Mark S. Davies

Mucus from Marine Molluscs

ABSTRACT Mucus functions in many invertebrate physiological processes and also influences structuring of the community and the ecosystem. Molluscan mucus is mostly water. The remaining components are protein, carbohydrate and lipid. The detailed structure of the protein–polysaccharide acidic glycosaminoglycan component is not yet known. Mucus is probably released in dehydrated form in distinct, membrane-bound packages, which then absorb water. A functioning mucus is probably formed by mixing of mucins from different types of gland. Under small deformities, hydrated mucus is a viscoelastic solid, able to function as a rope. As stress increases, it yields to become a liquid which can return to the solid state once the stress is released. It is these properties that allow locomotion by molluscs on what is seemingly an adhesive. On dehydration, the strength and stiffness of mucus increase such that molluscs can suspend their body by a thread of it. Mucus production has been studied quantitatively by various methods, some gravimetric and some colorimentric using pedal, faecal, pseudofaecal and hypobranchial mucus: there is much spatial and temporal variation. In locomotion mucus is a coupling agent between foot and substratum; a medium in which propulsive cilia beat; and a drogue. Mucus deposited as a trail by gastropods is an important facet of their environment. Many species follow mucus trails possibly contributing to the observed patchy distributions of gastropods. The methods by which the presence and polarity of mucus trails are detected is poorly understood. Mucus plays a vital role in feeding. In filter-feeding bivalves, mucus aids the transport of food from gill to mouth and is employed to cleanse the mantle cavity of particles rejected by the labial palps. In gastropods mucus nets and bags are used to trap food prior to ingestion and some groups roll their prey in mucus to prevent its escape. Pedal mucus may be ingested after it has become studded with organic material and perhaps act as a fertilizer for microbial growth. A copious secretion of epithelial mucus is used to isolate molluscs from their environment and mucus may also serve as an ionoregulator. Mucus may also contain specific products to render the animal poisonous, distasteful or irritating. Agglutinin and lysozyme have been found in mucus from marine molluscs. Mucus secretion can present a considerable drain of energy (up to 70% of consumed energy). The fate of molluscan mucus is largely unknown and probably makes a considerable contribution to POM in inshore waters, although its is readily degradable by marine microbes. Given the persistence of mucus, densities of benthic gastropods and their motility patterns, much of the gastropod-inhabited benthos is likely to be covered for most of the time with a layer of pedal mucus.

Snails and their trails: the multiple functions of trail-following in gastropods

Snails are highly unusual among multicellular animals in that they move on a layer of costly mucus, leaving behind a trail that can be followed and utilized for various purposes by themselves or by other animals. Here we review more than 40 years of experimental and theoretical research to try to understand the ecological and evolutionary rationales for trail‐following in gastropods. Data from over 30 genera are currently available, representing a broad taxonomic range living in both aquatic and terrestrial environments. The emerging picture is that the production of mucus trails, which initially was an adaptation to facilitate locomotion and/or habitat extension, has evolved to facilitate a multitude of additional functions. Trail‐following supports homing behaviours, and provides simple mechanisms for self‐organisation in groups of snails, promoting aggregation and thus relieving desiccation and predation pressures. In gastropods that copulate, trail‐following is an important component in mate‐searching, either as an alternative, or in addition to the release of water‐ or air‐borne pheromones. In some species, this includes a capacity of males not only to identify trails of conspecifics but also to discriminate between trails laid by females and males. Notably, trail discrimination seems important as a pre‐zygotic barrier to mating in some snail species. As production of a mucus trail is the most costly component of snail locomotion, it is also tempting to speculate that evolution has given rise to various ways to compensate for energy losses. Some snails, for example, increase energy intake by eating particles attached to the mucus of trails that they follow, whereas others save energy through reducing the production of their own mucus by moving over previously laid mucus trails. Trail‐following to locate a prey item or a mate is also a way to save energy. While the rationale for trail‐following in many cases appears clear, the basic mechanisms of trail discrimination, including the mechanisms by which many snails determine the polarity of the trail, are yet to be experimentally determined. Given the multiple functions of trail‐following we propose that future studies should adopt an integrated approach, taking into account the possibility of the simultaneous occurrence of many selectively advantageous roles of trail‐following behaviour in gastropods. We also believe that future opportunities to link phenotypic and genotypic traits will make possible a new generation of research projects in which gastropod trail‐following, its multitude of functions and evolutionary trade‐offs can be further elucidated.

