GROWTH AND SURVIVAL OF EARLY 



JUVENILE AMERICAN LOBSTERS, HOMARUS 



AMERICANUS, ON A DIET OF PLANKTON 



Larval American lobsters, Homarus americanus, 

 are planktonic and are known to feed raptorially on 

 zooplankton (Herrick 1895; Williams 1907; Temple- 

 man 1936). However, the benthic, postlarval stages 

 of the American lobster are not routinely found in 

 the field, and their natural habitat and feeding be- 

 havior are not known. Consequently, the natural diet 

 of these stages is unknown. Stomach content anal- 

 yses of larger juveniles and adult lobsters show that 

 they feed on a great variety of benthic animals, in- 

 cluding polychaetes, molluscs, macroalgae, and 

 other crustaceans (Leavitt et al. 1979; Carter and 

 Steele 1982). 



Older juvenile and adult American lobsters, how- 

 ever, seem to have fundamental differences in their 

 behavior compared with the early juvenile stages 

 used in this study. Laboratory studies and field ob- 

 servations indicate that early juveniles are more 

 (perhaps exclusively) shelter bound (as Cooper un- 

 published data in Cooper and LTzmann 1980; Lawton 

 1987; Barshaw and Bryant-Rich 1988). If the early 

 juveniles do not forage for food outside of their bur- 

 rows, they must feed in a different manner and on 

 a different diet than that of older lobsters. During 

 behavioral observations in naturalistic substrates, 

 early juvenile lobsters were seen to generate a cur- 

 rent through their U-shaped burrows by pleopod 

 fanning (Barshaw and Bryant- Rich 1988). They ap- 

 peared to catch and feed on the plankton that was 

 carried in by this current. These observations form 

 the basis for the hypothesis that early juvenile lob- 

 sters can feed upon plankton. 



Materials and Methods 



Stage IV lobster siblings from the Department of 

 Fisheries and Oceans Laboratory, St. Andrews, 

 New Brunswick, Canada were held in plankton 

 "kreisels" (Hughes et al. 1972) for one day after be- 

 ing transported to Woods Hole, MA. These lobsters 

 had all molted into Stage IV approximately two days 

 before they were transported and were all fed on 

 frozen brine shrimp until the experiment started. 

 At the start of the experiment, individual lobsters 

 were placed into 72 trays (22 cm long x 6.4 cm wide 

 X 5 cm deep; water volume = 750 mL). Each tray 

 was provided with filtered, ambient, running sea- 

 water, kept on a natural light/dark regime, and had 

 an artificial lobster shelter made of black tubing 

 glued to the bottom. The lobsters were allowed four 



days to acclimate to the trays before the experiment 

 began. During this time, all of the lobsters were fed 

 once on frozen brine shrimp (Artemia), and any dead 

 lobster was replaced by another sibling. The 72 

 lobsters were then randomly divided into three 

 groups of 24; one group was starved, one group was 

 fed daily on five frozen brine shrimp per lobster, and 

 the last group was fed daily on plankton. Dead, 

 settled plankton was not used; only plankton which 

 appeared living was presented to the lobsters. The 

 trays were cleaned daily and any uneaten shrimp 

 or plankton were removed. 



The plankton was collected every other day by 

 plankton tows in the Woods Hole area. After col- 

 lection, the plankton was sieved through a 1 mm 

 mesh. Half of the plankton was kept alive for 24 

 hours, while the rest was fed to the lobsters imme- 

 diately. Representative subsamples from the daily 

 portions were rinsed with distilled water, filtered, 

 dried, and weighed; the same was done with the 

 daily portion of frozen brine shrimp. 



For one hour at the onset of feeding, the flow of 

 seawater through all the trays was stopped so that 

 the plankton-fed lobsters had a chance to feed before 

 the plankton was flushed out of the trays. During 

 this hour, informal observations were made on the 

 behavior of the feeding lobsters. Movements of the 

 lobsters in the trays, pleopod-fanning and mouth 

 part activity were observed. 



The experiment continued until all surviving 

 lobsters had completed two molts; this took 65 days, 

 from 14 October to 17 December 1984. During that 

 time, all molts and deaths were recorded. The 

 lobsters' weight and carapace length (CL) were 

 taken after 40 days and at the end of the experi- 

 ment. To make these measurements, each lobster 

 was carefully removed from its tray and placed on 

 absorbent paper to remove excess water. The 

 lobster was then weighed to 0.01 mg on a Mettler 

 balance; CL was measured to the nearest 0.1 mm 

 using calipers. This procedure took less than two 

 minutes and did not appear to adversely affect the 

 lobsters. 



Results 



There was no significant difference in American 

 lobster survival between the group fed brine shrimp 

 (75% survival) and the group fed plankton (83% sur- 

 vival). All of the starved lobsters died by day 39 of 

 the experiment (Fig. 1). This group is significantly 

 different from the other two (x", P < 0.001). 



Nine of the starved lobsters molted to Stage V 

 before dying. All of the surviving lobsters in the two 



366 



Fishery Bulletin. U.S. 87:366-370, 1989. 



