BULLETIN OF MARINE SCIENCE, 58(1): 1-8, 1996

PREY SELECTION BY THE SOUTH AFRICAN CAPE ROCK LALANDI/: ECOLOGICAL AND PHYSIOLOGICAL APPROACHES

A. Barkai, C. L. Davis and S. Tugwell

ABSTRACT Certain aspects of the foraging behavior of the Cape Rock Lobster, , were investigated to determine whether this selects prey or merely feeds randomly on a variety of prey items in proportion to their abundance. We also investigated the ' ability to digest unusual food items, e.g., sponges and seaweeds, which are sometimes found in large quantities in their gut. The study combined field observations with analyses of gut contents and digestive enzyme activity. The results suggest that Jasus lalandii is primarily a carnivore with limited ability to digest plant tissue. Preferred foods are black and ribbed (Charamytilus meridianalis and Aulacamya ater correspondingly), and a large variety of marine ; activities of protease and chitinase are strong. Seaweeds are probably only incidentally caught by lobsters. They are only sporadically found in lobster guts. More- over, neither laminarinase nor alginase activity, which is essential for the digestion of plant tissue, was detected. It is possible, however, that Jasus lalandii extracts some starches from marine plants as amylase activity in their gut is strong. The most significant finding of this study is the ability of lobsters to feed on and digest sponges. This is evidenced by large quantities of sponge in certain lobster guts and strong activity of the protease gelatinase (which breaks down the sponge collagen skeleton).

The Cape Rock Lobster, Jasus lalandii (Milne Edwards), is an important sub- tidal predator along the west coast of (Heydorn, 1969; Newman and Pollock, 1974; Pollock, 1979, 1982, 1986; Barkai and Branch, 1988a, 1988b, 1988c; Barkai and McQuaid, 1988). These substantially modify subtidal benthic communities and, when present in large numbers, may eliminate most of their benthic prey (Barkai and Branch, 1988c). Barkai and Branch (1988b) found that dense populations of rock lobsters can sustain high growth rates at high densities even when the standing stock of conventionally recognised prey falls below the minimal energy requirements of the lobsters. In their study of a high-density population of lobsters near Malgas Island, Saldanha Bay (Fig. I), Barkai and Branch (I 988b ) noted that during the summer lobsters often scratched the substrata. They ascertained that the energetic requirements of the lobsters were met by the consumption of newly settled species such as and mussels. During the winter, when recruitment of prey was low, the rate of cannibalism among lobsters was high and their selection of prey centered on unorthodox spe- cies such as the mysid Mysidopsis schultzei. Barkai and Branch (1988b) concluded that the Cape Rock Lobster is an opportunistic predator and scavenger. The guts of the Cape Rock Lobster often contain fragments of sponge and sea weed, and it is unclear whether these items are selectively or incidentally con- sumed (Heydorn, 1969; Newman and Pollock, 1974; Pollock et aI., 1982; Barkai and Branch, 1988b). It is also unknown whether lobsters are capable of digesting and assimilating sponge and plant tissue. These questions are of great ecophysi- ological interest since both sponges and seaweed are often very abundant in lob- ster grounds and might constitute another important, yet unrecognised, source of energy for lobsters. Two methods were selected to address these questions. The first method utilised an analytical approach to potential food items, based on the "availability versus 2 BULLETIN OF MARINE SCIENCE. VOL. 58. NO. I. 1996

Durban t South Africa N

Port Elizabeth Cape Agulhas

Figure I. Map of the sampling areas and the sampling sites in each area (site numbers in parentheses as presented in Table 2). usage" analysis developed by Johnson (1980). The second method tested the activity of various digestive enzymes in the digestive fluids of the rock lobsters. The enzymes selected for this analysis (Table 1) were those necessary for the break-down of the component polymers characteristic of various potential lobster prey (i.e., chitin, starch, laminarin, alginates and proteins).

Table 1. List of five digestive enzymes for which the digestive fluid of the rock lobster Jasus lalandii was tested. Column 2 lists the different types of substrates that were used in the assays, expressed as weight of substrate per volume (WN) of phosphate buffer (PB). The third column lists the equivalent end products which were used to draw up the standard curve. The last column provides the method used for the analysis, with the literature source in parentheses.

