Adipicola Dautzenberg, 1927

Crespo, Enrique, 2017, First record of the genus Adipicola (Mollusca: Bivalvia: Mytilidae) and description of a new species from the Argentine SW Atlantic Ocean, Zootaxa 4318 (2), pp. 325-338 : 326-329

Genus Adipicola Dautzenberg, 1927

[= Myrina Adams & Adams, 1857 (non Fabricius, 1807)].

Type species (by monotypy). Modiolarca pelagica Forbes in Woodward, 1854 .

Diagnosis. Shell medium size, shell length up to 40 mm, equivalve, subequilateral, fragile, antero-posteriorly elongate; external surface smooth with irregular growth lines covered by a yellowish to brownish periostracum, periostracal hair absent; valves without gape; umbo subcentral to anterior; internally white to nacreous; hinge plate without teeth or crenulations in adults.

Posterior byssal retractors not divided; pedal retractors integrated with the posterior byssal retractors; demibranchs thick, outer and inner demibranchs equal in size. [Soft anatomical features after Coan et al. (2000); Gustafson et al. (1998); Ockelmann & Dinesen (2011).]

Species included. Species are listed in Table 1.

Distribution. NE Atlantic from Iceland to Portugal, Mediterranean Sea, Marmara Sea; SW and SE Atlantic ( Argentine Sea and West Coast of South Africa), Indo-Pacific (West Coast of Sumatra); Pacific Ocean from off Hawaii and Japan to off New Zealand.

Remarks. The genus Adipicola was proposed by Dautzenberg (1927) as a substitute name for Myrina Adams & Adams, 1857 , which was preoccupied by Myrina Fabricius, 1807 . Iredale (1939) suggested to replace Myrina by Miridas , but its type species Myrina coppingeri Smith, 1885 , is currently placed in Idas ( Dell 1987) . Miridas and Adipicola are not synonyms, therefore.

Warén (1991) regarded Adipicola to be synonymous with Idas stating that the main difference, the retention of "crenulated areas along the hinge line” in adult Idas but not in Adipicola , is only of infrageneric value. However, Coan et al. (2000), Gustafson et al. (1998) and Ockelmann & Dinesen (2011) found anatomical differences in pedal retractor muscle association, demibranch length and type of gills which would support the validity of two genera. Also, molecular data suggest that Adipicola and Idas are not monophyletic ( Duperron & Gros 2016; Fujita et al. 2009; Génio et al. 2012; Kyuno et al. 2009; Lorion et al. 2009, 2010, 2012, 2013; Rodrigues et al. 2013; Thubaut et al. 2013a & b). These contrasting points of view are well reflected by the two classification approaches of Huber (2010, 2015).

TABLE I. List of recent anđ fossil species that were or are placeđ in Adipicola Dautzenberg, 1927 . " Adipicola¨ species probably represent a new genus the proposeđ name of which is

unavailable, however ( Thubaut et al. 2013b).

It adds to the problem that the type species of Idas , I. argenteus Jeffreys, 1876 , is not representative of the genus because of its adaptations to carnivory and absence of symbiotic bacteria ( Ockelmann & Dinesen 2011; Rodrigues et al. 2014) whereas the type of Adipicola , A. pelagica , is a very rare species which has not been found again for a number of years. Hence, a revision including morphological and molecular investigations is currently not possible. This said, we can only state that our new species is morphologically closer to Adipicola than to Idas .

Analogous to the above problem, the validity of the genus Terua is still unresolved. Dell (1987) synonymized Terua with Adipicola after morphological comparisons of their type species and Duperron & Gros (2016) suggested a closer genetic relationship between A. pacifica and Terua n. sp. from Guadeloupe, in the Caribbean Sea, which raises the question of the relationship between Adipicola leticiae n. sp. and Terua . In this speculative context, we prefer to maintain the genus Adipicola .