INTERTIDAL ECOLOGY 



151 



Heterocentrotus trigonarius to be the most important her- 

 bivores. 



On the undersides of rocks, the amphipod Melita 

 celericula may be primarily herbivorous. In the laboratory it 

 fed on the algae Valonia and Acetabulaha as well as 

 detritus, fecal pellets, and small conspecific individuals 

 (Croker, 1971). 



Carnivores on Encrusting Animals 



Only the abundance of polychaetes in this category has 

 been quantified (Bailey-Brock, White, and Ward, 1980), 

 and their feeding biology has not been studied. The amphi- 

 nomids Eur[^thoe complanata and Pseudeuri;thoe oculifera 

 occur in the outer or encrusting zone of the Enewetak 

 Island platform 40 to 70 m from shore; they are closely 

 related to species known to feed primarily on corals. Many 

 members of the numerically dominant Syllidae probably 

 belong to this category; some are reputed to feed on 

 sponges, but despite their dominance little is known of 

 their biology. 



Chromodohs geomethca and other doridid nudibranch 

 gastropods probably also feed on sponges (Young, 1967). 

 Other nudibranchs whose food is known include Heruielh 

 mietta, which eats the eggs of the prosobranch Cerithium 

 sejunctum, and Okadaia elegans, which eats spirorbid 

 polychaetes after drilling a hole through the calcareous 

 tube with its radula (Young, 1967). 



Predators 



The most important predatory invertebrates at the pri- 

 mary carnivore level are probably gastropods of the fami- 

 lies Conidac, Muricidae, Mitridae, Vasidae, and Buccinidae. 

 The food subwebs they participate in were being studied 

 by the author and Paul J. Leviten at the time of the 

 letter's untimely death in 1980. It is hoped this study will 

 be concluded in the future, but preliminary results can be 

 mentioned here. The pattern of trophic relationships in the 

 food subweb in which gastropods are the primary car- 

 nivores (Fig. 4) suggests that species of Conus, the most 

 abundant genus, prey primarily on members of different 

 families from the other vermivorous gastropods. Within 

 Conus, preliminary results indicate a general pattern of 

 specialization on different polychaete taxa by the co- 

 occurring species. Most similar to Conus diets are those of 

 Drupa morum, which eats mainly eunicids, and D. arach- 

 noldes, which eats only nereids (Bernstein, 1974). As 

 noted above, these limpet-like muricids are better able to 

 exploit the more exp>osed, seaward portions of the plat- 

 form where nereid and eunicid polychaetes are inaccessible 

 to Conus. Drupa ricina eats mainly vermetid gastropods 

 (92% of the diet: Bernstein, 1974). Two vermivores of dif- 

 ferent families, Engina mendicaria (Buccinidae) and Vasum 

 turbinellus (Vasidae) prey on polychaetes of two families 

 not utilized by other predatory gastropods. Their diets are 

 very similar to each other, but their sizes differ strikingly; 



shells of the former average 12 mm long (range 8 to 18 

 mm) and those of the latter, 23 mm (range 15 to 32 mm). 

 Finally, Mitra litterata (family Mitridae) feeds exclusively on 

 sipunculans. 



One uncommon Conus, C. retifer, and several abun- 

 dant muricids prey primarily on herbivorous gastropods. 

 Morula granulata eats mainly cerithiids (author's unpub- 

 lished observations), and the diet of Thais arnnigera con- 

 sists almost entirely of Siphonaria normalis (Menge, 1973). 

 In addition, some of the species listed in Table 7 

 occasionally prey on molluscs. 



Several xanthid crabs arc also important primary car- 

 nivores on windward, seaward platforms, beachrock 

 outcrops, and lagoon-side rocky shores (Havens, 1974). 

 The upf>er intertidal Lydia annulipes is primarily a preda- 

 tor: 84% of stomachs examined contained animal remains 

 and 28% contained plant food. Sipunculans and Siphonaria 

 normalis are the main prey organisms (Table 7), either or 

 both occurring in two thirds of the individuals examined. In 

 contrast, these organisms were not commonly eaten by the 

 omnivorous xanthids Pseudozius cai/strus (4%) and Eriphia 

 scabricula (18%) or the partly carnivorous Dacri/opilumnus 

 rathbunae (1%). The diets of xanthid species that co-occur 

 in the same microhabitats differ strikingly. For example, 

 where L. annulipes and P. caysfrus use the same crevices 

 for shelter, the former feeds mainly on sipunculans and the 

 latter on large crustaceans and algae (Havens, 1974). 



A recent study of the macruran Panulirus penicillatus 

 (McCollum, 1981) has provided considerable information 

 on its prey. The several types of microhabitats frequented 

 by P. penicillatus during foraging have been noted. 

 McCollum's results suggest that P. penicillatus is a nearly 

 omnivorous predator (Table 8); its diet is very diverse and 

 did not differ significantly among locations studied at 

 Enewetak. Panulirus penicillatus crushes its prey and swal- 

 lows all the parts, so prey organisms with shells or other 

 hard structures can be enumerated from stomach contents. 

 Numerically, molluscs, crustaceans, and miscellaneous 

 items ranked about equally; polychaetes and echinoids 

 predominated in the last category. Although biomass esti- 

 mates were not possible, a measure of imp>ort^ce based 

 on number, volume, and occurrence indicated molluscs to 

 be about twice as important as crustaceans and about 

 eight times as important as miscellaneous items (McCol- 

 lum, 1981). The stomach of one female P. penicillatus con- 

 tained remains of 56 individual ccrithiid gastropods, 52 

 individuals of Strombus sp., three trochids, and one 

 Fragum fragum (Bivalvia); the gastropods were enumerated 

 by counts of opercula. Another female had eaten many 

 Fragum fragum, one Pinctada sp., at least two mytilids, 57 

 Strombus sp., 21 muricids, probably mainly Morula spp., 

 nine cerithiids, including Cerithium aloeolus, one each of 

 Mitra cucumerina, Pusia cancellarioides, Conus sp., and 

 Natica sp., and unidentified gastropods. On reef areas with 

 more polychaetes and hermit crabs, higher proportions of 

 these were consumed. In general, individual stomachs con- 

 tained a high diversity of prey items (McCoIIum, 1981). 



