Abstract of the Annual Meeting of The Malacological Society of London, at the Ecology Centre, University of Sunderland, Wednesday 25th March 1998.

The daily rhythm of slug activity in the field (Port et al)

How do terrestrial slugs obtain a balanced diet? (Cook et al)

Cephalopods: their role as consumers in the world's oceans . (Rodhouse)

Suspension feeding mechanisms in bivalves. (Beninger)

Mutualism between aquatic plants and snails. (Jones)

Foraging in intertidal grazing gastropods and its consequences for community structure. (Hawkins)

The daily rhythm of slug activity in the field

Gordon Port, Andrew Young and Mark Shirley.

Department of Agricultural and Environmental Science, Ridley Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU UK. Email: Gordon.Port@ncl.ac.uk

The purpose of our work is to understand slugs better so that we can improve the management of these pests. By "improve" we mean either achieving detailed biological information or using less toxic control methods. To do this we require detailed biological information about slugs. The widespread use of pellets to try and control slugs is testament to their importance. Apart from the use in gardens, pellets are used in horticultural crops and in arable crops such as wheat and rape.

In general we know that the damage caused by slugs increases with the activity of the animal. Rather neatly the opportunity to control slugs with poisoned bait pellets is also limited to those times when the slugs are active. So by understanding the limits on slug activity we may be able to predict when and where pellets could be used most effectively. We know that most slug species are primarily nocturnal because temperature and water constraints are minimal at that time. In ideal conditions they will forage shortly after dusk, feed two or three times each night, they may engage in mating activity, but then they will return to some suitable shelter.

We have already shown the importance of soil moisture in regulating this nocturnal activity. In general when conditions are moist and neither too hot or cold the slugs will be active and will feed on plants or indeed on slug pellets. This has enabled us to produce a decision system which will help minimise the unnecessary use of slug pellets (Figure 1).

During the course of this work we became interested in the annual variation in activity and used time lapse video to examine the numbers of slugs active on different nights throughout the year. The video was recording an area of open soil which slugs were able to enter and leave at will. We simply noted the time the first slugs became active and the time when the last slug activity was seen. Whilst we were recording slug activity we also monitored weather conditions.

Although the activity is controlled by a circadian rhythm, the proportion of time slugs remained inactive below the soil surface varied throughout the year. The greatest delay before the onset of activity was in the winter and in the summer with minimal delay in autumn and spring. The duration of activity was greatest in the autumn and spring, was slightly reduced in winter and was greatly reduced during the warm summer months.The time between the last slug seen active and dawn was recorded as the post-activity period.This was longest in the winter and shortest in the summer. Thus conditions for prolonged activity seem to be optimal in the spring and autumn, whilst in the summer and winter the activity is constrained and starts later in the night-time period and, in winter, finishes well before dawn. Our results suggest that these patterns are governed by weather variation. In the winter period activity is commonly curtailed by low temperature. In the summer period activity is restricted by low soil moisture.

FIGURE 1: Decision System for Pellet Application in Arable Crops
....

How do terrestrial slugs obtain a balanced diet?

Richard Cook*, S Bailey, C McCrohan, B Nash and R Woodhouse.

*School of Life Sciences, Kingston University

Although certain plant chemicals are known to deter feeding, and others are phagostimulatory, the food preferences of slugs appear to be affected by their recent dietary history, including a tendency to select novel food items (neophilia). By feeding Deroceras reticulatum artificial diets with known nutrient composition (protein or carbohydrate concentration) and taste (quinidine sulphate or amyl acetate) for 7 days before giving them a choice, we have investigated whether (1) they are programmed to prefer any novel food, or (2) they show a learned aversion to nutrient defficiency, or (3) they are able to detect essential nutrients.

There was no general inclination for neophilia based on taste alone. No significant learnt association was shown for either a familiar taste or for a novel one, although slugs fed high protein food flavoured with amylacetate slightly preferred the familiar food while slugs fed low protein food slightly preferred the novel food. Similar results were obtained with carbohydrate manipulation. However, the slugs could select food items to replenish nutrients which were deficient in earlier diets, despite the presence of the masking chemicals. Injection of missing nutrients into the haemocoel blocked these changes in food preference, indicating that preferences are influenced by internal state. Thus we conclude that the neophilia exhibited by D. reticulatum is a response to a nutritional imbalance in its recent history.

