Endoscopy of gastropods: A novel view of the mantle cavities and gills of the keyhole limpet Diodora aspera and the abalone Haliotis rufescens.

The gills, or ctenidia, of marine gastropods serve as the sites for respiratory gas exchange. Cilia on the surface provide the pump that moves water through the mantle cavity and enhance diffusion. Because the gills are housed inside the shell, it is difficult to view them while they are functioning. Published images of gills show contracted, fragile structures that are distorted by the processes of dissection and preservation. Members of the families Fissurellidae (keyhole limpets) and Haliotidae (abalone) have openings in their shells through which water enters and/or exits. I inserted an endoscope connected to a video camera into the openings of the shells of living, non-anaesthetized individuals of the fissurellid Diodora aspera and the haliotid Haliotis rufescens. In both species, the dorsal afferent branchial vessel of the afferent gill axis appeared large and inflated, as did the leaflets that extended from either side of the axis. In D. aspera, the leaflets appeared to fill the mantle cavity and responded to touch, particles, and dye in the water by contracting quickly and slowly re-extending. In contrast, the gills of H. rufescens did not noticeably respond to disturbance. On the other hand, these gills showed a regular pattern of pleats that had not been described in the extensive anatomical literature of these common and economically significant animals. These results provide a novel view of the gastropod mantle cavity as a dynamic space filled by the gills, which divide the mantle cavity into distinct incurrent and excurrent chambers and produce a laminar flow of water through the cavity.

Substrate attributes determine gait in a terrestrial gastropod.

Some terrestrial gastropods are able to move using two gaits: adhesive crawling, where the entire foot is separated from the substrate only by a thin layer of mucus and the snail leaves a continuous mucus trail; and loping, where regions of the foot arch above the substrate and the snail leaves a discontinuous mucus trail. Loping has been interpreted as a means of rapidly escaping predators. We found that the pulmonate Cornu aspersum moved using adhesive crawling on dry acrylic or glass substrates, but loped on dry concrete or wood. Loping snails did not move more rapidly than snails using adhesive crawling. Snails moving on concrete secreted a greater volume of pedal mucus per area of trail than those moving on acrylic; locomotion on concrete thus requires greater expenditure of mucus than does locomotion on acrylic. Because loping snails deposit a smaller area of mucus per distance traveled than do snails using adhesive crawling, loping may conserve mucus when moving on porous, absorbent substrates. Members of several other terrestrial pulmonate taxa can also lope on concrete, suggesting that this plasticity in gait is widespread among terrestrial snails.

Anatomy of and patterns of water currents through the mantle cavities of pleurotomariid gastropods.

The paired pallial organs of pleurotomariids (slit shells) are generally interpreted as representing the primitive gastropod condition. Relatively few individual pleurotomariids have been observed alive; most anatomical descriptions have been based on retracted, preserved material. In this study, live animals were observed in still and moving water, videotaped, photographed, and relaxed prior to anatomical description. The pleurotomariid Perotrochus maureri formed a siphon with its mantle at the posterior end of the slit in the shell. As expected, water was expelled from the mantle cavity through this siphon. Dye released along the aperture, the area traditionally considered to be the incurrent window, was never observed to enter the mantle cavity. Instead, water entered through the slit just anterior to the siphon. A similar pattern was observed in the deep-slit pleurotomariid Entemnotrochus adansonianus. Thus the "anal" slit provided the sites for the incurrent as well as the excurrent windows. This surprising flow pattern is inconsistent with the traditional scenario of early gastropod evolution.

Morphological evolution in sea urchin development: hybrids provide insights into the pace of evolution.

Hybridisations between related species with divergent ontogenies can provide insights into the bases for evolutionary change in development. One example of such hybridisations involves sea urchin species that exhibit either standard larval (pluteal) stages or those that develop directly from embryo to adult without an intervening feeding larval stage. In such crosses, pluteal features were found to be restored in fertilisations of the eggs of some direct developing sea urchins (Heliocidaris erythrogramma) with the sperm of closely (Heliocidaris tuberculata) and distantly (Pseudoboletia maculata) related species with feeding larvae. Such results can be argued to support the punctuated equilibrium model-conservation in pluteal regulatory systems and a comparatively rapid switch to direct development in evolution.1,2 Generation of hybrids between distantly related direct developers may, however, indicate evolutionary convergence. The 'rescue' of pluteal features by paternal genomes may require maternal factors from H. erythrogramma because the larva of this species has pluteal features. In contrast, pluteal features were not restored in hybridisations with the eggs of Holopneustes purpurescens, which lacks pluteal features. How much of pluteal development can be lost before it cannot be rescued in such crosses? The answer awaits hybridisations among indirect and direct developing sea urchins differing in developmental phenotype, in parallel with investigations of the genetic programs involved.