<I>Triplofusus Giganteus</I>

BULLETIN OF MARINE SCIENCE, 73(3): 757–761, 2003 NOTES FIRST REPORT OF CANNIBALISM IN TRIPLOFUSUS GIGANTEUS (GASTROPODA: FASCIOLARIIDAE) Gregory P. Dietl The horse conch Triplofusus giganteus Kiener is the largest predatory gastropod in the western Atlantic (Abbott, 1974). In Florida, the major prey items in the diet of T. giganteus are other gastropods and bivalves (Menzel and Nichy, 1958; Hathaway and Woodburn, 1961; Paine, 1963a,b; Kent, 1983; Kuhlmann, 1994). Observations of the feeding biol- ogy of T. giganteus and other large-sized fasciolariid species indicate a preference for gastropods over bivalves (Risbec, 1932; Wells, 1958; Paine, 1963a, 1966; Maes, 1967; Jory, 1985; Stupakoff, 1986; Edward et al., 1992). Wells (1958) was the first to report on the prey-handling behavior of T. giganteus, which is very similar to that of Fasciolaria hunteria. He stated that when attacking gastropod prey, the predator “attaches firmly to the operculum of its victim, preventing the characteristic withdrawal of soft parts to the relative safety of the shell. While holding the prey in the expanded condition, the predator rasps and devours the soft parts except the columellar muscle attached to the operculum.” Cannibalism, or intraspecific predation, has been observed in the genus Fasciolaria (Wells, 1958; Paine, 1963a; Wells 1970; Snyder and Snyder, 1971; Table 1). However, there are no reports of cannibalism in members of the closely related taxa ,Triplofusus and Pleuroploca. This brief note is the first to document the occurrence of cannibalism in Triplofusus. OBSERVATIONS On 15 May 2000, two specimens of Triplofusus giganteus were collected from a tidal flat on the Intracoastal Waterway, Masonboro Inlet, North Carolina (34∞ 11' N: 77∞ 49' W). The horse conchs were placed in a 75.7 l (20 gal) aquarium maintained at constant temperature (20–22∞C) and given oysters, Crassostrea virginica, as prey. These Triplofusus specimens, one 225 mm in length, and the other 157 mm long, remained in the aquarium feeding on the oysters for nearly a year without any signs of directed intraspecific aggres- sion. Occasionally during the summer and fall months, other gastropod prey species (e.g., F. hunteria) were introduced into the aquarium tank and were quickly seized and killed. During the late fall and winter (November–March) gastropod prey were harder to find in the field so that only oysters were offered as prey. After a five-month period of feeding exclusively on oysters, it was observed on 5 April 2001 that the larger Triplofusus specimen was attempting to prey on the smaller individual, in the manner described by Wells (1958) (Fig. 1). The opercular opening of the smaller individual was oriented so that the proboscis of the larger conspecific could be inserted into the aperture. The foot of the larger horse conch held onto the operculum of its victim. The prey was held in this position throughout the predation process, which lasted over three days. Upon completion of the predation process the predator abandoned the prey. At this time it was observed that the predator did not completely consume its Bulletin of Marine Science 757 © 2003 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 758 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 3, 2003 Figure 1. View of Triplofusus giganteus attacking a smaller conspecific. The prey’s foot and operculum are being held by the predator’s expanded foot to prevent withdrawal of the prey into its shell. Scale bar = 2 cm. prey. The predator left the muscular opercular regions of the prey. This portion of the prey was held in the foot, thus preventing any retraction of the prey during the struggle, or as stated by Paine (1963a), contest of “strength” between conspecifics. In this case cannibalism involved an asymmetric interaction: The larger Triplofusus remained relatively invulnerable to life-threatening injury or death during the predatory attack due to its size, and it was therefore only a matter of time before the smaller Triplofusus was overpow- ered and immobilized, once the attack was initiated. DISCUSSION Cannibalism in some animals is inversely related to the availability of alternate prey (Polis, 1981). According to Polis (1981), one factor that may explain why hunger, or a decrease in alternate food leads to cannibalism is that, as predicted by foraging theory, an animal may expand its diet beyond the normal limits of acceptable prey during periods of hunger, or limited availability of food. Quantitatively, the feeding history of the larger horse conch suggests that hunger may have played a role in stimulating intraspecific predation. The feeding rate of the larger T. giganteus was lower than a diet consisting of only gastropod prey. The large Triplofusus had a feeding rate of 0.29 oysters d-1. This feeding rate is equivalent to an average of 2.0 ml of oyster prey consumed per day, given that the average sized oyster eaten (60 mm) had an internal volume of 7 ml. This value is far less