Food consumption and growth of juvenile lobsters

Aquaculture, 24 (1981) 285-300 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

285

FOOD CONSUMPTION AND GROWTH OF JUVENILE LOBSTERS

CLARK E. BORDNER and DOUGLAS E. CONKLIN dquaculture Program, University of California, Bodega Marine Laboratory, 247, Bodega Bay, Cd 94923 (U.S.A.) (Accepted

P. 0. Box

2 December 1980)

ABSTRACT Bordner, C.E. and Conklin, D.E., 1981. Food consumption lobsters. Aquaculture, 24: 285-300.

and growth of juvenile

Experiments conducted on juvenile lobsters (Homarus americanus) measured the effects of several environmental variables on growth and ingestion of food. These variables consisted of feeding frequency, photoperiod, and temperature, and were selected on the basis of their potential importance to intensive culture systems. Specific consumption rates, defined as percent of body wet weight ingested daily, and determined by measurements of the ingestion of natural foods, were found to follow an inverse log-log relation to live weight. Significantly faster growth was obtained when juvenile lobsters were kept in near-constant darkness, and when they were fed 7 days per week instead of 5 days per week. Increased food consumption was observed when juvenile lobsters were maintained in near-constant darkness, as well as when temperature was increased. Feeding twice daily did not increase the amount of food consumed daily. The proportion of food consumed during the first 4h indicated that the lobsters ate periodically throughout the day. Although starvation for a 48-h period caused a significant increase in the proportion of food consumed during the next feeding period, this increase was not sustained more than 2 days. The evidence suggests that lobsters are slow, periodic feeders, and that food consumption and growth can be readily increased by manipulation of particular environmental factors.

INTRODUCTION

The possibility of intensive lobster rearing (Hand et al., 1977) has stimulated investigations into lobster feeding habits and nutritional needs. Along with behavioral and growth responses, measurements of food intake are required to evaluate performance on various diets. Although standard techniques have been developed to measure food intake of many different animals, such research on aquatic species has been limited, with fish (Halver, 1972) and shrimp (Shewbart et al., 1973; New, 1976) receiving the most attention. Some techniques have also been developed for insect larvae, filter-feeding crustaceans, and bivalves, but these procedures do not lend them-

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selves well to the study of lobsters. Some observations have been made on the feeding habits of lobsters (Shleser, 1974; Shleser and Gallagher, 1974; Conklin et al., 1975; Carlberg and Van Olst, 1976; Cobb, 1976; Logan and Epifanio, 1978), but data have been lacking on the responses to be expected from large numbers of lobsters reared in intensive aquaculture. Lobsters are known to be influenced by environmental factors, but neither the magnitude of this influence nor its relation to nutrition is clear at present. Such data are essential in determining the amount and cost of food required, in designing culture facilities, and in conducting detailed studies of lobster nutrition. Some of the important factors associated with measuring aquatic arthropod food intake and growth response to diets have been reviewed by Dadd (1972), Gordon (1972), Zein-Eldin and Meyers (1973), and Conklin et al. (1978). Briefly these include: (1) natural variability among progeny and individuals, (2) homogeneity and palatability of the diets, (3) quantitative recovery of uneaten food, (4) environmental effects, and (5) dissolution of diet in water and leaching of nutrients, particularly water-soluble vitamins. It was felt that the first four factors could be controlled by careful experimental design, but the effects of the last one have been little studied in relation to animals such as lobsters. Preliminary experiments showed that by using pieces of shrimp or live Artemiu which remained whole in water, the effects of diet leaching could be minimized to the extent that reliable measurements of average food consumption could be obtained under a variety of environmental conditions. Thus, the responses of lobsters subjected to different temperatures, photoperiods, and feeding schedules were evaluated by means of two types of experiments: growth studies and food consumption experiments. METHODS

