Aspects of the Biology of the Giant Isopod Bathynomus Giganteus a. Milne Edwards, 1879 (Flabellifera: Cirolanidae), Off the Yucatan Peninsula

Abstract

The giant isopod Bathynomus giganteus A. Milne Edwards was caught with wire traps in several locations at depths between 349 and 733 m off the Yucatan Peninsula. A total of 1,651 isopods, ranging from 4.4-36.5 cm in body length, were obtained on 5 cruises. Size distribution, sex ratio, and percentage of mature males (with appendices masculinae) and females (with functional oostegites) are given for each cruise. Length-weight and length-width relationships were obtained for 774 and 515 specimens, respectively. Reproductive aspects were studied in isopods captured on cruises 4 (August 1989) and 5 (February 1990). Percentages of mature adult males were 47 and 71%, respectively, and those of mature adult females were 15 and 53%. On cruise 4, many females had ovaries in intermediate stages of maturity, while on cruise 5 most had ovaries either in a resting stage or in a fully mature stage. These results suggest seasonal reproduction. Gut content analyses performed on 158 isopods showed that these animals feed on a wide variety of food items, the most abundant of which were fish, cephalopods, and decapods. The scavenging nature of B. giganteus is supported, although these isopods seem to feed also on sessile and slow-moving animals, such as echinoderms and tunicates. One large isopod was found in the stomach of a shark (Galeocerdo cuvierii). Epizoans found attached to B. giganteus included the cirripede Octolasmis aymonini geryonophila, the gastropod Mitrella amphisella var. rushii, and unidentified tube worms, branchiurans, and a hydrozoan. Mitrella amphisella var. rushii was more abundant inside the empty brood pouches of females, but the nature of this association is unclear. Bathynomus giganteus, a member of the family Cirolanidae, is the largest isopod known to date. The species has been de-scribed and reported widely (Milne Ed-wards, 1879a, 1879b;Agassiz, 1888; Wood-Mason and Alcock, 1891; Bouvier, 1901a, 1901b; Hansen, 1903; Boone, 1927; Schmitt, 1930, 1931; Nierstrasz, 1931; Bul-lis and Thompson, 1965; Lemos de Castro, 1978), and is found throughout the Gulf of Mexico and the Caribbean Sea down to the state of Ceara, Brazil, and in the north-western Indian Ocean (Holthuis and Mikul-ka, 1972; Lemos de Castro, 1978). Its mor-phology and internal anatomy (Milne Edwards and Bouvier, 1902; Lloyd, 1908; Cocke, 1987) and its physiology (Bouvier, 1901c; Steeves, 1969; Van Holde and Bre-nowitz, 1981; Chamberlain et al., 1986) have been investigated. However, few studies are based on large numbers of specimens. Hol-thuis and Mikulka (1972) reviewed the ex-isting literature on B. giganteus, and made some biological observations on 97 speci-mens collected by several vessels in the Gulf of Mexico and Caribbean Sea. Cocke (1987)