Competitive Exclusion between Two Intestinal Parasites of the Three-Spined Stickleback, Gasterosteus aculeatus L.

Abstract

When the cestode Proteocephalus filicollis occurred concurrently with the acanthocephalan Neoechinorhynchus rutili, in natural infections of the three-spined stickleback, the distribution of each species in the gut was significantly different from when these species occurred alone. In concurrent infections partial spatial separation of the two species was observed in which both adults and plerocercoids of P. filicollis attached more frequently in the anterior intestine while individuals of N. rutili attached more frequently in the rectum. In single species infections individuals of both species were distributed more widely throughout the gut. The data are thought to exemplify competitive exclusion and are possibly the result of the adverse effect of each species on the environment of the other. In a survey of the parasite fauna of the three-spined stickleback it was noted that, of the eight species that occurred, two species co-habited the alimentary tract of the fish. These two species were the cestode Proteocephalus filicollis (Rudolphi, 1819), of which both larval and adult worms were recovered, and an acanthocephalan, thought to be Neoechinorhynchus rutili (Miiller, 1780), of which no mature individual was found. It was decided to study the intestinal distribution of these two species when they occurred in the same fish for evidence of interspecific interaction, and to determine the role of any such interaction in the ecology of the two parasite species. MATERIALS AND METHODS The methods for the collection and examination of sticklebacks for parasites have been described previously (Chappell, 19'69). A brief synopsis of the methods employed is as follows: 6 bimonthly collections of sticklebacks were made from a small pond in the north of England during the period from September 1966 to August 1967. Fish were transported alive to the laboratory where they were maintained in aquaria at 11 C until required. All fish were killed and examined within 5 days of capture. Sticklebacks were killed by decapitation and opened by a ventral, longitudinal incision reaching posteriorly to the vent. The gut, from stomach to anus, was removed and placed in a dish of 0.9% saline. The gut was opened by a longitudinal incision and the location of gut parasites recorded. The gut was arbitrarily divided into 3 regions: anterior intestine, posterior intestine, and rectum. While the first 2 regions were approximately equal Received for publication 11 February 1969. * Present address: Biology Department, Rice University, Houston, Texas 77001, USA. in length (up to a maximum of 3 cm) the rectum was considerably shorter. In this study the designated position of a parasite in the gut refers explicitly to that region where the worm was attached.