Morphometrics and reproduction of Terebrasabella heterouncinata (Polychaeta : sabellidae), infesting abalone (Haliotis midae) from different culture environments

by Gray, Michael

In the late 1980’s abalone culturalists noticed reduced growth rate and shell deformities in some abalone stocks. These problems were the result of infestations by a shell boring polychaete, Terebrasabella heterouncinata. Under intensive abalone culture conditions the level of infestation can reach epidemic proportions and there are often severe consequences for the host abalone. Heavy sabellid infestation levels have placed the economic viability of several South African farms under threat. This study formed part of an ongoing project that is aimed at investigating the basic biology of Terebrasabella heterouncinata. The majority of abalone farmers in South Africa feed their abalone either naturally occurring kelp (Ecklonia maxima) or the formulated abalone feed, Abfeed. Farmers have suggested that the use of Abfeed is associated with higher sabellid infestation levels and changing the abalone diet from Abfeed to kelp helps reduce sabellid infestation. Speculation has arisen indicating that older, slower growing abalone are more susceptible to sabellid infestation. The effect of host abalone diet history and their growth on sabellid settlement success, morphometrics and reproduction was quantified. To better understand the plasticity of the expression of life history traits the variability of morphometric and reproductive characteristics was compared between different farm environments.

A change in abalone diet from kelp to Abfeed resulted in smaller adult sabellids (p?0.001), larvae (p?0.001) and eggs (p?0.0001), compared to sabellids from abalone that experienced a diet change from Abfeed to kelp. Abalone diet history and growth rate did not influence the occupation level of tubes (p>0.05) or the sabellid intensity (i.e. number of tubes per centimeter of the shell edge) (p>0.05). Sabellids from slower growing abalone were larger in various body measurements and other characteristics; (length (p<0.0001); base width (p<0.0001); stage 2 larvae length (p<0.001); egg volume (p<0.001); number of stage 1 and stage 2 larvae per adult (p?0.0004); and number of eggs per brood (p?0.0001). The combined effect of slow abalone growth and the feeding of Abfeed resulted in increased number and size of the sabellids, indicating a confounding effect of these two conditions.

This study suggests that sabellids are essentially K- selected, exhibiting variation in reproductive and morphometric characteristics under different conditions. The number of larvae per adult (CV= 113- 79%), number of eggs per brood (CV= 86- 58%), sabellid intensity (CV= 79- 39%) and number of larvae per egg (CV= 126- 84%) were the lifehistory-related variables that exhibited the greatest variation for all studies. The smallest variation in sabellid characteristics included the larval length (CV= 11- 17%), base width (CV= 12- 31%) and occupation level (CV= 19- 27%). Thus, in all studies the numbers of individuals of the life-cycle stages were more variable than the sizes. The greatest variation occurred between the farms with the least variation occurring between abalone of different growth rate. Larval settlement was greatest on the thinnest and fastest growing region of the shell edge. Larvae settled most successfully at high tides. This study suggests that sabellid larval settlement is principally determined by abalone shell region, then by a change in diet, and least by abalone growth.