Feature

Polychaetes that infest perlemoen shells - they're not just boring worms!

The anterior half of the Boccardia species, a polydorid worm. Note the tentacle on the head. Image by Carol Simon

Molluscs like oysters and perlemoen (abalone) are host to a wide range of fascinating worms called polychaetes. These worms live in tubes built on, or bored into, the host's shell. Under natural conditions these worms contribute to the recycling of calcium by breaking down the shells of their hosts when they die, but become problematic when they infest cultured shellfish, reducing their growth rate and market value and increasing their mortality.

With the growth of the perlemoen industry in Southern Africa, shell-infesting worms are becoming increasingly problematic. The most conspicuous shell-infesters belong to the polydorid group and were first recorded as pests 120 years ago. 

The feather-duster worm, Terebrasabella heterouncinata. This is an hermaphrodite with the eggs and sperm visible inside the body. Note the feeding crown which is closed. Image by Carol Simon

The first genus described in this group is Polydora, named after a mythical sea-nymph - probably because they look like dancing girls when they wave the tentacles on their heads in the water when looking for food. The larvae of most species settle on the surface of the shell and then bore into it.

When the burrow penetrates the inner surface of the shell, the host repairs the shell with nacre (mother of pearl shell), forming the blister from which these worms get their common name, blister worm. Because these worms are up to 4 cm long, they can cause considerable damage to the shell. 

Other species are called surface foulers because they build tubes of sediment on the surface of the shell. They do not harm the host, but they may shelter the larvae of boring species. Finally, some species are secondary borers - they don't normally bore into shell, but may settle in, and excavate, abandoned burrows. Cultured perlemoen in South Africa are infested by three species: the true borers Polydora hoplura and Dipolydora capensis, and an unidentified Boccardia species, a secondary borer. 

A perlemoen shell that was damaged by blister-worm infestation. The arrows point to the blisters. Image by Carol Simon

Another worm that affects cultured perlemoen is the small (< 5 mm) Terebrasabella heterouncinata. This type of polychaete has a characteristic feeding crown and is commonly called a feather-duster worm. The larvae of this worm settle on the inner lip of the shell-edge where they stimulate the host to cover them with nacre, forming the burrow. When high concentrations of these larvae settle on the shell-lip, the perlemoen produces so much nacre that the shell becomes deformed and brittle.

A perlemoen shell showing damage caused by the secondary borer, the Boccardia species. Image by Carol Simon

A perlemoen shell that had been infested by the feather-duster worm. Infestation weakened the shell, resulting in the breakage (arrow). Image by Carol Simon

The success of blister and feather-duster worms on farms may be attributed to two factors. The first is related to their reproductive mechanisms that lead to a high fertilisation success and the increased survival of larvae. Eggs are laid in the mother's burrow where they develop into larvae. The animals are therefore protected during the most sensitive stage of their development. 

The survival of the larvae is also increased because they don't have to hunt for food to survive; most rely on their yolk for food, but some are adelphophagic - only some of the eggs are fertilised and develop into larvae, and these feed on the undeveloped eggs which are called nurse eggs. When eggs are kept in the burrow, they need to be fertilised there - males therefore release their sperm into the water while the females collect them (with their tentacles or feeding crowns) and fertilise their eggs as they are laid. 

A perlemoen farm, showing the on-shore tanks in which the baskets of abalone are suspended. Image by Peter Britz

The result is that fertilisation success is high compared to the hit and miss affair that occurs when polychaetes release both eggs and sperm into the water.

The second factor is related to the nature of the farm environment. Farmed perlemoen are held in baskets suspended in huge on-shore tanks with water pumped in from the sea. Naturally, the density of the abalone in these baskets is significantly higher than it would be in the wild. This means that the larvae, which may crawl or swim, don't have to move far to find a new host, and this reduces the death rate of larvae (which are food for many sea creatures) after they leave their mothers' burrows. So, the density of the worms on the farms will keep increasing, producing an increased supply of larvae to re-infest the perlemoen. 

A close-up of a basket of abalone feeding on kelp. A basket like this can contain about 300 abalone at market size. Image by Peter Britz

Furthermore, the water in the tanks contains high concentrations of degraded perlemoen food (kelp or a nutritious artificial feed), as well as faeces which the worms love to feed off. Under such conditions, the worms thrive.

The presence of shell-infesting polychaetes on farmed perlemoen is unavoidable, but the perlemoen will not be damaged if the number of worms is kept low. To do this, farmers must keep the tanks clean and concentrations of 'worm food' low.

More information:

Dr Carol Simon is a Postdoctoral Fellow in the Department of Zoology and Entomology at Rhodes University in South Africa

Email: c.simon@ru.ac.za 

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