Enzyme Substrate (WN in PB) Standard curve Method (References) Chitinase Approx. 20 mg·ml-I N-acetyl glucosamine (Monreal and Reese, 1969) chitin* Amylase 1.0% starch Glucose Nelson Somogyi (Nelson, 1952) Laminarinase 0.1 % laminarin Glucose Nelson Somogyi (Nelson, 1952) Alginase 0.4% alginate Malon-dialdehyde Thiobarbituric acid (Jacober et aI., 1980) Protease a) 0.5% azocaseint Azocasein spectrophotometric (Long et aI., 1981) b) I% gelatine:!: Gelatine plate I 0.2% agar (Cowan and Steel, 1970)

*' Chitin slurry was prepared from powdered shell as described by Reichenbach and Dworkin (t 981). t Awcasein substrate made up in O.2M Tris-HCI. pH 7.8. :I:Agar plate assay involves monitoring zones of activity around wells cut in agar filled with "enzyme", BARKA! ET AL.: ROCK LOBSTER PREY SELECTION 3

MATERIALS AND METHODS

Sampling of the Benthos.- Twenty-eight sites along the South African and Namibian west coasts (Fig. 1) were sampled to obtain comparative data on the standing stocks of potential benthic prey and the gut contents of the lobsters. Sites were selected according to their accessibility to divers operating from the shore. In most cases, there was no prior knowledge of the composition of the benthos. Benthic species abundance was estimated as mean percent coverage from six 50 cm X 50 cm quadrats. Replication of samples was achieved by the haphazard positioning of quadrats on the sea floor after "blind diving" for not less than 30 sec between each of the replicates. Kelp abundance was assessed by estimating the percent coverage of holdfasts in each quadrat using a 50 cm X 50 cm three-sided quadrat. Rock lobster densities were estimated from six replicate belt-transects in which all animals within I m of a lO-m weighted rope were counted (20 m2 per replicate). Analysis of Gut Contents.-Samples of gut contents were taken from at least ]5 rock lobsters of similar size (90 mm to ]00 mm carapace length) that were caught at each of the 28 sites mentioned above. The proventriculus of each lobster was removed and the contents examincd microscopicaHy and identified to the level of species where possible. The relative abundance of each item was estimated from its contribution to the total volume of each gut (P, expressed as a percentage). This value was then multiplied by the estimated fullness of the gut (F, ranked from I to 10), summed for all guts, divided by the total relative value of all items from all the gut contents of the sample, and expressed as a percentage (IPiFilIPijFij)1100 (Shepherd, ]973). Comparison of Prey Availability Versus Usage.-The methods and computer program of Johnson (1980) were used to determine whether lobsters consume ("usage") their prey according to its relative abundance ("availability"), or whether they select prey items regardless of their relative abundance. To simplify the analysis, these methods were only applied to the seven most common benthic groups that were found in the lobster guts. These included the black Choromytilus meridionalis, the ribbed mussel Aulacomya ater, barnacles (mostly Notomegabalanus algicola), the sea-urchins Pare- chinus angulosus, sponges, kelp and other seaweed. The availability of these groups were determined at each site and ranked according to their average percent coverage. The usage of these groups by rock lobsters was determined from gut contents and ranked according to the relative proportions present in the gut. Hotelling's T2 test (multivariate t-test; see Harris, 1985) was used to test whether the prey groups are consumed with equal intensity. Waller and Duncan's (1969) multiple comparisons test was then used to declare significant differences between preferences of the lobsters for the various prey groups. In this regard the term "preference" should be treated with caution since the lobsters were not free to choose from different, equally available, prey species. Therefore, in this study, we use the term "preferred food items" to refer to those species which on a relative scale occur in larger quantities in the lobster gut, compared to their proportional abundance in the benthos. Lobster Digestive Enzyme Activity.-Eight lobsters were caught at Oudekraal on the west coast of the Cape Peninsula in South Africa (Fig. I). Each animal was immediately transported to the laboratory and placed in an ice-cold sea water slush until all tactile response had ceased. The animals were then dissected and digestive fluid was collected in Pasteur pipettes from the hepatopancreatic caeca at a point immediately proximal to their junction with the gut. Extracts of several milliliters of fluid were obtained from each animal in this manner. Each extract was diluted in phosphate buffer (67 mM with ]50 mM NaCl, pH 6.8). All substrates were also dissolved in phosphate buffer of the same specifi- cations. Enzyme activity was determined by measuring the rate of end-product formation in spectro- photometric assays (Table I). Standard curves for the end-product of each of the enzymes, excluding protease, were performed. Protein concentration of the digestive fluids was determined using the Folin- Lowry method (Lowry et aI., ]951) with bovine serum albumin as a standard. The rate of enzyme activity for each of the enzymes was expressed as the amount of product formed per hour per milligram of protein present in the digestive fluids. For asocaseinase, protease activity was expressed as the increase in optical density (00) per hour per milligram of protein. A second protease, gelatinase, was also detected using a plate method (Cowan and Steel, ]965). Enzyme activities were also measured in fluid extracted directly from the stomachs of the lobsters. As the enzyme activities in these fluids were always much weaker than the corresponding activity in fluids takcn from the caeca, only the latter was used for the analysis.

