CORRELATION BETWEEN SOFT AND HARD PARTS IN CYPRIDEIS (OSTRACODA): IMPLICATIONS FOR THE ANALYSIS OF SEXUAL SELECTION IN THE FOSSIL RECORD

Theory and limited empirical evidence indicate that sexual selection can influence rates of speciation and extinction in sexually reproducing organisms. However, studies of the influence of sexual selection on extinct organisms have been limited by a focus on mineralized hard parts of organisms, which are more readily preserved in the fossil record. Ostracodes offer an excellent system to explore sexual dimorphism as an indicator of sexual selection in both soft and hard parts. Male valves in cytheroidean ostracodes are more elongate than those of females, a shape difference hypothesized to result from a need to accommodate the very large male copulatory apparatus, which can account for up to 1/3 of body volume and may relate to the strength of sperm competition among males. Here, we examine the correlations between hard and soft parts in this group in order to test, in extant taxa, how male investment in primary sexual traits (soft parts) correlates with sexual dimorphism in ostracode valves (hard parts). We chose to study the genus Cyprideis because the group is characterized by strong and variable valve dimorphism.

Specimens were collected in the field (C. salebrosa, USA; C. torosa, UK) or obtained from collections of the National Museum of Natural History, Washington DC (C. mexicana). We digitized valve outlines to obtain measures of size (valve area) and shape (length to width ratio), and obtained several linear distances of the hemipenis, a primary sexual trait. Linear dimensions from four limbs were also measured as indicators of overall (soft-part) body size and allometry of the sexual structures was tested.

We verified that absolute size of soft parts is correlated with absolute valve size, suggesting that body size measured from hard parts is a good proxy for soft-part size, and therefore probably body mass as well. We also found that the absolute size of the hemipenis also correlated positively to valve area. Conversely, hemipenis size relative to other soft parts was not strongly related to valve elongation. Thus, results thus far indicate that sexual size dimorphism, perhaps as much or more so than shape dimorphism, may be an indicator of sexual selection acting on male reproductive investment. These findings can help to interpret the evolutionary significance of valve dimorphism as observed in the fossil record.