Bullia digitalis (Gastropoda) actively pursues moving prey by swash-riding

An individual's success at locating resource items is largely determined by the manner in which it searches. Studies on the movement trajectories of individuals have therefore been useful in elucidating mechanisms by which animals find their resources. Studies thus far have concentrated on animals that live in physically stable environments and search for stationary food items. Several of these studies revealed a surprising ineptitude of individuals in locating their resources. Our study on the other hand focuses on an animal living in a highly turbulent environment that needs to locate moving food items. We studied Bullia digitalis , a whelk that lives in the surf and swash zones of sandy beaches. In spite of constraints imposed by wave action on its movement and obvious difficulties in using positive chemotaxis in a turbulent environment, B. digitalis appears to be most efficient in tracking its prey through the surf. It uses wave-generated flow in a form of locomotion called swash-riding which is commonly used by a wide variety of sandy beach invertebrates.     

Proboscis extrusion in Bullia (Nassariidae): A study of fluid skeletons in Gastropoda

The pleurembolic proboscis of Bullia functions in the manner of a classical hydrostatic skeleton. Its protrusion occurs both with high pressure pulses (2 KPa) in the cephalic sinus and at a steady standing pressure (0-2 KPa). The proboscis probably functions as a haemocoelic chamber separate from the cephalic sinus and pressure pulses from within the everted proboscis, but not the sinus, suggest that muscle antagonism occurs in a manner common to hydrostatic skeletons. The advantages of the proboscis having a hydrostatic skeleton are discussed.     

Shell colour variation in Bullia digitalis, a sand-dwelling, intertidal whelk (Gastropoda: Prosobranchia)

Bullia digitalis is an intertidal whelk that lives on sandy beaches in South Africa. It is highly variable in shell colour, with individuals varying from white to dark brown. This paper describes shell colour variation of B. digitalis at seven sites, along a 230 km coastline east of the Cape Peninsula. Seven colour forms were found: striped, violet, banded violet, banded brown, orange, pale yellow and white. These forms are probably genetically determined morphs. The striped form is the most common at all sites, constituting 53–62% of each sample. The violet is the second most common morph. Its frequencies are remarkably stable at 15–17%. The striped form blends well into the sandy environment and may therefore be of considerable cryptic value in concealing B. digitalis from predators. The violet form is highly conspicuous. Its stable frequency throughout the study area may represent a genetic balance that is not relevant to any visual advantages of the violet colour.