Cephalopods: their role as consumers in the world's oceans

Paul Rodhouse

British Antarctic Survey, Cambridge

According to FAO, cephalopods are the third most important species group in the world catch, in terms of landed value, after shrimp and tuna (figure 1). Their importance continues to grow, replacing to some extent landings of finfish. Cephalopods take large prey relative to their own body size and often make extensive migrations over the course of their short life cycle. The two commercially exploited species Illex illecebrosus and argentinus in the north and south Atlantic respectively spawn in the sub-tropics, and sections of the population migrate as far as Newfoundland and the Falkland Islands in the Atlantic western boundary current systems (Gulf Stream and Brazil Current). Pelagic squid show diurnal vertical migrations, rising from 400-600 m to surface at night, thereby extending their trophic niche and avoiding predation. Like many fish, pelagic squid school. Squid eat a wide range of prey and diet changes with growth. Initially small planktonic forms such as cyclopoid crustaceans are consumed, giving way in turn to euphausids, then fish (especially lantern fish) and other cephalopods. For instance squid dominate the prey of Illex illecebrosus through the summer months off the east coast of USA and are also important in the diet of Loligo pealei. Cannibalism is usually intra-cohort and this is an important cause of natural mortality. The pelagic biomass spectrum typically shows a series of peaks of body size, making it difficult for a growing predator to transfer from one peak to the next. The allometry of squid arms changes with respect to body size during growth, and this apparently allows the growing squid to make the transition from one peak to the next.

Rodhouse P G and Nigmatullin C M (1996) Role as consumers. In "The role of cephalopods in the world's oceans." M R Clarke (ed). Phil Trans Roy Soc, Lond., 351, 1003-1022.

FIGURE 2. Ommastrephid squid being caught by a Japanese jigger in the Falkland Islands fishery

Suspension feeding mechanisms in bivalves

Faculté des Sciences, Université de Nantes, 44322 Nantes, Cedex 03 France. Email: Beninger@svt.univ-nantes.fr

Until recently, the mechanisms of suspension-feeding in bivalves have been largely and inadequately inferred from indirect observations such as clearance experiments and the behaviour of particles deposited on dissected specimens. In the past decade, four innovative techniques have been brought to bear on the problem: in vivo video endoscopy, mucocyte mapping, in vivo confocal laser microscopy, and flow cytometry. These techniques have produced a wealth of information, and we now understand a very great deal of the mechanisms of suspension-feeding. Four general gill types exist within the Bivalvia, and each imposes different processing particularities on the entire pallial organ system. Fortunately, general principles are emerging. Two levels of division of labour are apparent: anatomical and biochemical. Pallial organ anatomy determines the particle flow characteristics and processing routes, cilia type determines particle-pallial organ interaction, and mucopolysaccharide type determines the nature of particle processing. The current state of knowledge is reviewed, with supporting video sequences.

Mutualism between aquatic plants and snails

J I Jones*, J W Eaton, K Hardwick, G Haynes, B Moss & J O Young

*School of Biological Sciences, Queen Mary & Westfield College, Mile End Road, London

The presence of a layer of periphyton (attached algae) on the submerged parts of aquatic plants is ubiquitous. Controversy exists as to whether the plants are simply inert platforms, hosts which mildly affect the behaviour of the periphyton, or part of an interacting system in which the plants exert considerable influence. If plants can exert an influence over their periphyton, there is the potential that a mutualistic relationship exists between the plants and invertebrate grazers, particularly snails.

In this mutualistic relationship the plants benefit from a reduced periphyton load, whilst, in return for preferential grazing, the snails gain an improved food source. The snail may also benefit in other ways such as protection from predators, and sites for oviposition. Given the apparent advantages to either partner, the potential for coevolution is strong.

