Preliminary observations of gastropod predation in the western Niger Delta

An orientatory investigation of the microbenthos of parts of the western Niger Delta has indicated that drilling gastropods are locally abundant. In addition to a large number of pelecypod and gastropod species, drills attack other drilling gastropods, scaphopods, ostracods and, in rare cases, even foraminifers. It could be ascertained, that there is generally poor correlation between the dimensions of the hole drilled and the size of the prey, thus suggesting that even large drills will select small victims. Ostrea is attacked preferentially to any other forms and pelecypods are more desired than gastropods or ostracods. Most of the predation is due to naticids, although muricids are abundant in the area under consideration. In the main part of the area, the drills have only been found to have attacked Bythocypris, Cytherella and occasionally Costa among the ostracods. However, one sample from near Lagos showed that naticids there attacked not only these genera noted but also Leguminocythereis, Bradleya, Hemicythere and even such a small form as Cytheropteron. In some cases, the hole drilled is two thirds or more of the total length of the carapace. Larger larval stages of all species have been observed to have been drilled. Taken as a whole, the intensity of attack with respect to pelecypods and ostracods does not display regularity. However, a group comprising Pecten flabellum, Crassatella paeteli, Leda rostrata, Venerupis decussata, Mactra nitida, and Arca sp., shows approximately the same degree of susceptibility to naticids for all species. The statistical analysis suggests that right and left valves of ostracods and pelecypods are equally favored by the drills. It is also shown, that there is variation in the shape of the hole from sample to sample and this is thought to be possibly ascribable in part to the shell thickness. The ranges of variation found in some samples for the outer and inner dimensions of the hole may be connected with shifts in the composition of the predator fauna. The drills prefer to bore the central area of the pelecypod valve and many ostracod shells, although interest is concentrated to the posterior or anterior margin of such an elongated form as Bythocypris. Statistical analysis indicated a certain bias in locating the hole within the central field.