Energy saving through trail following in a marine snail

Most snails and slugs locomote over a layer of mucus and although the resultant mucus trail is expensive to produce, we show that this expense can be reduced by trail following. When tracking over fresh conspecific trails, the marine intertidal snail Littorina littorea (L.) produced only approximately 27% of the mucus laid by marker snails. When tracking over weathered trails, snails adjusted their mucus production to recreate a convex trail profile of similar shape and thickness to the trail as originally laid. Maximum energy saving occurs when following recently laid trails which are little weathered. Many and diverse ecological roles for trail following have been proposed. Energy saving is the only role that applies across the Gastropoda and so may help to explain why trail following is such a well-established behaviour.

The effect of rock‐type on the settlement of balanus balanoides (L.) cyprids

The nature of surfaces has been shown to have significant effects on the settlement of marine organisms. However, the few studies on the effect of natural rock surfaces on barnacle settlement have produced conflicting results. Here the settlement of Balanus balanoides (L.) cyprids on a variety of natural rock‐types is investigated in situ in the absence of strong colour cues. The potential effect of surface texture was minimised as only 5 of the 15 examined rocks showed significant differences in surface rugosity. Significant differences in settlement preferences between rock types were observed independent of any potential effect of surface rugosity or colour cues which were found to have no effect on settlement. Slate was the most attractive rock‐type followed by quartz and marble. Millstone grit sandstone and granitic gneiss were the least attractive and showed significant differences from slate, quartz and marble but not from all other rock‐types. It is proposed that these differences are due either i...

Effects of trematode parasitism on the behaviour and ecology of a common marine snail (Littorina littorea (L.))

Cryptocotyle lingua (Creplin) is a digenean trematode parasite of the littoral prosobranch gastropod Littorina littorea (L.). The literature suggests the snails become infected by grazing guano of the final host, the herring gull, Larus argentatus Pontoppidan. The parasite emerges from the snail as free-swimming cercariae. Interactions between the snail and the parasite at cellular and life-history levels are well established, but little is known of the influences the interaction has on the behaviour and the ecology of the snail. We tested the response of the snail to encounters with cercariae, examined the longevity of the guano on-shore and tested the responses of the snail to encounters with guano. Over half the L. littorea tested were able to detect both cercariae and a filtered homogenate of cercariae in conspecific mucus trails, approximately one-third of animals refusing to cross the treatments. Chemoreception by the mouth or foot is considered the most likely means of detection. Guano samples (mean weight 3.22 g) naturally deposited at approximately mid-tide level were completely washed away by one tidal inundation. We consider this period too brief to allow for ingestion of eggs in guano by the snail. Further, snails would not cross guano placed in conspecific trails. Most snails would not cross guano diluted by 10(3)x(10 mg ml(-1)) and some snails could still detect guano diluted by 10(6)x(10 &mgr;g ml(-1)), though all were prepared to cross it. Detection of guano is again believed to be by chemoreception by the mouth or foot. These results are discussed in terms of the mating and aggregating behaviour of L. littorea. Ingestion of the parasite by L. littorea is likely to take place once the guano has washed away as the eggs are negatively buoyant in seawater and may adhere to rock (biofilm) or algal fronds which may be grazed by the snail.

Effects of copper and zinc on the heart rate of the limpet Patella vulgata L.

Abstract Physiological modifications leading to death in marine organisms stressed by xenobiotics are poorly understood. Here we examine the effect of acute exposure to copper and zinc on the heart rate of the limpet Patella vulgata L. At 0.01 mg l−1 copper there was no effect on heart rate over a 1-week period of exposure. At 0.1 mg l−1 copper, a progressive bradycardia was evident after 24 h and the test animals were dead after 1 week. At 1 mg l−1 copper, the bradycardia was induced after 30 min and all animals were dead after 24 h. Limpets appeared to tolerate 0.5 mg l−1 zinc, though a bradycardia was evident after 24 h. At 5 mg l−1 zinc the heart rate slowed after 4 h and extrapolation indicates all animals would be dead by 1 week. The bradycardia may be adaptive in that it reduces blood flow and uptake of metal across the gills. Natural levels of metals in sea water indicate that heart rate will normally not be affected by the metals, but in areas of severe pollution there will be an effect. The current UK marine environmental quality standards for copper and zinc are well below the treatment levels used in this study and so are supported by this work.

