Meristogram Analysis of the Acanthocephalan Genus Pomphorhynchus in North America

Abstract

Meristogram analysis revealed that specimens of Pomphorhynchus bulbocolli and P. rocci can be easily distinguished by measuring the length of one hook in the 60-80 region of the proboscis. Percent-position of a hook is calculated by multiplying the counted posteriad position number of the hook by 100 and then dividing by 1 + the number of hooks in that longitudinal row. Hooks in the 60-80 percent-position region are longer than 37 aum in P. rocci and shorter than 37 ,um in P. bulbocolli. P. rocci was found not only in the coastal marine fishes to which it was formerly thought to be restricted, but also in freshwater fishes of coastal brackish and freshwaters. The Pomphorhynchus fauna of North American fishes is generally thought to consist of two species, P. bulbocolli Linkins in Van Cleave 1919, and P. rocci Cordonnier and Ward 1967. Pomphorhynchus bulbocolli was first described from fishes of the Illinois River by Linkins in an unpublished thesis. Van Cleave (1919) later published the description but did not provide diagnostic hook drawings and apparently did not designate type and paratype specimens. Cordonnier and Ward (1967) described Pomphorhynchus rocci from Morone (= Roccus) saxatilis caught commercially near Elizabeth City, North Carolina. Informative hook drawings accompanied the description and a type collection was deposited in the USNM Helm. Coll. P. rocci was distinguished as a new species primarily on the basis of possessing longer necks and bodies than P. bulbocolli and in having proboscides armed with 15-18 hooks in each longitudinal row rather than the 12-14 hooks/row reported in Van Cleave's description of P. bulbocolli. When subsequent collections produced Pomphorhynchus specimens from freshwater hosts with more than 14 hooks/row and specimens from marine hosts with fewer than 15 hooks/row, and the purported differences in neck and trunk lengths were shown not to exist (Samuel, Nickol, and Mayes, 1976), the validity of P. rocci was questioned. Samuel et al. (1976) studied Pomphorhynchus specimens from several hosts and localities in the hope that the additional morphological Received for publication 7 February 1978. data would provide characters that would distinguish P. bulbocolli from P. rocci. However, they were unable to sort the specimens into two groups using conventional characters such as number of longitudinal rows of proboscis hooks, number of hooks in each row, proboscis length, and trunk length, and were forced to assume that all specimens from freshwater hosts were P. bulbocolli. As will be shown later, this led to inclusion of some data from P. rocci specimens in their redescription of P. bulbocolli. Jensen (1952) found Pomphorhynchus larvae infecting Hyalella azteca. Johnson and Harkema (1971) found larvae in Gammarus tigrinus. Attempts by Johnson (1973, pers. comm.) to cross-infect were futile, suggesting that two distinct Pomphorhynchus populations were involved in spite of morphological similarity. A similar problem involving morphologically indistinguishable species of Echinorhynchus from the fishes of New England was resolved by meristogram analysis (Huffman and Bullock, 1975). Consequently, this technique was applied to several Pomphorhynchus collections with the hope that reliable morphological distinctions between the two Pomphorhynchus species could be discovered. MATERIALS AND METHODS Thirteen collections of North American Pomphorhynchus were exposed to meristogram analysis. Table I lists the code name, locality, and number of specimens analyzed for each collection. In order for a specimen to be suitable for meristo-