A.V. Zhirmunsky National Scientific Center of Marine Biology,
Far Eastern Branch of the Russian Academy of Sciences, Vladivostok 690041, Russia e-mail: allasilina@mail.ru
The phenomenon referred to as sexual size dimorphism (SSD), in which males and females differ in their body size, is common in many animal taxa and is usually interpreted as the result of sexual selection. SSD follows from two adaptive hypotheses:
(1) the intrasexual selection hypothesis, by which the competition between males leads to selection for a larger body size, and (2) the fecundity advantage hypothesis, according to which natural selection drives females to have larger bodies in order to produce a larger brood size. SSD is predicted to vary across mating systems. Mating systems with males larger than females occur when males compete for female access or guard territories, while mating systems with group mating tend to occur in spe- cies where females are the same size or larger than males. To date, it is known that at a given age, females are larger than males in Mytilus edulis and Calyptogena gallardoi, both having group mating systems. Studies on sexual size dimorphism in Pectinidae are limited. In natural populations bivalve growth rates vary under different environ- ments; therefore same-sized bivalves may have different ages, and this circumstance, in the absence of a method to determine individual age, hampers the comparison of results for the same aged mollusks. No data on sexual size monomorphism or dimor- phism have been reported for the mobile long-lived scallop Mizuhopecten yessoensis (Jay, 1857), a common (fished and cultured) species in the subtidal benthos of the Sea of Japan. It is a group spawner; therefore, it is expected that the species will have female-biased SSD or both sexes have the same size. Because this scallop is important to marine aquaculture and provides a food source for humans, it is important to under- stand the ability to differentiate males from females of this species to ensure proper management policies.
The goal of this study was a comparison of the shell characteristics, tissue weights and growth patterns of the same aged males and females in a wild population M. yessoen- sis for a research of its SSD. The comparisons of parameters of the same aged individuals in population of M. yessoensis can be studied as the method for age and shell growth determination of each scallop during its lifetime was devised by author.
The scallops, 209 specimens, were collected by SCUBA divers from wild popu- lation at Ozero Vtoroe Bay of Peter the Great Bay in the northwestern Sea of Japan (42°72′38′′ N, 133°03′05′′ E). Individuals were sampled just before the spawning, when their gonads were mature (at the beginning of June) and the sex of the scallops can
the male gonad is creamy.
It was found that both the males and females display the somatic growth during all their life. Gonad weight (in the pre-spawning period) increased with age too, until a threshold age was attained; and then gonad weight remained virtually unchanged.
The threshold age coincides with the beginning of the senile period of scallop ontoge- nesis development. In the population studied, the threshold age was found for individu- als of 6 years of age.
The shell height of the same aged scallops was not sexually dimorphic. The adduc- tor muscle weight of both scallop genders of the same age do not differ. The shell growth rates in the studied bivalve species were similar for both genders. The von Bertalanffy equations of the shell height (H, mm) growth for males (Hm) and females (Hf) were respectively:
Hm = 144.7 (1 − exp (−0.6079 (t − 0.6929))), Hf = 142.4 (1 − exp (−0.6308 (t − 0.6053))), where t is scallop age, years.
Due to insignificant difference between the mean shell sizes of male and female scal- lops, it is impossible to visually identify the sex of live specimens during non-breeding periods.
The differences become significant with scallop age increase for such parameters as the total and shell weights (male-biased, >5 years old). A similar tendency was found for the adductor muscle weight of the scallops from the wild population. The results in terms of reproductive effort were illustrated by the gonad weight. Statistical analyses showed female-biased dimorphism in the gonad weight for age groups >4 years old.
The fecundity advantage hypothesis for M. yessoensis with group mating and external fertilization is at least partly realized by physiological mechanisms, the larger gonads of older females than older males in a population, in order to produce a larger brood. Gregarious settlement of this bivalve contributes to the reproductive success of the population so that the energetically costly ovaries may all be fertilized.
69 Biodiversity and Evolution of Mollusks
Apodomenia – a solenogaster without a foot
Christiane Todt
Rådgivende Biologer AS and University Museum of Bergen, 5059 Bergen, 5007 Norway e-mail: Christiane.Todt@radgivende-biologer.no
Apodomenia enigmatica is a bizarre recently described species of worm-shaped mollusk that appears to be a morphological intermediate between the two major apla- cophoran groups, the Solenogastres (Neomeniomorpha) and Caudofoveata (Chaeto- dermomorpha). However, this animal is not a missing link; molecular and morpho- logical studies show that it is a derived solenogaster that lacks a foot, mantle cavity, and radula. The phylogenetic placement of the species is based on analyses of new transcriptomes from 25 aculiferans (Solenogastres, Caudofoveata and Polyplacophora) together with earlier published data. Phylogenomic analyses included datasets with up to 525 genes. Our results indicate that aplacophoran taxonomy is in need of revision as several traditionally recognized groups are non-monophyletic. Most notably, Cavi- belonia, the solenogaster taxon defined by hollow sclerites, is not monophyletic. Taken together, these results shed light on the evolutionary history of Aplacophora and reveal a surprising degree of morphological plasticity within the group.