Inter-Form Agonistic Contests in Male Crayfishes, Cambarus robustus (Decapoda, Cambaridae)

Abstract

In this study, we evaluate the aggressive interactions between breeding form (Form I) and non-breeding form (Form II) male crayfishes, Cambarus robustus, during inter-form contests, and compare these data to those from previous studies on intra-form contests in this species. The analysis of inter-form agonistic contests between size-matched crayfishes showed that Form I males were clearly dominant, in most cases, over Form II males. Furthermore, Form I males performed significantly more Lunge and Claws Raised initiation behaviors than Form II males during these contests. These two initiation behaviors strongly indicate the dominant status of individuals in both inter-form and intra-form contests. Form I winners performed significantly more aggressive Claws Raised initiation acts in the inter-form contests, against Form II conspecifics, than in intra-form contests against other Form I conspecifics. Male form is an important variable in the agonistic interactions of cambarid crayfishes. In C. robustus, adult males of both forms coexist in the same populations throughout the year. Thus, Form I and Form II males may compete with one another for access to potentially limiting resources. Additional key words: dominant, subordinate, initiation behaviors Cambarus robustus GIRARD 1852 (Decapoda, Cambaridae) is a large crayfish species whose distribution within Canada, at the northern edge of its range, is restricted to fast-flowing streams and some Precambrian Shield lakes located in southern and south-central Ontario (Guiasu et al. 1996a). In the United States, this species is found primarily in the Ohio River and Great Lakes (except for Lake Superior) drainage basins (Crocker & Barr 1968). Adult male crayfishes of the family Cambaridae exhibit sexual dimorphism (Fig. 1) characterized by cyclical alternation between a breeding form (Form I) and a non-breeding form (Form II) (Hobbs 1991). This sexual dimorphism and the corresponding cyclical alternation between male forms are unique among decapod crustaceans (Hobbs 1974, 1988). In at least one cambarid crayfish species, Procambarus spiculifer, the alternation between Form I and Form II appears to be absent, and Form II males seem to represent a developmentally intermediate stage between the juvenile and Form I stages (Taylor 1985). The complete cyclical alternation between the two male forms is, however, present in other cambarid crayfish species (Hobbs a Author for correspondence. E-mail: radu@zoo.toronto.edu 1974), including C. robustus (Hamr & Berrill 1985; Guiasu, unpubl. data). In Form I males, the distal elements of the copulatory stylets (highly specialized first male pleopods) are more heavily sclerotized and better differentiated (Fig. 2), the ischial hooks on one or more of the second to fourth pereiopods are more prominent, and the major chelae (claws) are larger, relative to the cephalothorax length, than in Form II males (Crocker & Barr 1968, Stein 1976, Hobbs 1991). Word & Hobbs (1958) showed that the sperm ducts of Form I males contained many recently formed spermatids. Such dimorphism in male morphology is often strongly influenced by sexual selection (Andersson 1994). Stein (1976) showed that, in the crayfish species Orconectes propinquus, males with relatively larger chelae were more successful in copulating with females and in the aggressive interactions with other males for access to females, compared to males with smaller chelae. Males with relatively larger chelae should also have an advantage over crayfishes with smaller chelae in the competition for potentially limiting resources such as