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Feeding Preferences of Echinoids for Plant and Animal Food Models

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Feeding Preferences of Echinoids for Plant and Animal Food Models SHORTPAPERS 365 Throndsen, J. 1972. Coccolithophorids from the Caribbean Sea. Norw. J. Bot. 19: 51-60. Zernova, V. V. 1970. Phytoplankton of the Gulf of Mexico and Caribbean Sea. Oceanological Re- search 20: 68-103. (In Russian.) --. 1974. Distributions of the phytoplankton biomass in the tropical Atlantic. Okeanologiya 14: 882-887. (rn Russian.) --, and V. Krylov. 1974. Species of monocellular algae in the Gulf of Mexico and Caribbean Sea. Pages 132-134 in Invest. Pesqueras Sovietico-Cubanas. (In Russian.) DATEACCEPTED: June II, 1980. ADDRESS: (HGM) Department of Biological Sciences, Old Dominion University, Norfolk. Virginia 23508; (lAS) lelm, Wyoming 82063. BULLETINOF MARINESCIENCE.32(1): 365-369, 1982 FEEDING PREFERENCES OF ECHINOIDS FOR PLANT AND ANIMAL FOOD MODELS J. B. McClintock, T. S. Klinger and J. M. Lawrence ABSTRACT-The echinoids, Echinometra lucunter, Lytechinus variegatus, and Eucidaris tribuloides showed a feeding response for prey models prepared with animal food (the bivalve, DOllax variabilis) as great or greater than that for prey models prepared with plant food (the seagrass, Thalassia testudinum). The feeding response of E. tribuloides was sig- nificantly greater for animal models as for plant models. The feeding response of L. vari- egallls was strong and as great for animal models as for plant models. The feeding response of E. II/cullter was very weak, but as great for animal as for plant models. These results indicate that although plant foods may be the predominant type offood ingested by echinoids, preference for animal food is high. This cannot be ignored when considering the evolutionary basis for food preference in echinoids. Animals may be under strong selective pressure to eat those foods in the pro- portion that will yield maximum "value" per unit time (Emlen, 1973). Feeding preference studies of echinoids generate interesting comparative information n:- garding diet in the field and may help provide insight into "optimizational" strat- egies. Growth and gonadal development in some echinoids is greater when they are fed preferred algal foods (Lilly, 1975; Vadas, 1977). This is believed to in- crease the inclusive fitness of these animals and therefore of evolutionary signif- icance. Numerous investigations of food preferences of echinoids offered differ- ent plant species have been conducted (Lawrence, 1975), but only McPherson (1968) has investigated preferences for animal food. The purpose of this study was to evaluate the relative preference for plant and animal food of three species of echinoids. Eucidaris tribuloides is a member of the primitive Perischoechinoidea, a predominately carnivorous group; Lytechinus variegatus and Echinometra lucunter are members of the Euechinoidea, a pre- dominately herbivorous group (Lawrence, 1975). Obviously, conclusions about the evolution of preferences among echinoids would be affected if a strong pref- erence for animal prey is present in echinoids but is ignored in consideration. 366 BULLETIN OF MARINE SCIENCE, VOL. 32. NO. I. 1982 MATERIALS AND METHODS Specimens of Eucidaris Iribuloides and Eehinomelra lueunler were collected in March 1980, from shallow subtidal areas off Key West, Florida (8]045'W, 24°33'N). Specimens of Lyleehinus var- iegatus were collected from a shallow seagrass bed near St. Petersburg, Florida (82°40'W, 27°37'N). Seven specimens of each species were placed into separate 40-1 aquaria equipped with recirculating seawater systems (25°C and 32%. respectively). The echinoids were allowed to adjust to laboratory conditions for I week prior to experimentation. Selectivity was examined by a procedure similar to that of Collins (1974). Flat, circular (0.2 cm depth; 2.0 cm diameter) prey models were made of plaster of Paris mixed with homogenized plant or animal tissue (5: I, wet plaster weight: wet tissue weight). Animal and plant homogenates were pre- pared from the tissues of the bivalve Donax variabilis and from cleaned blades of the seagrass Thalassia lesludinum. Individua]s were positioned so that their oral region was in direct contact with a prey model. The amount of time each individual spent covering the prey model and instances in which animals com- pletely ingested prey models were recorded. Covering time was considered terminated once the test of the individual moved completely off the prey model. All individuals were offered one prey model per day. Either plant or anima] models were offered on a given day. Although some echinoderms can be induced to change their hierarchy of feeding preferences with conditioning on a single food type, the conditioning time required is at least one month (Castilla, 1972), a time period far exceeding the time course of this experiment. In order to evaluate the tactile response (a response to a model without animal or plant homogenate) ten individuals of each species were placed on plaster of Paris discs containing no homogenate and the time individuals remained on the model (tactile time) was measured. Feeding