Size and Proportion Relationship Between the Beaked Sea Snake and Its Prey

Size comparisons are made between the beaked sea snake, Erhydrina schistosa, and its usual prey, an ariid catfish, Tachysurus maculatus. Of 108 catfish found in as many snake stomachs, most were partially digested. Estimates of catfish length, maximum diameter, and weight were made using linear regression relationships between these measures and index measures determined on entire specimens. Maximum prey diameters were one to two times the neck diameters of the snakes. Prey selection is implicated along both the upper and lower limits of prey size. Large snakes take a disproportionately large number of relatively small fish. This may be a function of prey availability alone. OPTIMAL FORAGJNG THEORIES have encouraged ecologists to look at feeding behavior and habits much more rigorously than in the past. To test theories of selective feeding it is necessary to define and gather data on factors such as foraging time, processing time, net energy gain, feeding frequency, etc. (see Schoener 1971, for a review of optimal foraging theory). Among terrestrial vertebrates snakes are the only major group which specializes in swallowing whole relatively large prey. The prey size of snakes relates directly to prey-handling factors as well as energy considerations. There are numerous reports in the literature on the taxonomic identity of the food of marine snakes (Klawe 1964, McCosker 1975, Voris 1972). The bulk of the data indicates that most species are specialists on some variety of fish. A study of the food and feeding behavior of the beaked sea snake, Enhydrina schistosa, in Malaysia (Voris et al. 1978) revealed behavioral specializations for ingesting spiny catfish (Ariidae). In this paper the relationship between the size and body proportions of E. schistosa relative to its catfish prey is explored. MATERIALS AND METHODS Beaked sea snakes (Enhydrina schistosa) were collected at Muar, Malaysia (2?3'20N, 102034'20E). Details of habitat and collection techniques are available elsewhere (Jeffries and Voris 1979). The measurements of snout-vent (s-v) length, girth at the neck, and girth at 3/4 the s-v length were made on snakes relaxed with sodium pentobarbital. The latter two girth measurements represent the minimum and maximum girths respectively for E. schistosa. The neck of E. schistosa is round, and thus girth measurements were converted to diameters by dividing each circumference by -r. All specimens were weighed several months after they had been preserved in formalin and stored in 70 percent ethanol. The stomach contents were removed from the relaxed snakes and preserved in 10 percent formalin. Several months later the fish were transferred to 70 percent ethanol. At Muar the most frequent prey of the beaked sea snakes was the ariid catfish, Tachysurus maculatus (Voris et al. 1978), and this paper deals mainly with this species. Measurements and weights were made on preserved catfish. Body length was measured from the most anterior part of the head to the base of the tail where the fin rays emerged from the body. The maximum diameter, excluding the extended dorsal or pectoral spines, was across the body at the base of the pectoral fins and will be hereafter referred to as the diameter. Tail height was measured as the depth of the caudal peduncle, excluding the fin rays, at the base of the tail. The dorsal fin to tail length was the distance from the posterior junction of the dorsal fin and the body to the base of the tail medio-laterally, where there was a small notch at the base of the tail. E. schistosa swallow catfish head first (Voris et al. 1978), and thus the posterior end of the fish is the last to undergo digestion. Fish lacking some part of the forebody comprised 75 percent of the stomach contents. Of the 108 catfish removed from stomachs, the length, diameter, and weight were measured directly for 27, 16, and 11 specimens respectively. For the remaining partly digested fish, tail height and dorsal fin to tail length measurements were used to estimate the original body dimensions and weight. This was done after first analyzing a series of 50 complete specimens to determine the relationship between the two index measurements (tail height and dorsal fin to tail length) and the length, diameter, and weight of the catfish. A linear relationship exists between tail height and body length, thus the linear regression was used BIOTROPICA 13(1): 15-19 1981 15 This content downloaded from 157.55.39.27 on Mon, 05 Sep 2016 06:03:11 UTC All use subject to http://about.jstor.org/terms TABLE 1. Data on relationships between catfish index measurements (dorsal fin-tail length and tail height in centimeters) and catfish parameters used in comparisons with snakes. Body Maximum Body length diameter weight (log) x n y r n y r n y r Fin-tail 50 1.87x 0.11 1.00 42 0.38x + 0.04 0.99 35 3.29x 1.27 0.99 length Tail height 50 10.03x + 0.91 .99 41 1.97x + 0.27 0.98 34 2.86x + 1.25 0.98