Paracellular water uptake and molecular sieving by the foot epithelium of terrestrial slugs

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.

Cutaneous application of an accessory-gland secretion after sperm exchange in a terrestrial slug (Mollusca: Pulmonata)

Competition for fertilisation in hermaphroditic animals seems to have led to many odd behaviours and complex morphologies involved in the transfer of accessory-gland products to the partner. Terrestrial slugs of the genus Deroceras show remarkably elaborate and interspecifically diverse penis morphologies and mating behaviours. Most species have an appending penial gland, which in Deroceras panormitanum consists of a few long fingers that are everted after sperm exchange and laid onto the partner's back. To investigate whether this gland transfers a secretion onto the partner's skin, we killed slugs at different mating stages and studied their penial glands and skin histologically. Two types of secretion granules appeared at a very early stage of courtship, and the penial gland was already filled 15min into the courtship. At copulation, the gland everted this secretion onto the partner's body, where it remained for at least 50min. No lysis of skin tissue or other effects on the skin were observed. The slugs tried to lick the received secretion off their own body, and some droplets were observed to be shed with the body mucus. Our results indicate the external application of a glandular substance that could function as either a pheromone or allohormone. The behaviours of the recipients suggest sexual conflict, although mutual interest cannot be ruled out.

Shrinking to fit: fluid jettison from a haemocoelic hydrostatic skeleton during defensive withdrawals of a gastropod larva

Although most of the basic animal body plans are supported by hydrostatic skeletons consisting of fluid maintained at constant volume, studies on how animals have solved biomechanical scaling dilemmas during evolution of large body size have emphasized cases where skeletons are formed by rigid solids. Larvae of gastropod molluscs swim using ciliated velar lobes supported by a constant volume hydrostatic skeleton. Defensive behaviour involves rapid withdrawal of the velar lobes and foot into a protective biomineralized shell. Some gastropod larvae grow to giant size and the velar lobes enlarge allometrically, but the lobes and foot of many can still withdraw completely into the mineral-stiffened shell. I dyed internal fluid of a large gastropod larva with fluorescein to show that fluid supporting the extended velar lobes is expelled from discrete release sites during defensive withdrawals. Scanning electron microscopy suggested that release sites are distinctive papillae on the upper velar epidermis. Ultrathin sections revealed that branched tracks of microvilli-free membrane on the surface of these papillae were formed by very thin epithelial cells, which may rupture and re-anneal during and after defensive withdrawals. Behaviours facilitated by fluid discharge from a haemocoelic (non-coelomic) body compartment have been rarely reported among aquatic invertebrates, but may be more widespread than currently recognized.

An endogenous SCP-related peptide modulates ciliary beating in the gills of a venerid clam, Mercenaria mercenaria

The activities of both the lateral and frontal cilia of Mercenaria mercenaria were unaffected, either by the two endogenous SCP-related peptides AMSFYFPRMamide and YFAFPRQamide, or by FMRFamide (all at 10(-6) M). Dopamine (DA) inhibited the lateral cilia; the mean EC50 was 2 x 10(-6) M. The peptide YFAFPRQamide--but neither AMSFYFPRMamide nor FMRFamide--antagonized the inhibition induced by DA; this effect was dependent on both time and dose. At a DA concentration of 5 x 10(-7) M, the effect of YFAFPRQamide appeared within 20 min and became maximal within 40-60 min; the mean EC50 at these times was 4.7 x 10(-11) M. If the concentration of DA was increased to 10(-6) M, the maximal effect of the peptide was delayed to 50 min, and the mean EC50 increased to 1.1 x 10(-7) M. Particle transport by the frontal cilia was inhibited by 5-hydroxytryptamine (5HT); the mean EC50 was 5.7 x 10(-7) M. Again, only YFAFPRQamide had an antagonistic effect on the 5HT-induced inhibition. At a 5HT concentration of 10(-6) M, the effects of YFAFPRQamide did not appear until 45 min; the mean EC50 was 10(-6) M. When radioimmunoassayed with an SCP antiserum, the elution profile of a gill extract overlapped those of the SCP-related peptides that had previously been identified in extracts of whole animals. These data suggest that all three SCP analogs occur in the gill. Immunohistochemistry of the gill, carried out with a monoclonal antibody raised to SCPB, stained many varicose neuronal fibers. Most of these were associated with the gill musculature, but a sparse innervation of the filaments underlying the cilia was also observed. Some fluorescent nerve cell bodies were also seen in the gill tissue. Our results are consistent with the hypothesis that YFAFPRQamide modulates branchial activities--muscular as well as ciliary--that are associated with feeding.