AND MATERIALS

The culture water used in all experiments consisted of natural seawater which was filtered through 5 pm pore filters and sterilized by ultraviolet light. In addition, the water was regularly monitored for ammonia, DO, salinity, nitrate, nitrite, and phosphate, and the amounts recorded did not vary widely between experiments, except as noted. In all experiments, light-held animals were exposed to “cool-white” fluorescent. lights of average room intensity (150-200 lux) maintained on a diurnal cycle of 15 h on and 9 h off (LD 15 : 9). Dark-held lobsters were exposed to less than 0.1 lux throughout the experimental period except when being cleaned or fed - a period of 15-30 min daily. For comparison, this lighting regime is referred to as LD 1 : 23. Except in the third consumption experiment, the food used in all experiments consisted of live, adult brine shrimp (Artemia .saZina) obtained twice weekly from Metaliame Corporation, San Francisco, California. In the third consumption experiment, pieces of frozen cocktail shrimp tail muscle were used as a diet, to reduce the variability encountered in the quality and aver-

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age size of brine shrimp of different batches. F’reliminary testing indicated that this food performed well in water and was freely consumed by lobsters. While the two foods are not identical in content, their nutritional value is similar. All experiments were conducted with postlarval lobsters (Homarus americanus) that had been hatched and reared through the larval stages at this laboratory by standard methods (Schuur et al., 1976). Growth study

The growth study was performed in a standard semi-recirculating system used in our laboratory for nutrition experiments (see Conklin et al., 1977, for complete description). The temperature was held at 20” f 1°C. Fourthstage lobsters were selected from rearing containers and randomly assigned to four groups of 24 animals each, and were fed brine shrimp daily until all had molted to the fifth stage, about 14 days. At that point they were weighed and measured (Shleser, 1974), and the experimental conditions were started. Two groups of lobsters were held in the light (LD 15 : 9); the other two groups of lobsters were kept covered with opaque plastic sheeting (LD 1 : 23). Additionally, half of the animals under each lighting condition were fed 5 days per week and the rest were fed 7 days per week. On each feeding day, the lobsters were fed once with an excess of live, adult brine shrimp. Uneaten food was removed by siphoning after each feeding period. Records were kept of all molts that occurred, and the lobsters were again weighed and measured at the end of the go-day experimental period. Analysis of variance on final wet weights was used to compare the effects. Food

consumption

studies

Lobsters used in consumption experiments ranged in weight from 0.8 to 6.4 g, and were chosen from large groups of siblings that had been fed Artemia since hatching. Within each experiment, groups were arranged to consist of lobsters of nearly equal size and molt stage, and growth was not measured in these consumption experiments. Efforts were made to select for measurement only lobsters in an intermolt stage (stages Cl to C4). This was accomplished by starting each experiment with lobsters that had molted more than 2 but less than 6 days before the experiment began. Occasionally an animal molted during an experiment; in such cases, data which were collected on the day of molt, or the days immediately before and after, were eliminated for calculation purposes. Two days after molting, a new wet weight was determined and used for subsequent calculations. Measurements of food consumption were obtained by collecting and determining the dry weight of uneaten food residues. In these experiments, lobsters were individually housed in 2-l glass battery jars held at a constant temperature in refrigerated water baths. Each jar was fitted with a plastic cover and supplied with air via an airstone. For animals tested in the dark,

