Feature

Wormy Cockle Feet

Traditionally, ecologists have recognized the important role of highly virulent pathogens, such as the RCD virus, in the functioning of ecosystems. However, they have often dismissed the role of parasitic worms as absolutely trivial. There is growing evidence that they may have been wrong. Researchers at the University of Otago are showing that the effects of parasitic worms extend well beyond mild pathology.

The New Zealand cockle, Austrovenus stutchburyi, common on mudflats at low tide, is frequently infected by the trematode Curtuteria australis. Adult worms live in the intestine of the pied oystercatcher. Eggs released in the intestine of the host pass out in the faeces and hatch into tiny free-swimming larval stages.

These larvae penetrate whelks in which they multiply asexually. Each larva can produce several thousand new larvae, which exit from whelks and swim near the bottom until they are sucked into a cockle through its inhalant siphon. Inside the cockle, larvae encyst in the foot of their host and nowhere else. Numbers of cysts range from very few to more than one thousand per cockle. The life cycle is completed when an oystercatcher ingests an infected cockle; the larval worm emerges from its cyst and attaches to the bird's intestine.

Normally, cockles use their foot to burrow one to two centimetres below the surface of the sediments, using their siphons to obtain water from the surface. Postdoctoral fellow Frédéric Thomas and senior lecturer Robert Poulin have shown that parasite cysts replace muscle tissue in the cockle's foot and decrease foot growth. This produces variability in the cockle population, with individuals ranging from lightly-parasitised big-footed cockles capable of burrowing, to heavily-parasitised small-footed cockles unable to burrow properly.

This direct effect of the parasite on the cockle raises interesting questions regarding the ecological or evolutionary implications of parasite-induced foot stunting and reductions in burrowing ability.

For instance, are cockles with impaired burrowing ability more susceptible to predation by oystercatchers? Many parasites depend on predation of their current host for the successful completion of their life cycle. Often parasites manipulate their current host to make it more prone to predation by their next host. In the present case, the parasite completes its cycle when an oystercatcher eats an infected cockle. By preventing cockles from burrowing, the parasites make them more visible to foraging birds. Indeed, a field experiment showed that surface cockles are more vulnerable to oystercatchers than the ones that successfully burrowed. Thus the small effect of the parasite on foot growth in cockles has important repercussions.

The biodiversity of the intertidal zone may also depend on the action of the parasite. Many species of invertebrates live attached to cockle shells. Some are very specific to cockles and are hardly ever found attached on other substrates; they include a species of polychaete, a sea anemone, a limpet and a barnacle. Others (chitons, bryozoans, mussels, even some algae) use cockles as well as rocks for substrate. Juvenile crabs also live as commensals inside living cockles.

All these invertebrates cannot easily locate and attach to buried cockles; they are also unable to feed themselves if buried in the sediments. Is it possible that the parasites make cockles available to these invertebrates? Is it possible that without the parasite, the diversity of organisms in the intertidal zone would be decreased?

If so, then the parasite could be what ecologists refer to as a keystone species -- a species essential for the existence of other species in an ecosystem. These questions are currently under study, but the evidence so far suggests that this tiny, apparently insignificant parasite is a cornerstone of biodiversity in the intertidal zone!

Dr Robert Poulin, Department of Zoology, University of Otago

Dr Robert Poulin Department of Zoology University of Otago