Effects of hyperosmotic challenge on the freshwater bivalveDreissena polymorpha: importance of K+

The effects of elevated ion concentrations in the bathing medium on ionoregulation and mortality rate were examined in the freshwater bivalve Dreissena polymorpha. The survival of mussels in saline water depended on the balance of Na+and K+in the medium. Animals acclimated to artificial pond water tolerated acute transfer to 108 mosmol artificial seawater (3.5‰) for months with virtually no mortality. Dreissena polymorpha rapidly (~ 12 h) became isosmotic to the bathing medium when exposed to approximately 100 mosmol hyperosmotic solution composed of 45 mM NaCl or 3.5‰ artificial seawater. This was primarily due to a rapid accumulation of ions with some osmotic loss of water. During a hyperosmotic challenge the intracellular compartment gained K+but mussels required exogenous K+to maintain an intracellular to extracellular gradient similar to that of control animals. The epithelia of D. polymorpha were remarkably "leaky" to solutes. Under hyperosmotic conditions, sucrose, glucose, and inulin penetrated the epithelial tissue and appeared in the blood. The principal route of solute entry was likely paracellular pathways. However, the estimated net movement of sucrose or glucose was less than 2% of the net flux of NaCl. Dreissena polymorpha have the capacity to tolerate oligohaline water where sufficient K+is available to promote cellular homeostasis.

Contact rehydration in the terrestrial slugLimax maximusL. in the field

Specimens of Limax maximus were observed in the field between the hours of 10:00 p.m. and 2:00 a.m. as they entered a pool of rain water approximately 0.5 cm deep. Hemolymph was withdrawn from slugs before they entered the water and after they exited it. A significant proportion of the animals exhibited locomotion to the water source (i.e., orientation to water; 16/16) and retraction of the superior and inferior tentacles (13/16; χ2 = 6.25, p < 0.02), and there was a trend for animals in the water to flare the foot (11/16); these are all behaviors associated with contact rehydration. The mean hemolymph osmolalities of the two groups (i.e., 179 ± 29.6 (SD) mosmol∙kg H2O−1prior to entering the water versus 104 ± 29.8 mosmol∙kg H2O−1after exiting the water; n = 16) were significantly different from one another (t = 6.92, p < 0.001). Time spent in the water was inversely correlated with hemolymph osmolality (r2 = 0.715, p < 0.01). The calculated rate of change in hemolymph osmolality for the slugs was 3.7 mosmol∙kg H2O−1∙min−1. These data support the hypothesis that contact rehydration plays a role in regulating water balance in slugs in the wild.

Morphological correlate of regional partitioning of integumental water absorption in terrestrial slugs

An ultrastructural study of the foot surface of the terrestrial mollusc, Limax maximus, has revealed a correlation of epithelial cell type with the functional partitioning of the surface. The lateral absorptive bands of the foot are comprised exclusively of microvillar epithelial cells, while those of the medial locomotor band are all ciliated. Thus, there is a clear partitioning of epithelial cell types between areas of the foot surface with distinct functional roles. Consistent with the proposed role for paracellular absorption, varying states of hydration are shown to affect the extent of the intercellular spaces, but not the intracellular architecture.