Meristic and Morphometric Differences in Populations of Rivulus marmoratus

Abstract

Rivulus marmoratus (Pisces: Aplocheilidae) is a small cyprinodontid found in mangroves of the western tropical Atlantic and is the only known self-fertilizing, hermaphroditic vertebrate. Populations normally consist of genetically diverse groups of homozygous clones. Rivulus marmoratus has the widest range of any member of the genus (southern Brazil to central Florida) and is the only marine representative of the genus. There has been considerable speculation about the "origin" of the species in an otherwise sexually reproducing genus and family. Although well studied in the laboratory, few specimens have been collected from the wild until recently. This study examined meristic and morphometric differences among 12 widespread populations and among individual clonal lineages reared in the laboratory. Thirty-two meristic and morphometric characters in 187 fish (ranging from Brazil to Florida) were examined with univariate and multivariate statistics for determination of overall differences among populations. To control for possible environmental effects, offspring of eight clones from two populations (Belize and Florida) were reared at 25 C and similarly analyzed. All characters with the exception of pectoral fin rays were significantly different among wild populations. In addition, Belizean male fish differed from hermaphrodites in several morphometric characters. Fish reared at constant temperature also displayed significant differences, indicating a probable genetic component to differences in wild fish. The multivariate analyses also confirmed considerable heterogeneity among R. marmoratus. Overall, fish from Brazil, the Florida Everglades, Belize, and Honduras appear to be distinct from the other groups. The characters that contributed most to group delineations were number of precaudal vertebrae, distance from pelvic fin origin to anal fin origin, pectoral fin length, and number of branched caudal rays. Discriminant function analysis was able to classify the fish reared at 25 C to individual clonal heritage, although with a different set of characters than in wild fish.