Trochus Biology

Trochus are mainly herbivorous and have separate sexes. However, males and females cannot be distinguished from external shell characteristics, but sex ratios in the wild are close to 1:1. Size at sexual maturity varies with location, being in the range of 50 - 70 mm. This is reached in two to three years and individuals are known to live for up to 12 years, but growth rates also vary with location (Nash, 1993). In WA, adults attain about 100 mm base shell diameter in about five years and their growth rate slows to below one millimetre per month.

Spawning

In their natural environment, trochus do not spawn as an entire adult population (that is, they spawn asynchronously). Individuals are believed to spawn once or twice each year, most actively in the summer months (Nash, 1985). Larger adults spawn less frequently, but can produce more than one million eggs. Animals may form spawning aggregates, usually in the evenings around the new moon or full moon phase. Spawning is initiated by males, which release sperm into the water column during the incoming tide, followed by release of eggs by nearby females.

A fertilised trochus egg measures about 200 mm and is surrounded by a pitted jelly layer of 200 - 300 mm thick (there are a thousand microns, µm, in a millimetre, mm).

See Crowe et al. (in prep) for detailed descriptions of developmental stages. At water temperatures of about 28 o - 30 o C, the first cell division occurs within 30 minutes after fertilisation; by the end of 12 hours, a fully developed trochophore larva is formed and is ready to hatch. Within 24 hours after fertilisation, the veliger larval stage is reached.

Larvae

Trochus larvae are 'planktonic', meaning they simply drift with the currents in the water column before they will settle onto a reef. During the planktonic phase, the larvae rely on egg reserves and do not feed actively. Larvae selectively settle, probably within crevices or holes on reefs with suitable algae and may use chemical cues from resident trochus. Under favourable conditions, the veliger larva settles onto a rocky surface and metamorphoses into a benthic (bottom-dwelling) trochus juvenile in three to four days ( Table 1). The settlement phase involves a high loss of larvae because of failure to settle and metamorphose.

The hatchery biology of trochus has much in common with abalone, for which great improvements have been made with settlement rates and supply of algae during the nursery phase (Daume, in prep.). Potential exists for further work by the Department of Fisheries on improving survival rates of trochus as well during the nursery phase in the Kimberley Region.

Informtation courtesy of the WA Department of Fisheries