Results were presented from a detailed experiment in which the nature of the plant (real and artificial plants of two growth forms, isoetid and elodeid), the nutrient loading, and the influence of the grazing snail Physa fontinalis (L.) were controlled. During, and at the end of, the controlled growth period, quantitative measures of all components were made, namely the the quantity and nutritional content (carbohydrate, protein and C:N) of the periphyton; the grazer's growth and population response; and the community composition of the periphyton.

Snail grazing greatly reduced periphyton abundance and encouraged plant growth and survival. Whilst in the absence of snails there were significant differences between the periphyton abundance on all four plant types, the real plants had little influence on the nutritional quality of the algae. The differential growth and reproduction of Phyta fontinalis found was best described by differences in the amount of periphyton available (see figure 1). The difference in periphyton community composition between the real and artificial plants was only as large as that between the two artificial plants.

It appears that submerged plants do not influence the nutritional quality of their periphyton, and that there has been no coevolution between the plants and snails. Any observed increases in the populations of snails and plants are the product of apparent mutualism. Such facilitation is a common phenomenon in food webs and is most easily described as "the enemy of my enemy is my friend".

For further detailed readings see:

Thomas JD (1990) Mutualistic interactions in freshwater modular systems with molluscan components. Advances in Ecological Research 20, 125-178.

Vandemer J, Hazlett B and Rathcke B (1985) Indirect facilitation and mutualism. In The biology of mutualism (D H Boucher, ed.) Croom Helm, London & Sydney.

FIGURE 3:

Ancovar shows that the influence of the four plant types on snail reproduction is not significantly different (Fslope = 0.09, Felev = 0.55, F0.05(1),3,43 = 2.84).

Foraging in intertidal grazing gastropods and its consequences for community structure.

Steve Hawkins*, P Della Santina, R C Thompson, M P Johnson, M S Davies, G Chelazzi, R N Hughes, M T Burrows & R G Hartnoll.

*University of Southampton and Port Erin Marine Lab, University of Liverpool

The traditional view of communities is one of functional groups interacting with other species or functional groups. This has been explored by the field experimental approach. Species can be experimentally removed or added to an assemblage and the consequences measured. An example of this approach is work spanning over 50 years on the Isle of Man started by Jones, Burrows, Lodge and Southward. This and follow-up work by ourselves (see Hawkins et al. 1992 for review) has shown that limpets prevent establishment of algae; patches of fucoids occasionally escape limpet grazing; these patches of Fucus can reduce barnacle settlement; the patches attract limpets and dogwhelks under them; eventually the patches of Fucus disappear leaving a local aggregation of limpets; new escapes of Fucus can occur in areas of low limpet density and high barnacle cover in the spaces between current patches of fucoids or groups of limpets in erstwhile patches.

In reality communities consist of groups (populations) of individuals of particular species. The EU-funded EUROROCK project intends to integrate studies at the individual, population and community level. It has taken advantage of technological advantages made at the University of Florence by Chelazzi and co-workers to monitor the spatial and temporal patterns of individual foraging excursions, particularly of limpets. A simpler approach to measure the spatial and temporal patterns of grazing intensity by limpets is the use of wax discs set into the rock on which radula marks are left (Thompson et al., 1977). Such information can be used as foraging rules in spatially explicit models or statistical analyses to explore the probabilities of escapes of algae from limpets or of barnacles from dogwhelks (e.g. Johnson et al., 1997). This is in contrast to the traditional approach of adding grazers or predators to give a spatial map of the probabilities of encounters. We are still, however, using manipulative field experiments in parallel with the individual-based approach.

This work was supported by NERC 'testable models in aquatic ecosystem' grant and the EU MAST EUROROCK project (MAS3-CT95-0012).

1. Hawkins SJ et al. (1992) In: Plant-Animal Interactions in the Marine Benthos. D M John, Hawkins SJ and Price JH (eds) Oxford University Press, 1-32.

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3. Chelazzi G et al. (1990) J. Moll. Stud. 56, 595-600.

4. Chelazzi G et al. (1983) Mar. Behav. Physiol. 10, 121-136.

5. Chelazzi G et al.(1994) J. Moll. Stud. 60, 123-128.

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