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the jars and covers were painted with opaque black fiberglass resin. Lighting was provided by a bank of three standard fluorescent tubes; the intensity was adjusted by raising or lowering the lights, and an effort was made to approximate normal room lighting. Measurements were made inside each jar at the water’s surface with an ISCO Model SR Spectroradiometer and averaged 150 lux for light jars and less than 0.03 lux for dark jars. An isolated location was used to minimize the effects of stray light and changing shadows. Each battery jar was filled with 1 or 2 1 of seawater, depending on animal size, and placed in the water bath for temperature equilibration. Weighed and measured lobsters were put into the jars and immediately fed excess amounts (approximately 20% dry weight diet/wet weight animal) of weighed portions of diet. Diets were weighed by rinsing with distilled water, draining briefly through a fine mesh net, then removing aliquots and weighing in aluminum pans on an analytical balance. The aliquots were quantitatively fed to the lobsters by rinsing the pans into the water. The lobsters were then left undisturbed for the experimental period of 4, 20, or 24 h, after which they were carefully removed and placed in clean jars containing fresh portions of seawater and diet. This procedure was followed for 3 or 5 days per week. After each experimental period, the uneaten food residues were washed onto Whatman paper filters and rinsed with 500 ml distilled water in two portions. Dry weights of diets, diet residues, and animals were determined by oven-drying at 80” C for 24 h. In these experiments the fecal matter present in the jars was removed by pipetting prior to filtration. Duplicate controls were run each day on the dry weight of the diet as fed, and experimental blanks were run to monitor the filters and the washing technique. The first three food consumption experiments were conducted at 20” C. Consumption experiment 1 tested the effect of light and dark on lobster feeding rates. Two groups of five individually housed juvenile lobsters each (average wet weight 1.6 g) were fed Artemiu, and their consumption measured 5 days/week for 4 weeks. The other 2 days/week the animals were starved. One group was kept under the diurnal photoperiod (LD 15 : 9) and the other was kept in darkness (LD 1 : 23). The second consumption experiment again tested the effect of light and dark, and also examined the effect of feeding schedule. Four groups of four individually housed juvenile lobsters each (average wet weight4.25 g) were fed and measured 5 days/week as in experiment 1, except that new groups were selected each week for 4 weeks. Thus a total of 16 animals was measured under each treatment. Two groups were maintained under each photoperiod, with one group of each photoperiod subjected to one daily feeding period of 24 h, while the other group of each photoperiod was fed twice each day, once for 4 h, and again for 20 h. For consumption experiment 3, the diet was changed to frozen shrimp tail muscle. Only the effect of light and dark was tested, with 33 animals measured under each photoperiod. Larger juveniles were again used (average wet weight 4.7 g). These animals were housed and fed in their customary habitats,

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7 days per week, and were placed in the battery jars only 3 days per week for food consumption measurements. Each lobster was measured for only 1 week. This procedure seemed to result in less stress to the lobsters, and 3 days was sufficient to measure average daily consumption. A fourth series of experiments examined food consumption at 15”) 18”, and 21” C. Five juvenile lobsters ranging from 1.1 to 4.1 g wet weight were tested at each temperature for 1 week with four replications. These animals were fed live Artemia 5 days per week and were starved the other 2 days, and they were maintained under the LD 15 : 9 lighting regime. This experiment was thus similar to experiment 1. In all experiments, food consumption was measured gravimetrically as the dry weight of food consumed during each experimental period. Consumption per body weight (wet weight) per unit time, termed “specific food consumption”, was calculated for each lobster using the following formula: Specific food consumption (% C) = (C(mg)/animal wet wt. (mg)) X 100 where C(mg) = I-F/T = dry wt. of food consumed/day; and I = initial dry weight of food fed; F = final dry weight of uneaten food; T = time in days of experimental period. Since specific food consumption is inversely proportional to body weight (Sick et al., 1973) it was expected (L. Botsford, personal communication, 1978) that this parameter would be described by a regression equation of the type % C = a/W where % C = specific food consumption, W = live (wet) weight (mg), and a and b are constants, Lobsters of similar average size were compared directly by their average specific consumption, but lobsters of different sizes were compared by first determining the regression equation of consumption versus weight for each group and then comparing the constants. Statistical procedures including Student-t, analysis of variance, and regression analysis were obtained from Snedecor and Cochran (1973). RESULTS

