Predation on Holothurians: A Literature Review

In the literature, 69 references altogether have reported 76 predators on holothurians. In terms of the number of predatory species, the most important predators are fishes (26 species), seastars (19 species), and crustaceans (17 species). Seastars are the predators most often cited as regularly ingesting large quantities of holothurians. Toxicity seems to be an effective defense against a generalized predator but, against a specialist on holothurians, escape by swimming movements or shedding of a piece of body wall are the only behaviors that occasionally end with a successful escape. Escape behaviors may be a factor in providing the apparent size refuge from predators. Impacts of predators on holothurian populations have rarely been reported or evaluated, and predation on the earliest life stage is unknown. Additional key words: Echinodermata, Holothuroidea, Asteroidea, predators, prey, fishes The importance of predation in determining or influencing community structure has been well documented from a wide range of latitudes, depths, and substrates (e.g., Dayton 1971; Virnstein 1979; Menge & Lubchenco 1981) and is considered to be often the principal factor controlling the spatial and temporal distribution of marine invertebrates (Connell 1975; Menge & Lubchenco 1981). Predation may be an important component of the population dynamics of holothurians, yet field experiments are sorely lacking. Holothurians have developed seven main types of anti-predator mechanisms (Bingham & Braithwaite 1986; Lawrence 1987): thickness of the body wall (including ossicle development), toxic and noxious skin and organs, body swelling or stiffening, evisceration or autotomy (including Cuvierian tubules), swimming, nocturnal activity, and cryptic or burrowing behavior. Kropp (1982) reported shedding of a small piece of the body wall by Stichopus in response to attack by a gastropod, which constitutes an uncommon anti-predator behavior. Brooding (i.e., the association of embryos and juveniles with an adult) is considered to increase the probability of an offspring's survival to the stage of independent existence (Lawrence 1987). Thus, brooding might be considered to be another type of anti-predator mechanism. But, as with some other marine invertebrates, the incidence of brooding in echinoderms, including holothurians, increases with latitude (Mileikovsky 1971) and is mainly restricted to antarctic and arctic holothurians (Lawrence 1987). Most holothurians display one or more of these features. These anti-predator mechanisms appear so well developed that some authors consider predation on adult holothurians to be infrequent (Massin & Jangoux 1976; De Vore & Brodie 1982; Da Silva et al. 1986). On the other hand, scarcity of young holothurians has also been attributed to predation on early life stages (Bakus 1968; Sloan 1986). Does a broad literature review support these assumptions? This paper identifies the important predators on holothurians and addresses specific as well as broader questions: is predation upon holothurians actually infrequent? which parts of holothurians are consumed? is there any evidence for or against particular behaviors being effective against predation? is predation an important component of the population dynamics of sea cucumbers? Answers to these questions could provide a better understanding of holothurian ecology and the basis for future experimental studies on the effects of predation on holothurian communities.