Invasive, shell-boring polychaetes are a word-wide problem where molluscs and bivalves are intensively farmed. So too in South Africa where farmers of cultured abalone have lost millions to polychaetes.
The infestation impacts negatively on production and the quality of the farmed animals. The conditions in an abalone farm are ideal for the proliferation of polychaetes.
Hygiene has been identified as the only way to limit the impact of these invasive worms.
Dr Carol Simon, polychaete researcher and lecturer at the University of Stellenbosch in South Africa explained that several different families of polychaete worms infest molluscs worldwide; in South Africa the most problematic are the sabellid terebrasabella heterouncinata and three species of spionids belonging to the polydora-group: boccardia proboscidea, polydora hoplura and dipolydora capensis. The last species is the least problematic. The general common name for sabellids is "feather duster" worms. Polydora-type worms that infest molluscs are collectively referred to as either mud-worms or blister-worms.
Infestation with shell-boring polychaetes has a far-reaching impact on the production of top quality abalone. All three species do not affect the abalone when present in low densities; however a high-density infestation could result in production and quality losses.
The worms infest only the shells but, because the host abalone has to repair the shell, energy is diverted from flesh growth to shell growth. Abalone infested by mud/blister worms cannot be sold and must be culled.
Control is of economic importance to the mariculture industry in South Africa where abalone (Haliotis midae) is produced on land-based farms. Of the in excess of 800 tonnes of abalone produced a year by the ten operational abalone farms in South Africa, approximately half is exported as live product and the other as canned product.
Abagold, one of three abalone farms in the Hermanus abalone farming hub, on the southern coast of the Western Cape Province of South Africa, has a production of 200 wet tonnes of live abalone a year, said Stoffel van Dyk, Operational Manager, Abagold.
The abalone are harvested at 100 to 200 grams, which requires three to four years of growth.
From Dr Simon's observations, it seems young abalone are far more susceptible to infestation impact than the older abalone.
The financial impact of polychaete infestation is difficult to quantify. What is important is that farms where polychaete controls are in place are experiencing fewer losses and lower culling levels. Proper farm management is able to limit polychaete infestation to low or moderate levels.
On an abalone farm, shell-infesting polychaetes are in a more secure and food-rich environment than in the wild. Water flow in an abalone farm is far slower, and less energetic, than in their wild habitat.
These worms thrive on the nutrient-rich degraded pellet feed and faeces. The use of a formulated feed, in comparison with fresh kelp (Ecklonia maxima), results in faeces with a higher level of protein and energy, promoting a higher rate of polychaete reproduction and quality of offspring.
The sabellids are purely filter feeders, Dr Simon pointed out. The spionids are deposit feeders with some degree of filter feeding. They "catch" food on their palps, then move it along the feeding groove of the palp to the mouth. They can also physically pick food up from the sediment around them.
Breeding and recruitment occurs throughout the year, said Dr Simon, with an increase in late winter and early spring as the water begins to warm up. All the worms are sedentary as adults. Thus the only time when they can disperse is as larvae.
"Suitable filtration of incoming water poses a problem," said Mr van Dyk. "We pump ashore seven million litres of seawater an hour. Our present filter has a 1,000-micron (one millimetre) screen but this is not effective against the minute eggs or larvae of pests being pumped in with the seawater. To be able to filter out these pests would be prohibitively costly."
Treatment
According to Dr Simon, treatment needs to be throughout the year, with an increase in attention and effort when recruitment increases, before the larvae have had time to begin making their burrows.
For the polydora-type species this depends on the water temperature. Farms on the southern cape coast receive a mixture of water from the Indian and Atlantic Oceans where the annual water temperature is higher than those on the western cape coast which draw their water directly by the cold Benguella Current of the Atlantic Ocean. Water temperature, and thus recruitment, on the southern coast increases from June onwards, peaking in August. On the west coast it is about a month later.
By contrast, Mike Gray, who did his MSc on sabellids at Rhodes University found that there were bimonthly peaks in recruitment which coincided with the full and new moons. Such cyclic patterns of recruitment are quite common in certain polychaete species.
Keeping it clean and simple
A fresh and clean environment has been identified as the best way to limit polychaete performance.
Water turnover in the 1900 production tanks at Abagold is once every two hours, thus requiring some seven million litres of seawater to be pumped ashore every hour. The water turnover and the sediment flushing system employed are considered suitable to limit sabellid success.
Medicating against shell-infesting polychaetes is not possible at present, said Mr van Dyk. Abalone are lower down on the ecological order than the polychaetes which means that anything that will kill the polychaetes will also kill the abalone. Because of this, no suitable medication has yet been developed.
There is also the situation that the worms are able to retreat into the microhabitat of their burrows while the abalone cannot seal themselves from the environment. This would be why, for example, an oyster is easier to treat with hot or fresh water because the animal can seal itself completely from the treatment.
The key, said Mr van Dyk, is simply to keep the abalone, and their living conditions, as clean as possible. On a farm the size of Abagold, which can have up to eight million abalone at any one time, and which draws up to seven million litres of seawater an hour from an area where many of the pest polychaetes naturally exist, this is a management nightmare.
As part of their polychaete management strategy, every abalone gets inspected for the presence of worm every four months. Clean and infested animals are kept in separate, clearly marked rows of tanks.
The equipment used to clean the different rows of tanks is kept separate. When staff move between the clean and infested rows or tanks, they are required to scrub their hands to ensure no eggs or worms are moved across to the clean tanks.
At the end of a production cycle, each tank is drained, the walls and floor scrubbed with a stiff broom and then washed down with high pressure water. No chemicals are used.
"We create an environment where the abalone perform well," said Mr van Dyk. "Unfortunately the polychaetes also perform well under those same conditions. The only way to limit worm infestation is to ensure organic waste is efficiently flushed out with the water flowing through the tank, thereby reducing their feeding and reproduction efficiency, and to isolate infested and non-infested animals."
Stephen James McVeigh
