Geographic Morphometric Variation in the American Lobster

Saila, S. B. , and Flowers, J. M. (Marine Expt. Sta., U. Rhode Island, Kingston, R. I.) 1969. Geographic morphometric variation in the American lobster. Syst. Zool. , 18:330-338. [Morphology; geographic variation; multivariate; lobster; Homarus americanws]. -An analysis of morphologic measurements from geographically separated samples of the American lobster, Homarus americanus, was made. The centour concept, which involved inferring an individual's group membership from multivariate data, was used. The study involved identification of inshore and offshore groups of lobsters by means of small shape differences based on simultaneous consideration of linear measurements from 16 body parts. The results of the study showed that definite profile differences existed between inshore and offshore groups when matched by size and sex. The observed differences were greater between inshore and offshore groups than those occurring within geographically separated samples of both inshore and offshore lobsters. The results of the study suggested that relatively little overlap was discernible between the inshore and offshore areas in the characters measured. These studies support inferences made from tagging studies to date. Sokal and Sneath (1963) have clearly pointed out the importance of numerical methods for evaluating the extent of affinity or similarity between taxonomic units. The scope of their work has been restricted primarily to methods intended to demonstrate taxonomic relationships and to create taxonomic groupings. The present study is related in approach to that of Thomas (1963), who analyzed morphological characters of the rabbit tick Haemaphysalis leporispalustris, with a view towards obtaining clues, to the operation of evolutionary mechanisms. In attempting to understand and measure causes, of fluctuations in the abundance of a species, it is imperative that the number and identity of any subsets of the species be established. The reason for this is obvious to, any rational management plan inasmuch as each subset of the population (species) may have a characteristic distribution and characteristic vital statistics, such as mortality and growth rates. These considerations are especially relevant to the lobster, Homarus americanus Milne-Edwards, because the fishery for this species in the offshore waters of the western 1 This research study was financed by the Bureau of Commercial Fisheries under Contract No. 14-17-0003-152. North Atlantic has rapidly expanded in recent years, and some segments of the lobster industry have contended that the offshore lobsters contribute significantly to the inshore pot fishery. Reports of the results of exploratory fishing in offshore areas have been made by Schroeder (1955; 1959) who also described depth and latitudinal distribution of offshore lobsters. In general, the inshore pot fishery is restricted to shoal waters not in excess of 20 fathoms, whereas the offshore lobsters are taken in depths from 60-250 fathoms, along the edge of the continental slope. No assessment of the relationship between inshore and offshore lobsters has been made to date. However, several approaches to this problem have been undertaken, and the work described herein is one segment of a comprehensive study conducted by the U. S. Fish and Wildlife Service, Bureau of Commercial Fisheries, Biological Laboratory, West Boothbay, Maine. The objectives of this study were: (a) to establish whether identifiable subsets (groups) of the lobster could be established on the basis of distinctive morphoimetric characters, and (b) to assign individuals on a probabilistic basis to these groups if they were found. The use of the term group