Effects of Acanthocephalus dirus (Acanthocephala) on Morphometrics and Reproduction of Caecidotea intermedius (Crustacea: Isopoda)

Abstract

Fifty-five percent of 201 Caecidotea intermedius (Crustacea: Isopoda) collected in McLean County, Illinois were infected with larvae of Acanthocephalus dirus. Through measurements of isopod total length and first pereiopod propodus and dactylus lengths, I found that mean total length of infected female isopods was significantly greater (Student's t-test, a = 0.05) than that of uninfected females. Mean total length of infected male isopods was significantly less (Student's t-test, a = 0.05) than that of uninfected isopods. Corresponding differences were noted for first pereiopod propodus and dactylus lengths. Regression analyses demonstrated significant (F-test, a = 0.001) linear relationships between first pereiopod propodus or dactylus lengths and isopod total lengths for uninfected, male worm-infected, and female worm-infected isopods of both sexes. The maintenance of such relationships during infection was indicated by the lack of statistically significant (a = 0.05) differences between regression or correlation coefficients. Acanthocephalan volume (mm3) did not differ significantly (Student's t-test, a = 0.05) for either sex of worm from male or female isopods. A statistically significant linear relationship was demonstrated (regression analysis) between female acanthocephalan volume and male isopod total length. Infection affected reproductive behavior of C. intermedius; infected isopods were never observed in amplexus and ovaries of infected females did not develop. Kuris (1974) and Baudoin (1975) reviewed the topic of parasitic castration for various invertebrates. There also are reports of parasite-induced host gigantism (see Cheng, 1971 for a review; Pearre, 1976). That the topics of parasitic castration and gigantism are complementary aspects of some host-parasite relationships has been suggested by Baudoin (1975) and supported by Pearre (1976, for a consideration of chaetognaths infected with larval trematodes). In an investigation (Oetinger & Nickol, 1981, 1982a,b) of altered pigmentation of Caecidotea intermedius Forbes, 1876 (Isopoda: Asellidae) infected with Acanthocephalus dirus (Van Cleave, 1931) (Acanthocephala: Echinorhynchidae), various additional features of the relationship were noted and are presented in this report in an attempt to evaluate some of the assumptions of parasitic castration/gigantism described by Baudoin (1975). MATERIALS AND METHODS Caecidotea intermedius were collected by dip net from overhanging vegetation along the banks of Sugar Creek, McLean County, Illinois. In the labo1 Publication costs, in part, are being met by a grant from the Spencer-Tolles Fund of the American Microscopical Society. 2 Present address: Department of Biology, Kentucky Wesleyan College, Owensboro, Kentucky 42302-1039, U.S.A. TRANS. AM. MICROSC. SOC., 106(3): 240-248. 1987. ? Copyright, 1987, by the American Microscopical Society, Inc. This content downloaded from 157.55.39.45 on Thu, 01 Sep 2016 04:47:07 UTC All use subject to http://about.jstor.org/terms VOL. 106, NO. 3, JULY 1987 ratory, isopods were placed in plastic boxes with aged tapwater and a source of aeration. Elodea, various filamentous algae, and decaying leaves were added for food. Water was changed daily and cultures were maintained at room temperature (20-25?C). All 201 isopods collected were examined within two weeks. Examination involved: (1) determination of isopod sex; (2) measurement of isopod total length (to the nearest 0.5 mm from the anterior margin of the cephalothorax to the posterior margin of the abdomen); and (3) dissection by separating body segments-first the cephalothorax and free thoracic segment 1 (T1), T2 and 3, T4 and 5, T6 and 7, and finally, the abdomen. Isopod body parts and acanthocephalans were processed as described previously (see Oetinger & Nickol, 1981). Outline drawings of propodus with dactylus, and acanthocephalans were made with a Rayoscope? microprojector to an enlargement of x 40. Propodus and dactylus lengths of left and right pereiopods were determined as indicated in Fig. 1. Acanthocephalan length was determined with a piece of waxed string placed along the mid-longitudinal axis of the drawing and subsequently measured with a metric rule. Width was measured at the point of greatest width directly on the drawing. Volumes of acanthocephalans were approximated with the formula of a prolate spheroid (V = 4/37rab2, where 2a = worm length and 2b = worm width-after Chen, 1934). Data for uninfected male isopods were reduced from the actual 70 such specimens to 25 using a random number table. Data for the female isopods and infected male isopods represent most of the isopods for each category occurring in the collection. Isopods lacking both first pereiopods were omitted from the data analyses (one male isopod with a male parasite and three male isopods with female worm infections). Analyses of data were carried out utilizing the conversational monitoring system statistical package of an IBM 360 computer. Weighted Student's t-tests were used when there were significant differences in variance.