15 February 2007

An occasional male ostracod

Ostracods are microscopic crustaceans with a pair of carapaces resembling clam shells. Until recently no males had been seen—at least since about the end of the 19th century—in darwinuloid ostracods (family Darwinulidae) and they were assumed not to exist. In fact, a paper I reviewed briefly (Martens et al., 2003), claimed that darwinuloids had been reproducing asexually for about 200 million years.

A paper by Smith et al.1 that came out in June 2006, and which I just had a chance to read, however, reported the finding of males in a new species of darwinuloid ostracod, Vestalenula cornelia, from Japan. The paper also showed that male carapace morphology of the new species was similar to that of juvenile females. This implies that the conclusions of previous studies that sexed fossil ostracod carapaces from their morphology (for example, Martens et al., 2003) may have been flawed.

Anatomy of Vestalenula cornelia (with right valve removed and only one pair of appendages drawn for clarity). Female (top) and male (bottom). The latter is identified by its penis (Hp) in the back. Figure from Smith et al.1

Interestingly, only 3 males, in contrast to more than 600 females, of the new ostracod species were collected. Smith et al. offer the following possibilities to explain the rarity of males:

1. Certain environmental or seasonal factors may induce the production of male V. cornelia. This is supported by the fact that all 3 males were collected during late winter and early spring.

2. Males may be non-functional relicts that occasionally appear but have no reproductive input.

3. Males may be rare but nevertheless, functional, that is contributing to the genetic diversity of their populations.

With the removal of ostracods from the list, bdelloid rotifers remain to be the largest group of animals in which no males have ever been recorded.

1Robin J. Smith, Takahiro Kamiya, David J. Horne. 2006. Living males of the 'ancient asexual' Darwinulidae (Ostracoda: Crustacea). Proceedings of the Royal Society B: Biological Sciences 273:1569-1578.

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