Sainte Mane and Chabot Natural diet of Homanis amencamis o\\ the Magdalen Islands 



113 



in spite of the fact that one or the other category occurred 

 in S8'/( of stomachs) and even less to stomach volume of 

 lobsters in the 10-mm size class (0.10, 86'^). During our 

 study, lobsters settled in August at sizes of 4.3-5.2 mm CL 

 and grew to 12-14.5 mm CL by October (Sainte-Marie et 

 al., 2001). Thus, we sampled the lobster population during 

 the only period of time when SRJs were present and sea- 

 sonal sampling bias cannot be invoked to explain the lack 

 of plankton in their diet. 



The other food items in the stomachs of SRJs, and es- 

 pecially the predominant bivalves and flesh (Figs. 4 and 

 5 ), probably were derived by predation and scavenging. Bi- 

 valves in the stomachs of SR.J lobsters were represented 

 by recently settled Modiolus and Myfilus. Mussel spat may 

 settle aggregatively and quite synchronously, forming dense 

 patches that can provide a short-term prey pool requiring 

 little or no search time (e.g. Auster, 1988). Furthermore, be- 

 cause mussel spat often settle in crevices or under rocks 

 (e.g. Nair et al., 1975), SRJs could access them with little or 

 no risk of exposure to predators. Lawton ( 1987 ) argued that 

 dominance and territoriality were likely to exist early in the 

 ontogeny of lobsters, as demonstrated subsequently ( James- 

 Pirri and Cobb. 1999; Paille and Sainte-Marie, 2001). and 

 that prolonged occupation and defense of shelters located 

 close to a food patch would be advantageous for juveniles. 

 Exploitation of mussel patches, inferred from the present 

 study, is consistent with that hypothesis. 



Flesh (tissue boluses) that could not be attributed to a 

 particular animal for lack of indicator fragments was a very 



important food item in the diet of SRIs, both in terms of 

 percent occurrence and of volumetric contribution (Figs. 4 

 and 5). Elner and Campbell ( 1987) also found that uniden- 

 tified animal tissue was one of the most frequent and most 

 volumetrically important foods in the stomachs, however, 

 of larger lobsters. Weiss (1970) observed that adolescent 

 and adult lobsters often captured crabs or other shelled 

 prey, cracked them open, and then selectively ingested only 

 soft tissue. Interestingly, the percent occurrence and volu- 

 metric contribution of flesh to diet was greater in lobsters 

 of size classes <30 mm CL (i.e. SRJs and emergent juve- 

 niles) than in larger lobsters (Fig. 2). It is unlikely that the 

 smallest of lobsters could find (within the confines of their 

 shelter) and subdue prey sufficiently large to provide tis- 

 sue boluses devoid of hard parts. Furthermore, claws are 

 not differentiated into cutter and crusher forms in SRJs 

 (Govind and Lang, 1978; Costello and Govind, 1984) and 

 early juveniles may be incapable of breaking open shelled 

 prey (Costello and Lang, 1979; Lawton and Lavalli, 1995). 

 Therefore, flesh ingested by SRJs and emergent juveniles 

 probably was obtained by scavenging animal remains. Con- 

 sidering that larger lobsters may hoard and bury food in 

 or nearby their dens (Herrick, 1895; Smith, 1976; Lawton, 

 1987; Wickins et al., 1996), we propose that early juveniles 

 exploit the meal scraps or food resei-ves of larger lobsters. 

 Indeed, we obsei-ved that small lobsters often occupied gal- 

 eries beneath, or in rock pilings nearby, the dens of larger 

 lobsters. This is consistent with reports that odor from con- 

 specific adults is a proximate cue for lobster settlement 

 (Boudreau et al.. 1993). Cohabitation of small lobsters with 

 large lobsters would offer the former protection from pred- 

 ators and a potentially abundant, high-quality, sheltered 

 food source, and would therefore represent a form of com- 

 mensalism. The risk of cannibalism for small lobsters liv- 

 ing in the vicinity of larger lobsters probably does not off- 

 set the benefits. Few lobster remains were found in lobster 

 stomachs in this (Fig. 4) as in other studies (Weiss, 1970; 

 Carter and Steele, 1982b; Elner and Campbell, 1987), and 

 an unknown proportion of those remains may have been 

 exuviae. 



Some other rarer food items found in the stomachs of 

 SRJ lobsters were probably taken by predation, possibly 

 within, but more likely in the neighborhood of, the lob- 

 sters" shelters. The most important of these prey by volu- 

 metric contribution were polychaetes, comprising juvenile 

 nereids and polynoids that are frequently found in soft 

 sediment or on the underside of rocks, and recently settled 

 rock crab. Similarly, amphipods and gastropods found in 

 the stomachs of SRJs were juveniles or small species that 

 may abound in crevices and in spaces beneath rocks. 



A carnivorous, high-energy diet such as the one demon- 

 strated for SRJs in our study would promote growth from 

 settlement time. By contrast, Lavalli ( 1991 ) demonstrated 

 that a diet of only diatomous algae was insufficient for ex- 

 tended growth and sui-vival of early juvenile lobster A diet 

 of mesozooplankton sustained growth of juvenile lobsters, 

 at least for some time after settlement (e.g. Daniel et al., 

 1985; Barshaw, 1989; Lavalli, 1991). However, Lawton and 

 Lavalli ( 1995) pointed out that intermolt periods tended to 

 be longer and molt increments smaller in laboratory-held. 



