Dianadema mascarenensis, P. G. Oliver & Holmes, 2004

Oliver, P. Graham & Holmes, Anna, 2004, Cryptic bivalves with descriptions of new species from the Rodrigues lagoon, Journal of Natural History 38 (23), pp. 3175-3227 : 3222-3224

Dianadema mascarenensis n. sp.

( figures 164–172 )

HOLOTYPE: one shell attached to paratype 1, attached to a Chama valve, off Totor, Port Mathurin , Rodrigues, attached to coral rubble at 10 m, 19 ‡ 40.23 ’ S, 63 ‡ 25.72 ’ E. NMW.Z.2000.086.00020.

PARATYPES: one shell attached to holotype, off Totor , Port Mathurin ,

NMW.Z.2000.086.00021; two shells from Mauritius , NMW.Z.2000.086.00022; 1 rv from RDS 52, Grand Pate´ , off Port Mathurin, 19 ‡ 39.307 ’ S, 63 ‡ 24.950 ’ E, 29 September 2001, sand in surge channel, 17 m, scuba, coll. N. L. Bruce and J. Smith , NMW.Z.2001.061.00044.

Measurements (mm, L 6 H). Holotype, right valve: 8.32 6 5.98. Paratype, Mauritius right valve: 9.48 6 7.01

Description. Crypt and tube to 15 mm. Crypt cemented to the inside of dead bivalve shells ( figures 164, 166 ). Tube short and terminally free from substrate, orientation at an upward angle to the crypt; with a median longitudinal depression and lacking frills. Left valve cemented into crypt but remains visible except for a dorsal overgrowth of a few tubules over the umbo ( figure 166 ); crypt tubules simple mostly around anterior and ventral regions ( figure 165 ). Right valve free in crypt ( figures 165, 168, 169 ), outline obliquely oval, strongly inequilateral with beaks near the anterior margin, anterior reduced, posterior expanded. Sculpture of widely separated radiating rows of short spikes ( figure 170 ), these fading towards margins where the shell is rather irregular with growth lines only. Hinge appears edentulous. Ligament internal on a shallow, posteriorly directed chondrophore ( figure 171 ). Adductor scars subequal, pallial sinus broadly rounded extending to midline ( figure 167 ).

Derivation of name. Latin suffix ensis denoting place of belonging and Mascarenes, the known range of the species.

Comparisons. Morton (2003) created the genus Dianadema , citing the unique features of a cemented mode of life and a dorsal crown of tubules. Both characters are present in the shells from Rodrigues and Mauritius, confirming that they do not belong to the only previously recorded clavagellid from the Mascarene region, Clavagella aperta minima Sowerby, 1889 .

Morton (2003) records that the genus Dianadema is restricted to Australian waters and includes the species D. multangularis (Tate, 1887) and D. torresi ( Smith, 1885) . From examinations of Indian Ocean material we would also include Clavagella mullerae Kilburn, 1974 , from South Africa, in the genus Dianadema . We are excluding D. torresi from further discussion because it appears to be primarily endobenthic in habit and has multiple longitudinal grooves on the tube. Dianadema multangularis , as the species name implies, also has multiple grooves on the tube but does share the epibenthic habit of the Mascarene material. Dianadema mullerae ( figures 173–177 ) is known from very few specimens and is most similar to the Mascarene shells but differs as follows. The radial rows of spikes are more closely spaced and the spikes are much smaller, appearing as granules ( figure 177 ). The pallial sinus is deep but is angular rather than rounded ( figure 174 ). For these reasons we conclude that while D. mullerae and the Mascarene shells are similar they do constitute different species.

Habitat. Cemented to bivalve shells ( Chama and Petricola ) that are themselves attached to or embedded in rocks and coral rubble, sublittoral.

Figures

FIGS. 164–169. Dianadema mascarenensis n. sp. (164, 165) Holotype, NMW.Z.2000.086.00020 (MontageTM). (166–169) Paratype from Mauritius, NMW.Z.2000.086.00022 (MontageTM).

FIGS. 170–177. (170–172) Dianadema mascarenensis n. sp. Paratype, NMW.Z.2001. 061.00044 (SEM): (170) external; (171) hinge view; (172) sculpture (SEM). (173–177) Dianadema mullerae, South Africa D1613.