INTERTIDAL ECOLOGY 



147 



bench with algal turf or natural or artificial depressions 

 than on adjacent smoother, barer bench of the type shown 

 in Fig. lb Species richness of Conus and of total preda- 

 tory gastropods is also significantly lower in the last 

 microhabitat type (Table 5; Kohn and Leviten, 1976; 

 Leviten and Kohn, 1980). The most common species of 

 Conus are C. ebraeus. C. sponsalis, C. chaldaeus, and 

 C. miliaris. Rarer species are C. frigidus. C. flavidus. 



C. catus. C. rattus. and C. retifer; the last two did not 

 occur in the quantitative samples summarized in Table 4. 

 The predominant muricids are Morula granulata. M. uva. 

 Drupa morum, and D. ricina; less common are 



D. arachnoides. Maculotriton serriale, and Thais fuberosa. 

 Drupa morum and D. ricina are as abundant on smooth, 

 bare bench as they are where refuges are present. Their 

 depressed, limpet-like shells and disc-shaped feet permit 

 more tenacious adhesion to the substrate than the longer, 

 narrower feet of most other gastropods present. Vasum 

 turbinellum (family Vasidae) is the most common other gas- 

 tropod present, followed by the mitrids Mitra litterata. 

 Mitra cucumerina. Vexillum cancellarioides, and Imbricaria 

 punctata (Kohn and Leviten, 1976). 



Co-occurring species of Conus do not use different 

 types of microhabitat (listed in Table 1) differentially. All 

 species observed are typically (96% of individuals) inactive 

 in refuges during daytime low tides, when physical stresses 

 are probably harshest. Activity increases abruptly to a high 

 level in all species in late afternoon and early evening, 

 unless water flow is too strong (50 cm s~'), as the animals 

 move over the smooth bench surface and feed during the 

 night. They do not home to the same site used the previ- 

 ous day, and we detected no evidence for competition for 

 protected sites. These results led to the conclusion that the 

 Conus species on the windward island platform at 

 Enewetak partition neither microhabitat nor temporal 

 resources (Leviten and Kohn, 1980). 



A few nudibranch gastropods occur in protected sites 

 on windward platforms. Chromodohs geometrica. Okadaia 

 elegans, and a few other members of the superfamily Dori- 

 dacea occur under rocks intertidally, and the aeolidiacean 

 Herviella mietta occurs in tide pools (Young, 1967). 

 Characteristic of inner zones of the windward platform is 

 the abundant pulmonate limpet Siphonaria normalis, which 

 occurs at population densities of 400 m~ on flat substrate 



TABLE 5 



Abundance and Diversity of Predatory Gastropods in Different Microhabitats 

 on Windward Reef Platforms at Enewetak* 



"Data from Kohn and Leviten, 1976. 



The different Conus species arc typically zoned across 

 island platforms, with the peak abundances of C. ebraeus 

 closest to shore, C. chaldaeus and C. coronatus intermedi- 

 ate, and C sponsalis closest to the outer edge. However, 

 distributions vary at different study sites and at different 

 times at the same site (Leviten and Kohn, 1980). In all 

 four of these species, shell length decreases significantly 

 with distance from shore, but in C. ebraeus and possibly 

 C. sponsalis it increases again near the outer limits of their 

 distribution. 



that dries at low tide and to 3500 m~ in shallow tide 

 pools. Its body size and population distribution are deter- 

 mined by the foraging behavior of its predator, the proso- 

 branch gastropod Thais armigera (Menge, 1973). 



Siphonaria normalis also occurs commonly on a 

 smooth, sloping beachrock shore on the lagoon side of 

 Enewetak Island at the +0.6- to +0.9-m level. It becomes 

 active when just covered by a rising tide or just uncovered 

 by a falling tide, moving from its home scar in an 

 unpredictable direction to forage on microalgae (Cook and 



