Lavalli: Survival and growth of Homarus amencanus fed plankton diets 



63 



grew more quickly and were more active in a nearly 

 constant dark regime (Bordner and Conklin 1981). The 

 water flow to the trays was turned off for 1 hour after 

 the introduction of food to allow the lobsters to more 

 easily capture the food. Filters were replaced during 

 these feeding periods if they were clogged. 



Lobsters were fed according to group; excess food 

 and other debris were removed daily with a kitchen 

 baster. All trays were thoroughly scrubbed each week 

 to remove algal growth from the sidewalls and bottom. 

 During cleaning the lobsters were held in a moist, 

 small-mesh fish net. Every attempt was made to feed 

 equal wet weights of food, and representative portions 

 of each diet were weighed each week. For the plankton 

 diets, representative portions were also photographed 

 using the technique of silhouette photography (Edger- 

 ton 1977, Ortner et al. 1979) so that identification of 

 the planktonic organisms could be made without the 

 aid of a microscope. 



Plankton was collected three to four times per week 

 by towing with a #10 plankton net (152 /mi) and a phy- 

 toplankton net (25^m) in the Waquoit Bay/Nantucket 

 Sound areas. After collection it was sieved to remove 

 objects > lOOO^m and to divide the plankton into each 

 size group. Half of the plankton was used immediately 

 while the other half was refrigerated overnight and 

 used the following day. 



Carapace lengths of Stage-V lobsters were measured 

 to the nearest 0.1 mm using calipers, and their weights 

 were recorded on a Brainweigh B300D scale to the 

 nearest 0.001 g. The lobsters were blotted with absor- 

 bent paper to remove excess water prior to weigh- 

 ing. The experiment ran until all surviving lobsters at- 

 tained Stage IX. During this time, the dates for all 

 molts (for the determination of intermolt periods) and 

 deaths were recorded. Although no post-mortems were 

 performed, it was noted whether lobsters died in the 

 process of molting or of unknown causes. Coloration 

 of the lobsters was also noted. After achieving Stage 

 IX, the lobsters were once again measured and 

 weighed. 



During the time of this experiment, a fifth group of 

 lobsters (also siblings of the other four groups of 

 lobsters) was raised in seawater tables for another ex- 

 periment. The lobsters in this fifth group were placed 

 individually into separate circular containers (85 mm 

 diameter; 200 mm high) consisting of a black plastic 

 bottom glued to a cylinder made of screening (1-mm 

 mesh). These lobsters were fed the same amount of 

 brine shrimp as the brine shrimp group of lobsters 

 above, but lived in unfiltered, ambient seawater and 

 were subject to ambient daylight plus overhead fluores- 

 cent lighting. Organic debris was cleaned out of the 

 seawater table and containers at least once per month. 

 While data on the initial (Stage V) weights and cara- 



pace lengths are unavailable for this fifth group of 

 lobsters, their final (Stage IX) weight and carapace 

 length were recorded. Intermolt periods were recorded 

 except for the period between Stages V and VI, since 

 this group was held communally until after they had 

 molted into Stage V. 



Data for each of the measurements taken (intermolt 

 period, initial (Stage V) and final (Stage IX) carapace 

 lengths and weights) were analyzed using the Student's 

 /-test when comparisons between two groups or two 

 measurements within a group (i.e., initial and final 

 weights or carapace lengths) were made, and by 1-way 

 ANOVA tests when more than two groups were com- 

 pared. Where ANOVA tests indicated significant dif- 

 ferences were present, the groups were compared to 

 determine which groups were different by using the 

 Tukey test with unequal sample sizes. Differences in 

 survival rates were tested with a 2 x 2 chi-square con- 

 tingency table. This experiment was conducted at the 

 Marine Biological Laboratory in Woods Hole, MA from 

 13 July to 27 October 1987. The ambient seawater 

 temperature ranged from 23 to 14.5°C and averaged 

 19.6°C. 



Results 



Survival was high in the groups fed brine shrimp in 

 unfiltered seawater (95% survival), brine shrimp in 

 filtered seawater (95% survival), and mesoplankton 

 (90% survival) for the molt between Stage V and VI. 

 During the subsequent molts, however, the group fed 

 brine shrimp in filtered seawater had significantly 

 higher mortality Or, P<0.001; Fig. 1), with only 15% 

 survival by the end of the experiment. The survival of 

 the brine shrimp-fed group in unfiltered seawater re- 

 mained unchanged, while that of the mesoplankton-fed 

 group fell to 80% by the end of the experiment. How- 

 ever, there was no significant difference in survival 

 between these two groups. Of the deaths noted for each 

 of the groups, one lobster fed brine shrimp in unfiltered 

 seawater and one fed mesoplankton died during its 

 molt; of the 17 lobsters which died on the brine shrimp 

 diet in filtered seawater, 14 died while in the process 

 of molting. Coloration of the surviving groups differed, 

 with the brine shrimp-fed group in filtered seawater 

 being pale blue, typical of brine shrimp-fed lobsters, 

 and the mesoplankton-fed group and brine shrimp-fed 

 group in unfiltered seawater being the wild-type 

 coloration. 



None of the starved or meso/microplankton com- 

 bination-fed lobsters molted beyond Stage V. All of the 

 lobsters in these two groups died within 36 days of the 

 107-day experiment, and although the lobsters fed the 

 meso/microplankton combination diet took slightly 