Perceptions and realities in the functions and processes of assessment

Assessment is acknowledged as a central motivator for learning, as being perhaps the most difficult and arduous task for tutors, and also, a defining component of institutional quality, curriculum, courses and degrees. Therefore, given this, surely our understanding of terms, processes and their relationships, which reveal our knowledge of theories, practices and research, would be expected to be coherent and critically defensible. Yet, this study supports other literature that demonstrates that this is not the case. What to do about resolving theoretical and practical issues in assessment is perhaps a key challenge for education and educationalists. One problem is that it is often perceived as being the realm of specialists and for specialist journals, when the reality is that understanding assessment is central to everyone in education.

EFFECTS OF COLLIERY WASTE ON LITTORAL COMMUNITIES IN NORTH-EAST ENGLAND

Colliery waste input has a detrimental effect on the species richness and alpha diversity of sandy and rocky shore communities in north-east England. On sandy shores at the shore levels Chart Datum (CD) + 1.2 to 1.5 m (low shore) and CD + 2.7 to 3.0 m (mid-shore), a maximum of two species of macroinvertebrates per shore level was found at sites heavily contaminated by colliery waste input. In contrast, typically about eight species were found at uncontaminated shores. At the shore level CD + 4.2 to 4.5 m (high shore), the species richness and diversity of fauna were not detectably affected by colliery waste input. On rocky shores that were uncontaminated, 12-15 species of macroalgae were found, whereas only five to eight species were found at contaminated shores. The absentees were usually ephemeral, early successional species. Macroalgal biomass, although less at contaminated shores, showed no significant relationship with colliery waste input. However, the alpha diversity of animal communities on rocky shores was, on occasions, significantly increased where colliery waste inputs occurred. The physical properties of colliery waste are likely to be the reason for the effects observed on both sandy and rocky shores, since many of the leachable chemicals in colliery waste are leached during the period that it spends on the sea bed before accumulating in the intertidal zone.

Diatom motility and low frequency electromagnetic fields—A new technique in the search for independent replication of results

The hypothesis that exposure to a certain combination of static and alternating electromagnetic fields (EMFs) results in an increase in motility of the marine diatom Amphora coffeaeformis was tested. Diatom motility in three strains of A. coffeaeformis was positively correlated with extracellular calcium ion (Ca2+) concentration. The test apparatus consisted of two pairs of Helmholtz coils supported around the stage of a microscope linked to a video recorder and monitor. This system allowed real-time in vivo recordings of diatom speed under EMF and control exposures. The EMFs were calculated at calcium resonance values, previously found to cause enhanced motility. Computerised image analysis was used to calculate the distance moved by individual diatoms in 2-min periods before, during and after EMF or sham-EMF (control) exposure. The addition of EMF caused no significant increase in diatom motility. The results are discussed in relation to the use of diatom motility to measure EMF exposure effects.

Mucus trail following as a mate-searching strategy in mangrove littorinid snails

Mate searching often involves chemical cues and is a key process in determining fitness in most sexually reproducing animals. Effective mate-searching strategies are, therefore, essential for individuals to avoid wasting resources as a result of misrecognition of mating partners. Marine snails in the genus Littoraria are among the most successful molluscan groups that live closely associated with mangroves. Their population densities are often low, and finding a mate within the complex three-dimensional habitat of tree leaves, branches and trunks requires an effective searching strategy. We tested whether males of L. ardouiniana and L. melanostoma located females by following their mucus trails. In the laboratory, male tracker snails followed mucus trails laid by conspecific female marker snails at a higher intensity compared with other markeretracker sex combinations in the mating season, but not in the nonmating season, and this was more pronounced in L. ardouiniana. Male trackers did not move faster when following the trails of conspecific female markers compared with other sex combinations; however, tracker snails moved faster in the mating than in the nonmating season, although this might be related to temperature. In both species, males tracked females regardless of trail complexity, and the majority of male trackers were able to detect the direction (polarity) of the trails of conspecific females. Together with previous studies on rocky shore Littorina species, these findings suggest that sex pheromones are incorporated into mucus trails to facilitate the reproductive success of these snails. Mucus trail following is, therefore, an adaptive mate-searching strategy in intertidal gastropod molluscs, and potentially in other gastropod groups in which trail-following behaviour is prevalent.