Feature

Warmer water boosts perlemoen pests, researchers show

When large numbers of worms settle and form burrows on the inner curve of the shell, the abalone covers it with shell, trying to wall them off. This process will eventually weaken the shell, causing it to break. Photo: Carol Simon

The warmer the sea water, the more vigilant abalone (perlemoen) farmers should be in controlling worm pests called polydorids that are found on abalone and other commercially valuable molluscs.

This is the finding of Dr Carol Simon of the Department of Botany and Zoology at Stellenbosch University, and Prof Tony Booth of Rhodes University, in a recent study published in the African Journal of Marine Science.

Molluscs like oysters and abalone are host to a wide range of worms called polychaetes that live in tubes built on, or bored into, the host’s shell. The most conspicuous shell-infesters belong to the polydorid group and were first recorded as pests 120 years ago.

The worms, which can be up to 4cm long, can cause considerable damage to the 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,” Dr Simon explains.

The current study was undertaken on a West Coast abalone farm and another along the Overberg coast.

The researchers say that abalone feeding regime and water temperature can influence the growth rate, size of maturity, maximum size, infestation intensity, recruitment, percentage of the population brooding and mortality of the pest worms.

“Infestation numbers were significantly higher during mid- to late winter/early spring when water temperature and day length increased,” Dr Simon explains. “Treatment measures should therefore be implemented throughout the year but with increased effort when water temperature increases.

Farm-specific conditions such as water temperature, the energy content of particulate organic matter in the raceways in which the abalone are kept, and the location of the farm may all influence the intensity of infestation and the levels of reproductive activity of the worms.

“These factors need to be considered in controlling infestation and may also influence which species become problematic.”

In a previous study, Dr Simon and her colleagues also found that the abalone will not be damaged if the number of worms is kept low. To ensure this, farmers must clean their holding tanks regularly and keep the concentration of “worm food” low, to remove high concentrations of degraded perlemoen food (such as kelp or a nutritional artificial feed) and faeces on which the worms thrive.

“With the growth of the abalone industry in South Africa, shell-infesting worms are becoming increasingly problematic,” Dr Simon believes.

Pest polydorids also infest cultured shellfish in Australia and New Zealand.

More information:

 Dr Carol Simon, Department of Botany and Zoology, Stellenbosch University(021) 808 3068 csimon@sun.ac.za 

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