Intraspecific and Ontogenetic Radular Variation in Opisthobranch Systematics (Mollusca: Gastropoda)

Abstract

Bertsch, H. (Galeta MaI'ine Laboraton], Sm.ithsonian Tropical Research. Institute, Canal Zone) 1976. Intraspeciiio and ontogenetic radular variation in opisthobranch system.atics (Mollusca: Gastropoda), Syst. Zool. 25:117-122,-New species of opisthobranch mollusks often have been erected on numerical differences of the radular teeth, Regression analyses of radular counts of Discodoris ecelinae (comprising also the synonymized species, D, hedgpethi) indicate intraspecific variation is ontogenetic. Predictions are given regarding the occurrence of this phenomenon among opisthobranchs, and suggestions are made for the use of radular characteristics in opisthobranch systematics. [Mollusca; Gastropoda; Discodoris; radula.] Many older taxonomic groups have turned out to be little more than size categories (Ghiselin, 1971). In particular, species of opisthobranch mollusks have often been differentiated based on size differences of the radulae or other parts of the anatomy, only later to be synonymized because researchers collect intermediate forms. Certain opisthobranch species have a wide range of radular variation in the number of rows of teeth, and the number of teeth per half-row. However, except for the anaspidean Phyllaplysia taylori Dall, 1900 (see Beeman, 1963) and phanerobranch nudibranchs of the genus Triopha Bergh, 1880 (Ferreira, in press), it has not been statistically proven with regression analyses that these variations do indeed indicate merely differences in size, not in taxa. The knowledge that such intraspecific variations occur should be applied to opisthobranch taxonomy in order to replace typological thinking with modern systematics (Mayr, 1969). This paper examines intraspecific radular variations between specimens of a cryptobranch dorid nudibranch as a growth sequence, and comments on the structural ontogeny of its radular teeth. 1Donner Laboratory and (mailing address:) Department of Zoology, University of California, Berkeley, California 94720. INTRASPECIFIC VABlATION Discodoris evelinae Marcus, 1955, was characterized as having 35 rows of teeth, and more than 77 teeth per half-row. Later, Discodoris hedgpethi Marcus, 1959, was described as a separate species because its radular formula was 26 rows of teeth and 44 teeth per half-row. With additional collections of specimens, animals with an intermediate number of teeth were found, and the radular difference became less. Eventually the two were synonymized (Marcus, 1971). Published records list the radular formula as 22-50 rows of teeth, with 40-80 teeth per half-row (Meyer, in press). Examination of 15 specimens of Discodoris evelinae that I collected from the Caribbean coast of Panama (July 29-September 1, 1974) reveals an even greater range of variation. The combined radular formula of these specimens is 16-35 rows, with maxima of 25-69 teeth per half-row. Some of these were smaller specimens than had previously been examined by other workers. A combined radular formula, although it prevents critical analysis, suggests that the number of tooth rows and individual teeth is dependent on the size of the specimen. This relationship can be demonstrated by using the data from each animal separately. FOl.U· size parameters were measured or counted: total length of living animal,