Do the Bivalvia demonstrate environment‐specific sexual strategies? A Hong Kong model

Abstract

The expression of sexuality, overall sex ratio and variation in sex ratio with age of 14 southern Chinese freshwater, brackish and marine bivalves have been investigated. Freshwater species are often hermaphroditic and brooding, e.g. Musculium and Pisidium (Pisidiidae), while Corbicula fluminea (Corbiculidae) demonstrates wide intraspecific variability in sexual expression, but also broods. Anodontu woodiunu (Unionidae) and Limnoperna fortunei (Mytilidae) are dioecious, with low incidence of hermaphrodites in the former, but both have strong female‐biased overall sex ratio which does not change with age. Juvenile age classes of Corbicula cf. fluminalis have female‐biased sex ratio. A pronounced female bias is means to optimize reproductive success, by maximizing resource allocation into more energy‐demanding oogenesis, in temporally and spatially heterogeneous lacustrine habitats. In such habitats, moreover, there are probably diminishing returns to producing sons. Mangrove and brackish water species are all dioecious (except Succostrea cuculluta) with slight male‐biased sex ratio overall, but are strongly male‐biased as juveniles. This strategy similarly optimizes success in fluctuating intertidal environment, where high rates of juvenile mortality may be expected possibly, for example, through enhanced predation. In such case, therefore, the greater investment in sons is prerequisite to an overall balanced sex ratio. The marine intertidal species, Perna ciridis (Mytilidae) and Donax semigranosus (Donacidae) have stable I:I sex ratio, notvvarying with age, and are representative of the ‘typical’ dioecious condition recorded for most marine bivalves. Thus, although intraspecific variations in life history tactics and sexual strategies equip all species for variable environment, it is postulated that permutations of the ‘dioecious’ condition, mediated through variations in the sex ratio either overall or with age, broadly equip the Bivalvia for the total range of aquatic habitats.

Sex Determining Mechanisms in Bivalves

“…Romo Piñera et al (2009) investigated sex ratio and occurrence of hermaphroditism in two populations of this commercially exploited species, which has been subjected to an approximately 30 times increase in fishing activity in recent years. An unusually large proportion of hermaphrodites was found (21.8% and 23.5%), much higher compared to earlier reports from different localities (0%, 0.25%, and 2.6%; see references in Romo Piñera et al 2009) and to classical estimates (0.1-7%) of incidental hermaphroditism in other gonochoric species by Morton (1991). Also, a sex ratio skewed toward females was observed for the first time in this species.…”

“…Coe (1943) suggested that fewer than 400 of the 10,000 described species of bivalves deviated from strict gonochorism and Heller (1993) stated that hermaphroditism is present in only 13 out of 117 bivalve families (taxonomy sensu Vaught 1989). These two reviews, as well as Morton (1991), pointed out that freshwater bivalves are more prone to this reproductive strategy than are marine species. That said, some marine groups, such as oysters, appear to be particularly plastic in their reproductive strategies and show many different variants of hermaphroditism; e.g., at any one time, some oyster populations may be made up of simultaneous hermaphroditic individuals, which also have the potential to change sex between seasons (sequential hermaphroditism) (see Collin 2013).…”

Aspectos morfológicos e de incubação em três espécies de Corbicula Mühlfeld, no lago Guaíba, Rio Grande do Sul, Brasil (Bivalvia, Corbiculidae)

“…Os bivalves de água doce apresentam uma reprodução altamente especializada para exploração deste ambiente (Morton 1991). Enquanto a maioria dos bivalves marinhos é ovípara, muitos bivalves de água doce exibem várias formas de ovoviviparidade e viviparidade (Korniushin & Glaubrecht 2003), como por exemplo as famílias da ordem Unionoida e duas famílias de Veneroida: Sphaeriidae e Corbiculidae.…”

“…Os corbiculídeos são típicos exemplos de bivalves que incubam seus embriões (Morton 1991). Segundo Ituarte (1984a) tal comportamento pode estar relacionado à proteção e desenvolvimento da prole como resposta às variações bióticas e abióticas dos ambientes límnicos.…”

Stable isotope composition of the modern freshwater bivalve Corbicula fluminea

“…The life span of Corbicula is about 1-5 years and adult individual shell height about 35 mm, normally Corbicula is dioecism, but some hermaphrodites, Corbicula are filter feeder and its primary food is phytoplankton. The spread of Corbicula is related to its rapid growth rate, early onset of maturity, high fecundity, and its ability to tolerate a wide range of environmental conditions (Morton, 1991;McMahon, 2000;Cataldo et al, 2001).…”