RESULTS Availability Versus Usage.-Only seven prey species and taxonomic groups were included in the "availability versus usage" comparative analysis. The mean values of those sampled and found in the gut are summarized in Table 2. Other species whose remains were found in the gut in very small quantities (e.g., very small 4 BULLETIN OF MARINE SCIENCE, VOL. 58, NO. I, 1996

Table 2. The mean % cover of major benthic species (S) sampled at 28 sites along the west South African coast (1 to 22) and 6 sites along the Namibian coast (23-28) (see Fig. 1) and their mean relative amount in lobster gut (G)

Ribbed Black mussels mussels Sea-urchins Kelp Sponges Barnacles Algae

Site S G S G S G S G S G S G S G I 0 0 ]6 21 22 0 65 4 17 0 0 1 42 51 2 0 0 12 0 15 0 35 17 2 0 10 7 66 33 3 0 0 24 0 26 0 28 0 12 100 25 0 53 0 4 11 59 33 29 23 1 0 0 1 0 0 0 0 0 5 0 1 100 68 0 3 0 14 2 0 0 0 0 0 6 76 14 0 0 32 8 0 0 0 0 19 32 9 0 7 0 0 I 24 0 0 7 0 31 0 17 61 73 0 8 I 2 34 I 5 0 5 97 11 0 2 0 4 0 9 14 58 0 0 14 26 1 1 1 0 10 3 0 3 10 30 46 19 0 12 9 2 0 0 0 13 14 16 0 II 0 0 0 31 3 0 44 6 11 0 9 28 20 13 12 0 0 18 56 30 28 10 0 0 0 5 13 23 0 13 0 2 58 37 5 31 4 0 1 a 5 a 30 0 14 0 a 18 a 45 50 a a 2 0 7 0 0 0 15 0 a a 100 4 a 4 0 3 a a 0 50 0 16 0 0 I 91 0 0 0 9 1 0 0 0 0 0 17 0 0 0 1 0 6 0 6 5 0 23 0 0 69 18 0 0 2 a 0 0 0 72 2 3 8 0 42 0 19 0 0 a 1 0 4 6 0 9 0 25 73 76 0 20 0 0 72 100 4 a 6 0 2 0 22 0 27 0 21 0 0 0 0 0 0 6 0 0 0 2 7 32 31 22 0 0 0 6 0 0 48 15 0 32 16 29 40 0 23 67 23 9 0 3 0 0 18 0 0 11 4 5 0 24 58 45 15 5 0 0 10 0 13 29 16 11 10 0 25 0 14 65 35 1 0 4 10 2 1 7 1 0 1 26 19 42 64 12 0 a 11 0 2 6 28 15 19 0 27 30 50 0 24 0 0 4 a 0 18 II 0 0 I 28 0 0 14 64 0 0 0 0 2 2 16 0 15 0