Growth The growth studies revealed a significant (P < 0.05) difference in go-day weights between light- and dark-held lobsters, and between 7 and 5 days per week feeding schedules. Lobsters reared under LD 1: 23 were 14.5% heavier on average and were observed to be more active, more uniform in size, and of lighter color than were their siblings reared under LD 15 : 9. Experiments under LD 1 : 23 discouraged the growth of algae, and culture trays were easier to keep clean. Lobsters exposed to LD 15 : 9 were inative, tended to huddle in the darkest corner of their cubicle, and often did not commence feeding until some time after the introduction of food. Animals kept in the dark, however, usually began eating immediately after the introduction of food. In this experiment, lobsters fed 7 days per week were on average 31% heavier after 90 days than those fed 5 days per week. Table I shows the re-

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TABLE

I

Results of go-day growth study Treatment

n’

Final weight2

LD 15 : 9/5 day feed LD 15 : 9/7 day feed LD 1 : 2315 day feed LD 1 : 23/7 day feed

24 24 24 24

945.7 1245.0 1086.9 1413.3

f 115.4* * 133.6” ?: 132.0* f 136.1*

a-factor ANOVA Source of variation Photoperiod Feed schedule Interactive

d.f. 1 1 1

Residual

92

Total

95

M.S.

Variance (F) ratio

473274.4 2578442.0 19343.2

4.48” 24.4’ 0.18

105687.0 -

-

’ Survival was 100% on all treatments. 2Mean wet weight f S.E. (mg). *P < 0.05.

sults of this experiment. An analysis of variance indicated no significant interactive effects; i.e., the growth response to darkness was not influenced by feeding schedule. Food consumption The results of three consumption experiments are summarized in Table II. The first experiment showed that the presence of light significantly inhibited consumption of Artemia. Lobsters held in LD 15 : 9 consumed food at an average rate 30% less than that of lobsters in LD 1 : 23. Fig.1 shows that when data for the same days of each week were averaged, differences in consumption were significant for 4 out of 5 days, with the greatest differences on days of highest consumption. Consumption was high after 2 days of starvation but leveled off after 2 days of feeding. In all cases, food consumption was reduced with exposure to light. The second consumption experiment examined lobsters that were larger (approximately 14th stage, compared to 12th stage in experiment 1 - see Table II), and which were held in 2 1 of seawater, instead of 1 1. A new group of lobsters was selected each week for 4 weeks, and a daily pattern similar to that in experiment 1 was apparent (Fig-B), with higher consumption after 48 h of starvation. Two feedings of live food per day did not significantly increase food consumption over one feeding per day. After 48 h of starva-

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DAY

LD I

LD

LD

LD

LD

2

Fig.1. Specific daily food consumption of juvenile lobsters fed 5 days/week. Values indicated are means f 95% confidence intervals. L = LD 15 :9,D = LD 1 : 23. TABLE

II

Results of three consumption

Experiment 1: (see Fig.1) Experiment 2: (see Fig.2)

Experiment 3 :

experiments Observations per animal

Specific daily consumption mean i S.E.

5

20

2.25 f 1.14***

1650

5

20

3.44 f 1.27***

4330 4200

16 16

5 5

4390

16 16 33 33

Treatment

Mean wet weight (mg)

LD15:9

1650

LD 1 LD15: fed 1 fed 2 LD 1 fed 1 fed 2 LD 15 LD 1

:23 9 x day x day :23 x day X day :9 : 23

4190 4750 4630

Number of animals

2.06 + 0.43 1.90 + 0.47 1.68 1.91 1.46 1.74

f f f f

0.74 0.56 0.49* 0.57*

“P < 0.05. ***p4 0.001.

tion, lobsters in either dark or light consumed about 34% of their daily intake in the first 4 h of feeding, whereas after daily feeding, consumption during the first 4.h was reduced to 17% of total. Lobsters in the dark tended to consume more food in the first 4 h of feeding, but this difference was not significant. In fact, no significant difference in food consumption between light- and dark-held animals was observed in this experiment. A factorial analysis of variance of the second consumption experiment showed that most of the total variation came from the wide range of response of each

