THE ONTOGENY AND EVOLUTION OF FOSSAE ARRANGEMENT IN THE DEEP-SEA OSTRACODE GENUS POSEIDONAMICUS

The carapace of ostracodes in the genus Poseidonamicus is characterized by a reticulum of intersecting mural ridges. These ridges outline compartments called fossae, which have been found to correspond to underlying epidermal cells (Okada, 1981). In the current study, a database of over 3,000 SEM images of adult through A-4 individuals was used to investigate the ontogeny and evolution of the pattern of fossae arrangement in this genus.

The progressive increase in the number of fossae with each molt is described, and from this progression, a hypothesized sequence of cell divisions is inferred. It appears that this sequence of cell divisions is quite conserved, although two relatively large evolutionary changes have occurred that are useful for delimiting clades within the genus. Whereas all Poseidonamicus species have nearly the same configuration of fossae as adults, it is fairly common to find aberrant individuals within species that do not show the modal arrangement. To explore the possible causes of evolutionary conservation in the face of seemingly abundant within-population variation, the fitness consequences of aberrant fossae patterns were estimated. This was done by calculating the proportion of individuals in the A-1 and adult instars that showed identifiable fossae aberrations; if there is a selective disadvantage to the aberrant fossae patterns, one expects that the proportion of individuals showing these patterns should decrease from the A-1 to adult instar. At least one type of deviation from the typical fossae pattern shows this decrease, suggesting that stabilizing natural selection may be acting to eliminate arrangements of fossae that deviate from the norm. Finally, the evolution of more modest changes in fossae arrangement was investigated by tracking evolutionary changes in the position of a subset of fossae. Evolutionary rates calculated based on the position of these fossae are far lower than what is expected under neutral genetic drift, an observation that is also consistent with stabilizing selection on fossae arrangement.