Malacological Society of London

An Evening Meeting of The Malacological Society of London and the Linnean Society of London at Burlington House, Piccadilly, London, on Thursday 21st January 1999, organised by Hugh Jones (University of Manchester).

Conus and the evolution of the hypodermic feeding mechanism

John Taylor (Natural History Museum, London)

Gastropods of the superfamily Conoidea (Toxoglossa) are notable for the possession of a large venom gland. together withhighly modified radular teeth which are used at the proboscis tip to inject the venom into the prey. The feeding process has been most studied in species of Conus which possess elongate, harpoon-like teeth. Most species feed upon polychaetes, some consume other gastropods and a few species eat fish. The Conoidea are a very speciose and disparate group of gastropods and it is now known that most species possess a venom gland and may use a wide variety of different kinds of radular teeth at the proboscis tip to envenomate their prey. Over the last ten years or so anatomical investigations of conoidean gastropods have revealed a great disparity of foregut anatomies, and characters defined in these studies have been used to help unravel phylogenetic relationships. However, the evolutionary pathways whereby hollow, harpoon-like teeth are derived from "normal" gastropod teeth are still unclear, as are the homologies between the different types of teeth found within the Conoidea.

Phylogenetic analyses of conoideans suggest the independent derivation of various forms of hypodermic teeth in six different clades. Some of these clades mostly comprise gastropods with rather different sorts of teeth, in particular the so-called "wishbbone" teeth of the Turridae, where each tooth appears divided into two limbs. It is difficult to envisage how enrolled teeth could be derived from these wishbone forms. However, study of the maturation of the wishbone teeth in the radular sac (with Yuri Kantor, Moscow) shows that they develop from initial flat plates by thickening of the tooth edges and elevation of one edge off the radular membrane. Various types of semi-enrolled and enrolled teeth develop in a similar way and there is thus no great difficulty in deriving hypodermic teeth from clades with wishbone teeth.

Sectioning of proboscis tips in many species of conoideans shows that different types of wishbone teeth are held at the proboscis tip. Thickening of the edges of the wishbone teeth probably imparts strength and stiffness, whilst the central groove may serve to channel venom. From this it can be envisaged that enrollment of the tooth would impart greater stiffness and the hollow centre would ensure more precise delivery of the venom to the tip of the tooth. In conclusion it is therefore not surprising that repeated evolution of hypodermic teeth has occurredin conoideans.

The venom apparatus of conoideans is considered to be homologous with the glandular dorsal folds and mid-oesophageal gland as found in other neogastropods. In conoideans the glandular folds are detached from the oesophagus and shortening of the anterior oesophagus would result in the gland opening into the posterior part of the buccal mass. This hypothesis is being tested (with Alex Ball, NHM London) by a study of the ontogeny of the foregut in the direct-developing Conus anemone.

Assessing predation levels in the fossil record

Elizabeth Harper (Department of Earth Sciences, University of Cambridge)

Predation, along with competition, has been fundamental in shaping the course of evolution. The increase in predation pressure at the beginning of the Phanerozoic has been at least partly implicated in the polyphyletic evolution of hard parts at the beginning of the Cambrian and it is believed that the morphology and life-habits of certain prey taxa, e.g. bivalves and brachiopods, have been greatly influenced by the evolution of their predators. It follows, therefore, that there is a legitimate interest in attempting to assess the evels of predation in the fossil record.

Unfortunately, many types of predatory activity are seldom traceable in the fossil record and many molluscan predators - such as the toxin-producing conoideans, wedging whelks and crushing cephalopods - fall into this category. However, there is hope when considering gastropods which bore holes into their prey.

Predatory gastropod boreholes are fairly easily recognised, being sharp-sided and circular in cross section, and generally more than 1 mm in diameter. Both muricid and naticid gastropods are well known Recent exponents of the habit, but in the '90s it has been noted that at least two other families have members which also make predatory boreholes.

The accepted fossil record of predatory gastropods dates back to the Lower Cretaceous. However, exceptionally preserved infaunal bivalves from a range of Jurassic localities in the UK have produced about 60 gastropod-like boreholes. These boreholes are so similar to those produced by Recent gastropods that, were they younger in age, the gastropod tag would not be questioned. The bored valves collected from Blockley occurred in sufficient numbers to allow assessment of the predation rate; at about 20% the level is comparable to that suffered by Recent malacofauna.
Although these beds do not contain members of families known to bore today, this does not preclude the possibility that other gastropods possessed the ability to bore 90 million years before the currently accepted date.

It is, of course, possible that non-gastropod taxa were responsible for these boreholes. If this is true, then we need to question the identification of other more widely accepted fossil gastropod boreholes.

Feeding Behaviour of Euglandina

Anthony Cook (Educational Development Unit, University of Ulster)

Euglandina is a native of the South-Eastern United States and a highly specialised predator of gastropods. It has a long narrow body for penetrating the whorls of snail shells, a radula reduced to a few stabbing teeth and elongated lips which are involved in prey trail detection.

Feeding commences with trail following and the inflated mouth parts are everted after contact has been made with the prey, but the subsequent events vary with the type of prey being consumed. Small snails are swallowed whole, large snail shells are rotated to expose the soft parts which are then extracted, and slugs are sucked up and swallowed either whole or piecemeal.

Snails have no active defence against predation and all snails offered as prey which were attacked, were consumed. Deroceras species move rapidly and respond with tail flicking and mantle flaring to being cntacted. This is an effective defence and about 20% of slugs escape being eaten. This is not the only form of defence, however, since some trails are followed less than expected. Some native species might be protected by their spatial distribution.

Observations on feeding preferences of Euglandina show that small species are consumed in preference to large, especially if the shell can be swallowed whole. Euglandina will spend an apparently disproportionate time in attempting to consume prey remnants and whole shells. These observations and analyses of prey content indicate that a component of feeding behaviour is controlled by a high demand for calcium.

Introductions of Euglandina in attempts to control large pest species such as Achatina have largely failed and are posing a major threat to native island gastropod faunas. Most threatened species are relatively small and the hope that Euglandina will control large species in the presence of smaller ones is ill founded.

Euglandina approaching a Trichia in the laboratory

The pros and cons of fishing disturbance for whelks

Michel J Kaiser (School of Ocean Sciences, University of Wales - Bangor)

Bottom fishing is one of the most wide-spread forms of human disturbance of seabed habitats. Fishing removes non-target species from the seabed and damages and kills animals in the trawl path. This re-directs energy within the marine ecosystem such that it is then available to scavenging and predatory species such as whelks. Experimental and field studies have demonstrated that whelks respond to the odours of animals killed by trawling and are able to capitalise on this additional food source. However, whelks are unable to compete with or are deterred by dense aggregations of competitor species such as hermit crabs and starfish that also predate whelks. Contrary to predictions based on previous studies, whelk survival after contact with fishing gears is severely reduced as a result of their increased vulnerability to predators. The reason why apparently undamaged whelks are less able to effect escape responses after they have passed through a fishing gear is unclear. Whelks are now extinct from the Wadden Sea and evidence collected in the field suggests that this is almost certainly due to bottom fishing activities and not the occurrence of TBT in the local environment.

Figure: Bag baited with animals killed in the catch of a beam trawl attracted a variety of scavengers including whelks, hermit crabs and starfish.

					..Click An Image To Visit Society Website