DAY

I

2

4

3

5

Fig.2. Specific daily food consumption of juvenile lobsters. Values indicated are means + 95% confidence intervals. Ll = LD 15 : 9, fed once daily; Dl = LD 1 : 23, fed once daily; L2 = LD 15 : 9, fed twice daily; D2 = LD 1 : 23, fed twice daily.

animal. The food consumption of each lobster ranged from nearly zero to two or three times the average value, and no pattern was obvious except the weekly pattern observed in Fig.2. The lobsters used in this experiment weighed 4-5 g, and may have overloaded the battery jar system, as suggested by ammonia analysis. After a 4-g lobster had spent 24 h in 2 1 of water with live Artemia, the concentration of ammonia had increased over 50 times. Thus, it is nearly certain that these animals were stressed. Individual variations in consumption also may have resulted from variable quality of the brine shrimp and from additional stress induced by the extra handling necessary to feed and measure these lobsters. The data, however, yielded average consumption figures considered typical for lobsters of this size, and the data for this experiment are reported in Table II for comparative purposes. TABLE

III

Food consumption

at three temperatures

Trial

n

1

2 3 4

Weight’

4110 1790 2390 1140

5 5 5 5

Specific daily consumption 15” c

18°C

21°C

S.D.*

1.43 2.52 2.65 4.35

1.42 2.90 3.05 5.04

1.77 3.04 2.93 5.17

0.61 1.25 1.14 1.91

Regression equations for pooled data: 21” C % C.= 2S0.3/W”‘9 n = 20 15°C % c = 230.9/W”O n = 20

’ mean wet weight (mg). ‘SD. obtained from ANOVA.

r = -0.77 r = -0.79

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Consumption experiment 3, which incorporated several changes in technique to reduce variability, is also summarized in Table II. Once again, food consumption was significantly decreased under the long photoperiod, and lobsters held under LD 15 : 9 consumed at a rate 16% lower than those under LD 1 : 23. The fourth series of consumption experiments, which measured consumption of Artemiu at three temperatures, is summarized in Table III. With the small groups used (a total of 20 animals at each temperature), the difference in specific food consumption between 15” and 18”C, or between 18” and 21”C, was not significant at the 95% confidence level; however, the difference between 15” and 21°C was significant at this level. Nested factorial analysis of variance indicated wide individual variation in response to the same conditions, as reflected by non-homogeneity of variances in several trialsand variation in the same animal from day to day. Other trials were more uniform, and the overall regressions were significant (r = -0.78, P < 0.05). The slopes (b) of these regression equations were nearly identical, while the intercepts (a) varied according to temperature, and in general, lobsters maintained at 21°C consumed food at a rate 15-25s higher than did lobsters at 15” C. 0 lO.O-

I 5x102

1x103 WET

5x103 WEIGHT - mg

I 1x104

I 2x104

Fig.3. Specific daily consumption of juvenile lobsters. Least squares regression equation is % C = 403/w’65 (see text,). Dotted lines indicate 95% confidence limits of regression. Data points were obtained under various environmental conditions.

A log-log plot of specific food consumption versus body weight for 162 juvenile lobsters, measured under the various experimental conditions outlined above, yielded a significant regression (r = -0.83, P d 0.05). Although significant consumption-wet weight regressions were obtained for separate temperatures (see Table III), the other consumption experiments consisted of similar-sized groups, which made individual weight-specific regressions impossible. The most valid and useful overall estimate of specific food consumption in juvenile lobsters is given by the sum of these specific consumption data, since the various environmental conditions under which the data

294

were derived represent a range likely to be encountered by others in the field. More than 80% of these data were obtained at 20°C f 1°C; the rest were at 15” or 18°C. The regression coefficients yielded by the above data are a = 403 + 14 SE. and b = 0.65 + 0.03 S.E. The graph of this equation is shown in Fig. 3. DISCUSSION

