41 



is associated with specific conditions of existence. In the superfamily Veneroidea, 

 predominance of the lecithotrophic phase is related to colonization of polar 

 waters (Ockelmann, 1958; Dell, 1972) in one line, and in another to reduction 

 in body size (Hansen, 1953; Oldfield, 1964). In the superfamilies Tellinoidea 

 and Mytiloidea, development with lecithotrophic larvae is observed in species 

 colonizing arctic waters. Lecithotrophic larvae and direct development are also 

 characteristic of the suborder Erycinina, in which brooding of young up to the 

 juvenile stage is observed, as well as male dwarfism an uncommon phenom- 

 enon in bivalves. In the family Galeommatidae, male dwarfism is observed in 

 Ephippodonta oedipus and Chlamydoconcha orcutti. Male dwarfism evolved 

 from secondary brooding of juveniles under the protective mantle of the adult 

 mollusk (Morton, 1976, 1981). According to Morton, male dwarfism is a 

 temporary phase in the development of these mollusks, which later grow into 

 animals of definitive size. In the family Montacutidae, dwarf males are found 

 in commensal forms — Montacuta floridiana, M. percompressa ( Jenner and 

 McCrary, 1968), and M. phascolionis (Deroux, 1960). Recently, dwarf males 

 were reported for two species of teredinides — Zachsia zenkewitchi and Z. 

 serenei — inhabiting the roots of sea grass (Temer and Yakovlev, 1981, 1983; 

 Yakovlev, 1988). Yakovlev and Malakhov (1985, 1988) have shown that dwarf 

 males of Z zenkewitchi combine features of neotany, regression and special- 

 ization. 



Mollusks of the suborder Erycinina are the smallest bivalves (less than 1 

 cm), exhibiting a sharp fall in fecundity with extended periods of brooding. 

 Thus Arthritica bifurca, living in silt singly or with the polychaeta Pectinaria 

 australis (Wear, 1966), incubates numerous larvae only up to a length of 

 120 ^.m, while Lasaea rubra cares for its progeny up to the juvenile stage 

 (Oldfield, 1955, 1964); in this species there is a maximum of 33 embryos, 

 which reach a length of 500 |a.m. Booth (1979) observed one sexually mature 

 L. rubra hinemoa, 1 mm in length, brooding two embryos, each 400 |im long. 



Table 3 presents data from tables compiled by Sellmer (1967), Blacknell 

 and Ansell (1974), Sastry (1979), as well as data from other researchers, on 

 species of marine and brackish- water mollusks with some form of brood care. 

 The list, of course, is not exhaustive. The classification of bivalves is that 

 given by Skarlato and Starobogatov (1979), except for the genus 

 Chlamydoconcha, which is included in the family Galeommatidae. 



IDENTIFICATION OF PELAGIC LARVAE OF BIVALVES 

 (Terminology and Taxonomic Characters) 



In larvae, a distinction is made between the anterior and posterior, and upper 

 and lower (dorsal and ventral) margins of the shell. The velum and adductor 