gastropods of all kinds, amphipods, isopods and polychaetes) were excluded from the analysis, since they seem to have made an insignificant contribution to the overall lobster gut content). Lobster remains were also both common and present in relatively large quantities. However, since the consumption of their own exuvia may have led to an over-estimation of the degree of cannibalistic behavior (Hey- dorn, 1969; Barkai and Branch, 1988b), these were also excluded from the pref- erence analysis. The scatter distribution and the large number of "zeros" in the benthic samples (Table 2) is typical of marine communities of the highly dynamic sublittoral hab- itats along the southern African west coast (Branch and Griffiths, 1988). A large variety of benthic species including conspecifics was recorded from the lobster guts. These included ribbed mussels (Aulacomya ater), black mussels (Choromytilus meridionalis), barnacles (Notomegabalanus algicola), sea-urchins () and a variety of small gastropods, amphipods, isopods and polychaetes. Sponges and kelp and other algae were rarely present, but when present they sometimes contributed to a large proportion of the total gut content. Measurements of prey availability and consumption suggest that lobsters are primarily predators that actively select their prey species. Black and ribbed mus- sels were the most preferred prey species followed by sea urchins, kelp, sponges and barnacles. Algae were by far the least preferred "prey" (Table 3). BARKAI ET AL.: ROCK LOBSTER PREY SELECTION 5

Table 3. Results of the Hotelling's T2 analysis (Harris. 1985) testing the null hypothesis that all food components are equally preferred. T is the average difference in ranks for the different food compo- nents, i.e .. availability minus usage. Statistically indistinguishable groups of food components are connected by vertical lines. (Waller and Duncan (1969) multiple comparisons test: for K = \00 and for K = 50 (the equivalent for lX = 0.05 and lX = 0.1 with critical W values 2.25 and 1.89 corre- spondingly».

Type of food

K = 100 K = 50 T

Black mussel Black mussel -1.20 I (most preferred) Ribbed mussel Ribbed mussel -0.42 2 Sea-urchin Sea-urchin -0.28 3 Kelp Kelp -0.18 4 Sponges Sponges 0.40 5 Barnacles Barnacles 0.46 6 Algae Algae 1.22 7 (least preferred)

Lobster Digestive Enzyme Activity.- Table 4 summarizes the levels of enzyme activity on a relative scale according to the rate of end-product formation in spectrophotometric assays (see Methods), that were measured in the digestive fluid extracts. High chitinase, amylase and protease activities were recorded from the digestive fluid of each of the lobsters, regardless of the composition of the gut contents of the animals. Very weak laminarinase and no alginase activities were detected in the digestive fluid.

DISCUSSION Availability Versus Usage.- The South African sublittoral west coast is a highly productive and dynamic system. It is exposed to severe and frequent wave action and violent storms. The result is a variable ecosystem which provides complex and often unstable habitats for many benthic species. It is thus not surprisIng that lobster foraging behavior and selection of food are critically dependent on weather conditions, depth and the availability of shelter (Heydorn, 1969). Nevertheless, other studies (Barkai and Branch, 1988a, 1988b, 1988c) have shown that the intensity of lobster predation is probably the most critical factor determining com- munity richness and species diversity along the South African west coast. As a result, many of the lobsters' "favorite" prey species occur only in very small numbers in areas which are densely populated by lobsters. This probably means that lobsters' gut contents are very likely to reflect what is available rather than their dietetic preference. For example, ribbed mussels Aulacomya ater, which are more common around lobster grounds and are traditionally considered to be fa·-

Table 4. Results of the enzyme activity assays (- no enzyme activity; :!: trace of activity; + + + very strong acti vity)

Enzyme Activity Units of activity Chitinase +++ 5.71 fLg N-acetylglucosamine released·hr-I·mg-I pro- tein Amylase +++ 41 fLg reducing sugar released·h-I·mg-I protein Laminarinase <0.4 fLm reducing sugar releasedlhr/mg protein Alginase Protease a) azocasein +++ 0.9 OD units·h-I·mg-I protein b) gelatine +++ Plate assay. Large zones even with fluid diluted 1/100 6 BULLETIN OF MARINE SCIENCE, VOL. 58, NO. I, 1996 vorite lobster prey (Table 2, Heydorn, 1969; Pollock, 1979), are less desired as a food item (probably because of their stronger shell and smaller size (Griffiths and King, 1979)) than black mussels Choromytilus meridionalis, which are rarely found in large numbers in the vicinity of lobster grounds (Tables 2, 3; Barkai and Branch, 1988a; Barkai, unpub. data). It is noteworthy that the sea-urchin Parechinus angulosus was poorly repre- sented in the gut samples despite their abundance in areas with lobsters (Barkai, unpub.). This is probably due to their poor energetic value. For example, Evans artd Mann (1977) found that the Romarus americanus may gain 15 times more energy and 4 times more protein from a diet of than from a diet of sea-urchins.