An important aspect of food consumption is its observed variability, which can stem from differences between progeny (Hedgecock et al., 1976), among siblings (Hedgecock and Nelson, 1978), and from “irreducible biological variation” (Gordon, 1972). Food consumption of juvenile lobsters is particularly affected by ecdysis, which can occur three or more times within 60 days after the fourth stage at 20” C, and continues at lengthier intervals thereafter. Ecdysis is characterized by loss of appetite prior to and increased appetite after shedding the exoskeleton, and food consumption measurements made during this time would not reflect the rate of intermolt food consumption. The extent of appetite changes and the lengths of time involved at each molt have not been formally studied in Homarus, but work with other decapods (Travis, 1955) showed that the non-feeding period may represent 25% or more of the total molt cycle of juveniles. During this time the animal lives on stored reserves accumulated during the period of active feeding. Therefore, the same cyclical condition exists with respect to food consumption as with growth, and the onset of the non-feeding period can be a good predictor of impending molt, and vice versa. Although our data may include a few instances of zero or reduced consumption due to impending molt, the large numbers of measurements and associated behavoral observations allowed accurate estimates of intermolt food consumption. The food used for testing can also be a cause of wide variation. Natural foods in particular are subject to changes in nutrient composition from batch to batch, and thus the feeding response may also change. With commercial live brine shrimp, one daily feeding ensured a constant supply of live food for a 24-h period, even though the average size and quality of the brine shrimp varied from day to day. Lobsters usually killed most of the brine shrimp and consumed parts of them, but for calculation, it was assumed that the food remained complete and palatable for the 24-h period, since some brine shrimp were usually still alive after the feeding period. The shrimp tail muscle pieces used in experiment 3 were quite stable in water, the only disintegration being small pieces of tissue removed by the lobster during feeding, and which were easily recovered for measurement. It should be noted that nutritional needs and preferences may be different at different stages in the molt cycle; thus it is possible that neither Artemiu nor shrimp tails stimulated optimum ingestion rates.

295

Food consumption The equation described in Fig.3, relating food consumption to weight, is similar to previous estimates of consumption by lobsters (Botsford, 1977). This equation, with its associated error, adequately describes natural food consumption by lobsters of the size range studied and has yielded information useful in the formulation of purified diets and the establishment of feeding levels. However, inspection of Fig.3 shows that some presumably faster-growing animals may be underfed at this level. Extrapolation to larger lobsters indicates reasonable agreement with other estimates of consumption by large lobsters. Jahnig (1973) estimated that adult lobsters would require less than 1% of their body weight daily as food to maintain their slow growth. Branford (1979) measured consumption by male adults fed natural diet and obtained figures in the 0.5 to 1.5% range. At this laboratory, preliminary studies of consumption of shrimp muscle by lobsters weighing 30 to 50 g showed specific consumption ranging from 0.35 to 1.4% (unpublished results, 1978). It is likely that for larger lobsters, these figures may represent a lower limit to specific consumption rates, and the equation of Fig.3 is not expected to be valid for lobsters above 100 g. Further experiments are planned on lobsters in the 50-500 g class. In our consumption experiments, no lobsters smaller than 0.8 g wet weight were tested, although the methods could be used for such lobsters. Therefore, no direct comparisons can be made with the data reported by other investigators for larval lobsters (Carlberg and Van Olst, 1976; Logan and Epifanio, 1978). Extrapolation of our consumption equation to smaller lobsters, however, suggests that fifth-stage juveniles (40-150 mg wet weight) could consume more than 10% of their own (wet) weight in food daily. This is higher than the 2-4% reported by Carlberg and Van Olst (1976) for fourth-stage lobsters feeding on brine shrimp, but lower than the almost 20% shown by third-stage larvae in the same study. Our extrapolated figure is also higher than the data given by Logan and Epifanio (1978). The cause of these discrepancies is unknown but may result from small lobster size or variable brine shrimp size. In our experiments, even 800 mg juvenile lobsters occasionally consumed more than 10% of their weight per day, which indicates that lobsters, at times, are capable of at least this high rate of consumption. Feeding schedule In earlier work at this laboratory, Shleser (1974) determined that maximum growth resulted from daily feeding rather than feeding at intervals of 2 or 3 days. Our experiments confirm these findings; feeding 5 days out of 7 resulted in significantly smaller lobsters than those fed 7 days out of 7. Even though consumption was significantly increased after 2 days of starvation, growth on the 5 day per week schedule was reduced in proportion to the