than that reported by Paine (1963b) for comparably sized Triplofusus feeding on gastropod prey; the average predator consumed 15.1 ml of gastropod-prey d-1. When converted to relative volume, Triplofusus that fed on gastropods consumed 3% of their body volume per day (Paine 1963b). For comparison, the horse conch in this study only consumed 0.4% of its body volume per day while feeding on oysters. NOTES 759 Table 1. Evidence of cannibalism in large-sized fasciolariids. WA = western Atlantic, EP = eastern Pacific, IP = Indo-Pacific. Smpecies Ccannibalis Geographi Source(s) region Fasciolaria hunteria yAes W)Wells (1958); Wells (1970 Fasciolaria tulipa yAes W)Paine (1963a); Snyder and Snyder (1971 Triplofusus giganteus yAes** WyThis stud Triplofusus princeps nPo* E)Stupakoff (1986 Pleuroploca ?P*** I * This was a short-term (2-wk) laboratory study. Detailed field observations are lacking, so whether the behavior is absent under natural conditions remains unclear. ** No instances of cannibalism have been reported from field observation of T. giganteus prey selection (Paine, 1963a; Kent, 1983; Kuhlmann, 1994). *** It is not known if any members of the Indo-Pacific genus Pleuroploca are cannibalistic. Unsuccessful attacks of oysters by the larger horse conch also were observed. The valve margins of oysters have to be wedged apart by the predator to allow insertion of the proboscis into the prey’ s shell. This behavior requires the predator to grip the oyster so that the apertural lip can be applied to the margin of the oyster’s valves (see Wells, 1958). The predator’s large size may have constrained successful attachment of the foot to the prey’s shell while attempting to wedge open small (< 50 mm) oysters. The decreased effectiveness in handling prey as well as feeding rate of the larger Triplofusus may have favored cannibalism. The energy gained from cannibalism would have enhanced the sur- vivorship of the individual when resources were limited. Limitation of resources also was indicated by the fact that the larger individual was observed on one occasion to mistak- enly attack a hermit crab-occupied Fasciolaria shell, despite available oysters as potential prey. Paine (1963a) stated that feeding in Triplofusus occurs “opportunistically within the limits set by prey size.” The largest prey Fasciolaria tulipa could capture was approxi- mately 85% of its shell length (Paine, 1963a). If this ratio of prey to predator size, which sets the effective upper limit to the scope of predation, is close to that of T. giganteus, the 225 mm horse conch in this study is predicted to be able to capture a gastropod prey about 195 mm in length. This predicted size is nearly 40 mm longer than the conspecific individual the larger horse conch attacked. Size limits in prey handling do not seem to have been a factor in the delayed predation. Paine (1963a) also reported that T. giganteus selects gastropod prey according to their relative abundance (i.e., prey abundance is correlated with frequency in the diet) and that there was little selection of gastropod prey. However, Paine did not observe any instances of cannibalism despite the fact that he made observations during periods of maximum Triplofusus abundance and intertidal activity (Paine, 1963a). A lack of cannibalism in the field also was corroborated by Kent’s (1983) and Kuhlmann’s (1994) observations of Triplofusus prey selection. The lack of cannibalism in these long-term studies suggests that the behavior may be avoided under natural conditions. That cannibalism was avoided for nearly a year in the interaction history involving the two individuals in this study, and that it seems to have occurred under stressed conditions of food deprivation, again suggests, albeit anecdotally, that the behavior is avoided usually. Why avoid cannibalism? The role cannibalism plays in the population ecology of a species may help to understand why the behavior is common in some species and rare in others. Cannibalism may be advantageous in large populations, in which the encounter 760 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 3, 2003 rate between conspecifics increases, because it removes potential competitors for resources (this may be the case in species in which the behavior is more common, such as F. tulipa). On the other hand, cannibalism may be disadvantageous when population size is small (e.g., T. giganteus: Paine, 1963a) because the behavior may decrease the reproductive success, or fitness of an individual by removing potential mates. It is not known if population size (i.e., encounter rate with conspecifics) has been important to the evolution of cannibalistic behavior in the Fasciolariidae. In conclusion, whether further observation yields results that corroborate or refute the hypothesis that cannibalism is avoided in Triplofusus, the preliminary results of this study highlight the fact that we still have much to learn about these conspicuous predators.

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