responses were calculated by subtracting the mean tactile time from the total covering time. Only times greater than these means were considered as positive feeding responses. A "persistence coefficient" was calculated to eliminate species' differences in activity levels and thus to facilitate interspecific comparisons. This was defined as the ratio of time spent covering a disc containing homogenate to the time spent covering a disc containing no homogenate. RESULTS Eucidaris tribuloides, Lytechinus variegatus, and Echinometra lucunter showed at least as strong, or a stronger, feeding response for animal rather than plant models. Feeding response times of E. lucunter, L. variegatus, and E. tri- buloides offered plant and animal models are presented in Figure lAo Eucidaris tribuloides showed a significant preference for models mixed with animal tissue (P ~ 0.05). Lytechinus variegatus did not show a significant preference for plant or animal models and spent twice as much time covering plant models as did E. tribuloides. Echinometra lucunter did not show a significant preference for plant or animal models and spent the least amount of time covering either prey. Ly- techinus variegatus and E. tribuloides ingested more models when offered discs prepared with animal tissue (Fig. IB). Eucidaris tribuloides consumed the most models. All three species showed positive feeding responses to both plant and animal models (Fig. IC). Positive responses were least for E. lucunter offered plant models, and greatest for L. variegatus offered animal models. Mean per- sistence coefficients were significantly higher for E. tribuloides offered animal models as opposed to plant models (P ~ 0.05) (Fig. ID). Mean persistence coef- ficients for L. variegatus were not significantly different between plant and animal models, and somewhat lower than E. tribuloides. Echinometra lucunter showed the least persistence for either plant or animal models. Mean persistence times for E. /ucunter, L. variegatus, and E. tribu/oides for control models (no homog- enate) were 4 min 58 sec ± I min 28 sec, 8 min 19 sec ± 2 min 54 sec and 4 min 8 sec ± 2 min 10 sec, respectively. DISCUSSION Feeding preference studies in echinoids have been conducted using a variety of techniques: Y-tubes (Vadas, 1968), passing of algal stimulant over the oral SHORT PAPERS 367 60 40 A B c c <ll C;j E 30 <ll 20 <ll >- E <ll i-= 0. eR E.I. L.V. E.!. E.!. L.v. E.!. 100 10 C 0 <ll C (/J <ll c 0 g a. Qi (/J 0 ~ () 50 <ll <ll () 5 ~ c 'Vi <ll 0 1i5 a. 'iii eR ~ Ix E.I. L.v. E.!. E.1. L.v. E.!. Figure 1. Feeding responses of Echinometra lucunter (E.I.), Lytechinus variegatus (L.v.), and Eu- cidaris tribuloides (E.t.) offered plant (Thalassia testudinum) models (shaded) and animal (Donax variabilis) models (unshaded). (n = 24 for Echinometra lucunter and Lytechinus variegatus; 18 for Eucidaris tribuloides.) A, Mean covering times ±95% confidence limits. B, Percent of prey models totally ingested. C, Individuals showing a positive feeding response expressed as a percentage of the total number of individuals tested. D, Mean persistence coefficients (time spent covering a disc containing homogenate to the time spent covering a disc containing no homogenate) ±95% confidence limits. region of the echinoid and observing the characteristic feeding motion of the teeth (Sammarco, pers. comm.), position of the echinoids relative to the food sources (McPherson, 1968; Himmelman and Steele, 1971), and predominately, relative feeding rates (Lewis, J958; Gezelius, 1962; Boolotian and Cantor, 1965; Leighton, 1966; 1968; Pearse, 1972). The methodological approach to this preference study is different in that individuals are placed in direct contact with the stimulant instead of requiring them to move to the stimulant. This method reduces the high degree of variability often noted in Y-tube experiments by removing the "search" component. Of the three species, Eucidaris tribuloides showed the strongest feeding re- sponse for animal models. This is consistent with the carnivorous nature of the Perischoechinoidea as shown by field studies, and supports McPherson's (1968) demonstration that E. tribuloides showed a preference for rock infested with the sponge Cliona lampa over marine plants. Eucidaris thouarsii feeds preferentially on corals when they are available and predation pressures reduced (Glynn et aI., 1979). Lytechinus variegatus showed a strong feeding response for both plant and 368 BULLETIN OF MARINE SCIENCE, VOL. 32, NO. I, 1982 animal models. Although we found no significant difference between mean per- sistence coefficients and time spent covering prey models, individuals did con- sume more animal than plant models. Lytechinus variegatus is a common inhab- itant of seagrass beds where it is primarily herbivorous and feeds extensively on Thalassia testudinum (Moore et at., 1963). The data indicate that models prepared with T. testudinum are not preferred over animal models; however a strong feed- ing response for T. testudinum is not surprising. Echinometra lucunter showed less response to prey models than the other two species. Its low level of activity may be related to its burrowing habit.
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