¹National Research Institute of Aquaculture, Nansei, Mie 516-01, Japan
²National Research Institute of Fisheries Science Kanazawa, Yokohama 236, Japan
ABSTRACT Predation of juvenile abalones by decapod crustaceans has been poorly known, especially in anomuran crabs. We examined predation of artificially bred juvenile abalones (Nordotis) by Japanese hermit crabs in three diogenid (Aniculus aniculus, Dardanus crassimanus, D. pedunculatus) and two pagurid (Pagurus similis, P. japonicus) species under laboratory conditions. Diogenid crabs preyed on abalones whereas pagurid crabs did not. The predation rate of Dardanus differed for two abalone species at 15oC.
INTRODUCTION Abalone is one of the most important commercial species with a high economic value in the coastal fisheries of Japan. Recently, artificial seedlings have been released to enhance the abalone fisheries. There are 30 prefectural facilities for abalone seedling production, and approximately 30 million seedlings of 15-30 mm in shell length (SL) were produced artificially in 1988 (Uki 1989). The survival rate of juvenile abalone, however, is remarkably low after release. It has been pointed out that predation is chiefly responsible for the high mortality of juvenile abalone. Decapod crustaceans have been listed as one of the predators (Cox 1962, Shibui 1971, Momma 1972, Shepherd 1973, Kojima 1981, Hayashi 1988). Tegner and Butler (1985) reported that from one-third to one-half of the total mortality of young abalone was attributable to predation by crabs. However, few previous studies on seedling mortality in abalones focused on hermit crabs, while many brachyuran predators have been known as cited in Table 1. This study gives a preliminary description of predation of juvenile abalones by hermit crabs in two major taxonomic groups, families Diogenidae and Paguridae, under laboratory conditions.
MATERIALS AND METHODS
We examined predatory behavior of hermit crabs in five Japanese species: three diogenid (Anculus aniculus, Dardanus crassimanus, D. pedunculatus) and two pagurid (Pagurus similis, P. japonicus) species. For comparison, two brachyuran crabs, Thalamita prymna and Plagusia dentipes, were also examined. These crabs are commonly found in the abalone fishing ground at Taso, Mie Prefecture. The animals were collected by crab pots at Taso. Two species of artificially bred juvenile abalones, Nordotis discus discus (= Haliotis discus discus) and Nordotis gigantea (= Haliotis sieboldii), were used for the prey. Twenty seedlings of the abalone, 20-30 mm, were placed in a tank filled with 20 liters of seawater with a rectangular shelter 3 days before the test. Two hermit crabs were put into each tank; before the experiment, these crabs had been starved for a week. A test tank without crabs was used as a control. Each experiment lasted for 10 days; three tanks were maintained at three water temperatures: 15oC, 20oC, and 25oC. The condition of the animals was checked every day, and the behavior of the hermit crabs was also monitored using a VTR system with optical intensifier. Damage to the shell of the consumed abalone was classified into three types: 1) no damage, 2) chipped margin, and 3) broken into pieces.
RESULTS
In general, brachyuran crabs tend to break or crush the shell of the abalone they attack. We can see in Figure 1 that abalone shells of N. discus discus preyed by the brachyuran crab Thalamita prymna were broken into pieces, but most of those eaten by hermit crabs of Dardanus usually suffered no damage. In addition, Aniculus aniculus also broke the shells. Three diogenid species preyed on juvenile abalones, whereas the pagurid crabs did not (Table 2). Figures 2a and 2b show daily consumption of seedlings by A. aniculus and D. crassimanus at different temperatures. Predation by the hermit crab decreased below 20oC for all species examined. In the case of D. crassipes, the predation rate is higher in N. discus discus than in N. gigantea at 15oC. A similar result was obtained for D. pedunculatus, while no difference was recorded for A. aniculus.
DISCUSSION
Kojima (1981) stated that predatory brachyuran crabs broke the abalone shells into pieces when they attacked. In the present result, predation occurred in a similar manner for two brachyurans and A. aniculus. In contrast, diogenids cause no damage to the shells. Shepherd (1973) listed six decapod crustaceans including an anomuran Paguristes frontalis as predator of Australian abalones. His study showed that diogenid crabs did not break but only slightly chipped the margin of abalone shells. Hayashi (1988) reviewed predators of artificially reared juveniles of Nordotis discus discus and included hermit crabs as predatory animals. Thus, in the field, it is difficult to distinguish abalone that have been killed by hermit crabs from those that died naturally.
Predator hermit crabs fed above 20oC during this study. The predation rate seems to be related to temperature. Furthermore, in D. crassimanus the predation rate of N. discus discus is higher than that of N. gigantea. It is suggested that differences in escape behavior of abalones affect their predation mortality: N. discus discus usually lift the shell and move quickly from the site whereas N. gigantea adhered firmly to the substrata when attacked by predators.
The hermit crabs are classified into two major groups: the Coenobitoidea including the family Diogenidae with a large left cheliped, and the Paguroidea including the family Paguridae with a large right cheliped. Although the two families are similar in morphology, their phylogenetic relationship is different, as has been evidenced by the process of larval development (McDonald et al. 1957) and by comparison of mitochondrial DNA (Cunningham et al. 1992). It is interesting that the predation behavior of hermit crabs on abalones is clearly different between these left-handed and right-handed families.
Mechanisms of predation behavior of hermit crabs are still unknown, although they have been studied in detail in spiny lobsters (e.g. Zimmer-Faust and Case 1983). To improve survival of released abalone seedlings, more information is needed on the predator-prey relationship.
ACKNOWLEDGMENTS
Special thanks are due the Taso Fisheries Cooperative Association for helping us collect specimens. This work was supported in part by a Grant-in-Aid (Bio Cosmos Program) from the ministry of Agriculture, Forestry and Fisheries (BCP94-IV-D-4).
LITERATURE CITED
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