296

amount of food fed; that is, a 29% reduction in food fed resulted in 31% smaller animals. This suggests that lobsters, like other poikilotherms, do not store appreciable amounts of energy and require a nearly constant supply of nutrients to achieve maximum growth. The fact that daily feeding was essential to rapid growth implies that a diurnal cycle of food consumption may exist as suggested by Carlberg (1975). No further evidence was found for such an exogenous cycle, however, as lobsters maintained in near-constant darkness routinely grew faster and consumed more than those on a diurnal photoperiod. Also, lobsters in either light or dark consumed similar proportions of their daily intake in the first 4 h of feeding, which suggests that darkness per se did not stimulate feeding. Further experiments showed that the total daily consumption remained the same whether lobsters were fed a natural diet once or twice per day, suggesting that the introduction of food, at least brine shrimp, does not necessarily stimulate feeding. An exception occurred after 2 days of starvation, at which time the lobsters consumed nearly twice their daily average, with a third eaten in the first 4 h of feeding. By comparison, unstarved lobsters consumed about one-sixth of their total daily intake in the first 4 h of feeding, which is the expected rate assuming constant intake or periodic feeding. It seems probable from these data that lobsters are intermittent feeders that return to the diet many times over the period that the food remains palatable. This feeding habit also occurs in other nocturnal crustaceans (Hill, 1976) and may result from rapid clearance of food from the foregut. According to this theory, initial feeding consists of filling the foregut with relatively large pieces of food, followed by grinding of the food by the gastric mill. Eighty percent of the ingested food may be passed out of the foregut in only 2 h, leaving the animal hungry for another “meal”. If the animal has an excess of palatable food available at all times, as in these experiments, it can be assumed that one “meal” follows another until satiety occurs, possibly with the fulfilling of caloric requirements. Under these conditions, replacement of one portion of food with another of identical nutritional composition would be unlikely to influence total food consumption. Artificial diets unstable in water, however, may not be a palatable food source after an initial feeding or two (Sick and Baptist, 1973), and preliminary observations have suggested that growth might be improved by multiple daily feedings of such diets. . Response

to temperature

Hughes et al. (1972) have documented the direct relation between growth and temperature for juvenile lobsters. Our experiments indicate that such growth increases are accompanied by proportionate increases in food consumption. Although in our experiments consumption was not significantly different at 18” C from 21” C, the direct relation is evident, and it is probable that the difference is real and would be significant given larger numbers of

