Microgloma macaron, Benaim, Natalia Pereira & Absalao, Ricardo Silva, 2011

Benaim, Natalia Pereira & Absalao, Ricardo Silva, 2011, Microgloma Sanders & Allen, 1973 (Nuculanidae) and Pristigloma Dall, 1900 (Pristiglomidae) (Pelecypoda) in the Campos Basin of Brazil, ZooKeys 152, pp. 1-20 : 9-12

Microgloma macaron sp. n. Figs 1422

Holotype.

MNRJ 19.112 (Figs 14, 16, 18).

Type locality.

Campos Basin, #54, 12/12/2002, 750m, 21°57'17,5"S, 39°56' 01,1"W.

Paratypes. IBUFRJ 15297, 8 valves and 2 specimens; MZUSP 99979, 4 valves; USNM 1156943, 6 valves; MNHN 24596, 6 valves; all from the type locality.

Etymology.

The species epithet refers to the French macaroon cookie ("macaron" in the French language), which has a similar appearance to the articulated valves. The species epithet is proposed as a noun in apposition.

Material examined.

IBUFRJ 19145 (# 32, 2002), 3 valves; IBUFRJ 19146 (# 33, 2002), 1 valve; IBUFRJ 19147 (# 34, 2002), 1 valve; IBUFRJ 19148 (# 36, 2002), 1 valve; IBUFRJ 15482 (# 54, 2002), 6 valves; IBUFRJ 17033 (# 59, 2002), 1 specimen; IBUFRJ 19150 (# 61, 2002), 3 specimens; IBUFRJ 16074 (# 64, 2002), 14 valves and 1 specimen; IBUFRJ 15141 (# 69, 2002), 7 valves; IBUFRJ 15285 (# 74, 2002), 2 valves; IBUFRJ 15635 (# 54, 2003), 32 valves and 5 specimens; IBUFRJ 19152 (# 59, 2003), 6 valves and 1 specimen; IBUFRJ 19153 (# 61, 2003), 1 valve; IBUFRJ 19153 (# 61, 2003), 1 valve; IBUFRJ 19154 (# 64, 2003), 18 valves and 4 specimens; IBUFRJ 19155 (# 69, 2003), 4 valves; IBUFRJ 19156 (# D11, 2008), 1 specimen; IBUFRJ 19157 (# G12, 2008), 1 specimen; IBUFRJ 19158 (# A7, 2008), 10 specimens; IBUFRJ 19159 (# A7, 2008), 1 specimen; IBUFRJ 19160 (# A7, 2008), 2 valves and 5 specimens; IBUFRJ 19161 (# D7, 2008), 1 specimen; IBUFRJ 19162 (# D7, 2008), 6 specimens; IBUFRJ 19163 (# CANAC7, 2008), 1 specimen; IBUFRJ 19164 (# H7, 2008), 1 specimen; IBUFRJ 19165 (# H7, 2008), 1 specimen; IBUFRJ 19166 (# H7, 2008), 5 specimens; IBUFRJ 19167 (# I7, 2008), 2 specimens; IBUFRJ 19168 (# A7, 2009), 3 specimens; IBUFRJ 19169 (# A7, 2009), 5 specimens; IBUFRJ 19170 (# D6, 2009), 1 specimen; IBUFRJ 19171 (# D7, 2009), 1 specimen; IBUFRJ 19172 (# H7, 2009), 12 specimens; IBUFRJ 19173 (# CANAC7, 2009), 1 specimen; IBUFRJ 19174 (# CANAC7, 2009), 1 specimen; IBUFRJ 19175 (# CANG7, 2009), 3 specimens; IBUFRJ 19177 (# 64, 2003), 4 specimens.

Diagnosis.

Shell minute, ovate. Entire shell margin thickened and covered by an extension of the periostracum. Prodissoconch surface with one subtle radial striae.

Description.

Shell minute, ovate, H/L ratio about 0.81 (n 38, min 0.75, max 0.92), W/H ratio about 0.84 (n 14, min 0.77, max 0.87), glossy, translucent, robust for its size, equilateral; umbones prominent, large, posterior to midline, orthogyrous. Antero–dorsal margin straight, oblique; anterior margin rounded, extended. Antero–ventral margin, postero–ventral margin shorter and rising up to the short posterior end; posterior margin slightly truncated, forming a small shoulder. Entire shell margin thickened and covered by an extension of the periostracum, resembling a macaroon in ventral view (Figs 21-22). Surface with commarginal growth lines. Hinge plate with 5-7 anterior and 4-5 posterior teeth, interrupted by a large, rectangular and shallow resilifer. Width of both anterior and posterior row of teeth occupies about 70% of width of hinge plate, which is thick for its size (whp/H approximately Campos Basin, #54, 12/12/2002 0.14). Posterior part of the hinge plate shorter than anterior one. Prodissoconch surface nacreous, with one subtle radial striae (Fig 20), and length approximately 244 µm. Maximum adult shell length 1.20 mm.

Remarks.

In some other species of Microgloma , the shell is expanded ventrally, around the valve, which enlarges the internal volume of the animal and counterbalances the effects of miniaturization ( Ockelmann and Warén 1998, La Perna 2008). However, in Microgloma macaron this expansion is thicker, and the periostracum surrounds the entire margin (Fig. 22). This characteristic of the periostracum deserves special attention since it not only covers the shell to the margins in the usual way, but is more conspicuous in this area, giving the impression that valves do not articulate. We are not sure how this system works, and how the animal, in spite of having these fibers surrounding the valve apertures, can have water flux in the mantle cavity.

Compared to Microgloma pusilla and Microgloma mirmidina , Microgloma macaron is distinguished by the ovate outline, umbones at midline and much more projecting, and a thicker hinge plate. Microgloma macaron is similar to Microgloma yongei in outline, but compared with the paratypes figured by Ockelmann and Warén (1998, page 14, fig 6-D), the former has more prominent umbones, a thicker hinge plate, as well as a larger resilifer. Microgloma macaron can be distinguished from Microgloma tumidula by the shape of the teeth, which are not as inclined as in this latter species. The anterior and posterior areas of the hinge plate form a less obtuse angle compared to those of Microgloma tumidula .

Except for two, probably worn, valves found at twostations at approx. 3000 m depth, and one at 1970 m, this species is concentrated at depths between 400-750 m. Untill now this species is reccorded solely in Campos Basin.