Lobster Digestive Enzyme Activity.-The detection of strong chitinase activity is not surprising as this enzyme is essential for digestion of the chitin present in the exoskeleton of marine arthropods (e.g., crabs, isopods, amphipods, cannibalized conspecifics and lobster exuvia (Funke and Spindler, 1989)). In addition, chitinase is also an important enzyme during ecdysis (Spindler and Buchholz, 1988). The presence of amylase is also not surprising as this enzyme has been found in virtually all animals and probably functions as a catabolic enzyme for the cellular breakdown of glycogen. In the case of the rock lobster, however, it might also enable the breakdown of any starches present in low concentrations in marine plants (Kooiman, 1964; Mayzaud, 1985). Laminarinase and alginase activities are characteristic of marine herbivores. For example, the herbivorous gastropod Raliotis midae shows very strong activity for these two enzymes (C. L. Davis, unpubl. data). The absence of these enzymes from the digestive fluid of lobsters suggests that they are primarily carnivorous. It cannot be ruled out, however, that lobsters could utilize simple, non-polymeric sugars, which may be released during the mechanical breakdown of ingested algae (Kooiman, 1964), Kelp, despite being ranked high on the preference list (Table 3), and other seaweeds are probably not essential components of the lobster's diet, but merely incidental to its foraging activities. Fair et al. (1980) suggested that plant material might have beneficial effects for some , either by in- creasing their microbial gut flora, or by increasing gut retention rates. Joll and Phillips (1984) consider both options as very unlikely for the western , since it exhibits very rapid evacuation of plant material from its foregut. Joll and Phillips (1984) found, however, that P. cygnus is capable of utilizing foliose coralline algae (Corallina cuvieri) although they suspect that the large amount of plant material is a result of incidental feeding of lobsters preying on the rich infaunal community. We tend to support this later assumption as being the most likely for Jasus lalandii, considering the virtual lack of laminarinase and a1ginase activities in its digestive fluids (regardless of whether its source is bac- terial or from the lobster's hepatopancreas). Most animals have a number of proteolytic enzymes that are used for the breakdown of proteins into peptides and amino acids. Being primarily carnivo- rous, lobsters feed on a high protein diet for which a high protease activity is necessary (Glass et aI., 1989). These proteases are also probably capable of di- gesting the organic skeletons of sponges, which consist of a collagen (spongin) (Brown, 1975), This may be particularly true of the gelatinase that we found active in the lobster digestive fluid (Table 4), since both gelatin and spongin are structurally similar. Sponges sometimes constitute a major proportion of lobster gut contents (Table 2), and we believe that, unlike seaweeds, these items are BARKAI ET AL.: ROCK LOBSTER PREY SELECTION 7 actively targeted as prey by lobsters and constitute an important, previously un- recognized, food source for these animals.

SUMMARY The most interesting finding of this study is probably that lobsters actively select sponges and have the ability to digest them. Sponges have never been considered as lobsters' intended prey. They occur in fairly large quantities in many lobster sites and might be a much more important component of their diet than has hitherto been assumed. That Jasus lalandii actively selects its food is evidenced by the higher propor- tions of certain food items present in the gut than of the same species in the benthos (Tables 2, 3; Phillips et aI., 1980). Like other palinurid species, 1. lalan- dii's most preferable food is mussels (Joll and Phillips, 1984). However, J. lalandii is also an opportunistic predator or scavenger which may shift to other, less pal- atable or less energetically rewarding, food items when necessary (Barkai and Branch, 1988b). As a result, the gut of J. lalandii may occasionally be filled with species that would normally be considered as incidental prey (e.g., sponges, bar- nacles and small crustaceans). It is also possible, however, that J. lalandii might seek out these species to fulfil certain metabolic requirements. For example, the collagen that sponges yield may be essential to lobsters (despite the poor calorillc value of sponge tissue). Similarly, lobster exuvia and barnacles may be sought for their high calcium content to replenish depleted calcium reserves following ecdysis or for regeneration of injured body parts.

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DATEACCEPTED:October 21, 1994.

ADDRESS:Marine Biology Research Institute, Department of Zoology, University of Cape Town, 7700, Rondebosch, Cape Town, South Africa.