297

observations. In experiments conducted on larval lobsters (Carlberg and Van Olst, 1976), growth was significantly faster at 19°C than at 15” or 16°C; food consumption was also slightly increased, although this difference was not signficant. Response to light and dark Many crustaceans, such as some shrimp, are known to be attracted to light, exhibiting better growth (Wormhoudt and Ceccaldi, 1975) and food consumption (Sick et al., 1973) with a diurnal light cycle than with constant darkness or reduced photoperiod. Lobsters, however, are primarily nocturnal and tend actively to shun light. Hatching of eggs usually occurs at night, and studies have shown that locomotor activity (Krekorian et al., 1974) and feeding time (Carlberg, 1975) increased in darkness. In their natural habitat, which extends to depths of 150 fathoms and more in the Atlantic Ocean, lobsters are exposed to very little light and probably depend upon chemosensory and tactile stimuli to locate their food. Thus, locomotor activity and feeding activity are closely related in lobsters. In behavioral studies, Zeitlin-Hale and Sastry (1978) showed that most locomotor activity was inhibited by light, and Krekorian et al. (1974) demonstrated that from 68 to 81% of lobster activities took place in darkness. Both of the above studies used light/dark periods of approximately 12 : 12, and observations were made during darkness with the aid of red incandescent lights not visible to the lobsters due to their limited range of spectral sensitivity (Kennedy and Bruno, 1961). While the nocturnal preferences of lobsters are now accepted by most culturists, and the data in these experiments show improved growth and food consumption in darkness, some evidence has accumulated to suggest that manipulation of photoperiod may increase growth and consumption responses even more. The observation that most feeding activity occurred immediately after darkness on a cycle of LD 13 : 11 (Carlberg, 1975) has led some investigators to suggest that increased photoperiods might result in improved growth by limiting activity to that required for feeding and inhibiting other activity wasteful of energy (Zeitlin-Hale and Sastry, 1978). Aiken and Waddy (1976) showed that at temperatures lower than 20” C, an increased photoperiod (LD 20 : 4) enhanced growth as compared with growth under a short photoperiod (LD 1 : 23). This effect, presumably caused by inhibited locomotor activity, disappeared at 20” C, and it was felt that photoperiod effects would be minimal at this temperature. These experiments were conducted on communally-held mature lobsters, and it is quite possible that larger lobsters respond differently to photoperiod than do juveniles. Also, communally-held lobsters are known to be influenced by agonistic interactions, and this influence may overshadow the effect of photoperiod. In our experiments, growth of juveniles at 20°C (5th to 12th stage, see Table I) was enhanced by short photoperiod, and food consumption by

298

juveniles at 20°C (12th to 14th stage, see Table II) was increased under short photoperiod. These results do not necessarily conflict with those of Aiken and Waddy (1976) since juvenile lobsters, particularly stages 5-12, often appear more solitary and light-sensitive than their older counterparts. After about eight postlarval molts, the lobsters appear to become more aggressive, and are perhaps less influenced by photoperiod. The fact that our 12th-stage juveniles under LD 15 : 9 consumed food at a rate 30% lower than those under LD 1 : 23, while 14th~stage juveniles under LD 15 : 9 consumed at a rate only 16% lower (consumption experiments 1 and 3) may indicate that the reponse to short photoperiod decreases with age. Furthermore, light quality and intensity probably exert some influence on lobster behavior, and locomotor activity, while necessary for obtaining food, may also have other important functions. Carlberg (1975) showed that, while feeding occurred principally in the first half of the dark period, locomotor activity continued throughout. Other factors besides light and feeding frequency probably influence activity and food consumption in lobsters and identification of these factors would allow more precise definition of optimum lobster feeding conditions. CONCLUSION

The evidence accumulated from these experiments shows that food consumption and growth of lobsters can be markedly influenced by environmental factors. Of the variables studied, feeding frequency exerted the greatest effect on growth, with daily feeding required for maximum growth. Feeding twice daily with a natural diet did not increase food consumption, but the proportion of food consumed in the first 4 h indicated that feeding is a slow, intermittent process in these animals and that diet stability and palatability are important criteria in evaluating diets. Food consumption increased directly with temperature, while a long photoperiod caused reduced growth and food consumption of juvenile lobsters. The implications of our results are twofold: first, food consumption by individually-held juvenile lobsters occurs over a broad span of time in either light or dark, and not merely in a 4-h period. Second, growth and food consumption are reduced with increased exposure to light. The combination of these implications suggests that optimum growth can be obtained by providing a relatively constant supply of food, and by decreased photoperiod. Further study is required on the interactions of feeding frequency, photoperiod, and light intensity in relation to behavior and food consumption. ACKNOWLEDGMENTS

The authors would like to thank C. Hand and A. McGuire for manuscript review and Lorraine Andrade and Louisa Otis for preparation. Nancy Baum and Tom McCormick of the Bodega Marine Laboratory provided technical assistance.

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