pacific and Atlantic.

Tromikosoma koehleri Mortensen, 1903, by original designation.

• T. koehleri Mortensen, 1903; Recent, Davis Straight.
• T. uranus (Wyville Thomson, 1877); Recent, East Atlantic.
• T. tenue (Agassiz, 1879); Recent, North Pacific.
• T. australe (Koehler, 1922); Recent, New Zealand.
• T. hispidum (Agassiz); Recent, West Pacific.

Euechinoidea; Echinothurioida; Echinothuriidae.

Monophyletic

Differs from other Echinothuriidae except for Sperosoma in having the primary ambulacral plate commonly formed of two parts. In Tromikosoma the buccal notches and 'gills' around the peristomial margin are absent, whereas in Sperosoma they are always developed according to Mortensen (1935).

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 Sperosoma 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 monogphyletic genus.  

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. 1903. Danish Ingolf Expedition 4(1) Echinoidea. p. 65.

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