Relevance of sperm ultrastructure to the classification of giant clams (Mollusca, Cardioidea, Cardiidae, Tridacninae)

Abstract

Examination of sperm ultrastructure in six out of eight of the living species of giant clams ('Tridacnidae') supports reduction in status of the Tridacnidae to a subfamily of the Cardiidae (Tridacninae), as suggested by recent cladistic analyses based on shell, anatomical and molecular characters. Tridacninae spermatozoa are all of the aquasperm type, featuring, in anterior-posterior sequence: a conical acrosomal vesicle, an oblong to rod-shaped nucleus, a short midpiece region (proximal and distal centrioles surrounded by four round mitochondria); and a flagellum (9 + 2 pattern axoneme). Substantial differences occur between species with respect to the shape and length of the nucleus, and in the spatial relationship between the acrosomal complex and the nuclear apex. Although the two extant genera can be distinguished on sperm features -Tridacna, nuclear peg associated with acrosome, centriolar connective absent; Hippopus, nuclear peg absent, connective present -no defining feature of the Tridacninae can be detected. Within Tridacna, the species T. (Chametrachea) maxima and T. (C.) crocea are distinguished from other species of the genus by a much finer nuclear peg and a considerably smaller acrosome. In contrast, and against expectation, T. (C.) squamosa shows acrosomal and nuclear dimensions very close to those obtained for T. (Tridacna) gigas. Recently proposed phylogenies and classificatory changes for 'tridacnids' are discussed in the light of available sperm data.

Sperm ultrastructure in Hemidonax pictus (Hemidonacidae, Bivalvia, Mollusca): comparison with other heterodonts, especially Cardiidae, Donacidae and Crassatelloidea

“…Sources of data: A–E, L, S–V (this paper; J. M. Healy, P. M. Mikkelsen & R. Bieler, unpubl. data); F (Popham, 1979); G, H [composite reconstruction based on Cerastoderma lamarcki (Reeve, 1844) from Drozdov et al ., 2001 and Cerastoderma edule (Linnaeus, 1758) from Sousa & Azevedo, 1988 and Sousa et al ., 1998]; I–K (Keys & Healy, 1999, 2000); M–O (Hodgson et al ., 1990); P (Sousa & Oliveira, 1994); Q, R (Pochon‐Masson & Gharagozlou, 1970; Gharagozlou‐Van Ginneken & Pochon‐Masson, 1971).…”

“…Sources of data: A–D, F, I, M–O (this paper; J. M. Healy, P. M. Mikkelsen & R. Bieler, unpubl. data); E (Popham, 1979); G, H (Keys & Healy, 1999, 2000); P (Gharagozlou‐Van Ginneken & Pochon‐Masson, 1971); J, K (Hodgson et al ., 1990); L (Sousa & Oliveira, 1994).…”

“…In order better to assess the two strongest claims regarding Hemidonax affinities (i.e. with the Donacidae or with the Cardiidae) we also present data for both of these families, which will help to supplement the available literature (Donacidae –Hodgson, Bernard & Van der Horst, 1990; Sousa & Oliveira, 1994; Healy, 1995b; Cardiidae –Popham, 1979; Sousa & Azevedo, 1988; Healy, 1995b, 1996a; Sousa et al ., 1998; Keys & Healy, 1999, 2000; Drozdov, Frolenko & Ferraguti, 2001). Aside from Pelseneer's (1911) statement that in Hemidonax donaciformis (Schröter, 1786) the sexes are separate, nothing appears to be known concerning the reproductive biology of Hemidonax .…”

The evolution of molluscan photosymbioses: a critical appraisal

“…Velates has been interpreted either as a suspension-feeder (Savazzi, 1992) or herbivore (Vermeij, 2011), although its large size, exposure of the mantle to light, and distribution in well-lit tropical carbonate-dominated environments are also consistent with photosymbiosis. Yonge, 1936;Rosewater, 1965;Hamner, 1978;Beckvar, 1981;Trench, Wethey & Porter, 1981;Lucas et al, 1991;Schneider, 1992Schneider, , 1998aGriffiths & Klumpp, 1996;Schneider & Ó Foighil, 1999;Keys & Healy, 2000;Harzhauser et al, 2008;Schwartzmann et al, 2011;deBoer et al, 2012 Fraginae Phylogenetic position: Bivalvia, Heterodonta, Cardiidae Placement of symbionts: Fragum and relatives with expanded inner fold of mantle in posterior part, often extending over exterior; Corculum: beneath windows on dorsal shell face Characteristics: Fragum partially enveloped posterior part of shell; all species with keel from umbo to posteroventral margin; Corculum dorsoventral flattened Ecology: tropical, infaunal in sand, Indo-West Pacific Maximum size: Fragum 65 mm; Corculum: 70 mm Age: Late Miocene to Recent References: Kawaguti, 1950Kawaguti, , 1983Watson & Signor, 1986;Ohno et al, 1995;Kobayashi, 1996;Carter & Schneider, 1997;Morton, 2000;Kirkendale, 2009;ter Poorten, 2007ter Poorten, , 2009 Clinocardium nuttalli Phylogenetic position: Bivalvia, Heterodonta, Cardiidae, Clinocaradiinae Placement of symbionts: in posterior mantle of adults Characteristics: no specializations Ecology: cool-temperate, infaunal in sand, northeastern Pacific Maximum size: 140 mm Age: Recent References: Jones & Jacobs, 1992; Coan et al, 2000 Controversy surrounds many of the groups listed in Table 1. Arguments for and against photosymbiosis in these groups are discussed below in the context of potential specializations to photosymbiosis in living species, with particular attention to the seemingly contradictory ecological distributions of many of the fossil candidates.…”

Taxonomy of Indonesian giant clams (Cardiidae, Tridacninae)

“…Globally, various studies focusing on many aspects of the clams have been done, from biological experiments to mariculture and conservation strategy, for example Keys and Healy (2000), Buck et al (2002), Kinch (2002), Harzhauser et al (2008), and Teitelbaum and Friedman (2008). In context of Indonesia, taxonomic notes of giant clams are rare although Indonesian waters are the habitat for most of giant clams species in the world (bin Othman et al 2010).…”