The Larval Development and Juvenile Growth of the Silver Mouth Turban, Turbo argyrostomus.

Abstract

The silver mouth turban, Turbo argyrostomus L. 1758, is an archeogastropod widely distributed throughout the Indo-Pacific region. It is an important marine resource, harvested for food, marine souvenirs and ornamental items. Mass seed production techniques for Turbo marmoratus and T. argyrostomus, based on the techniques developed for Trochus niloticus in Palau, are currently being developed in the Okinawa prefecture, Japan, to replenish the declining populations of these commercial turbinids. Here, I report on the larval development, metamorphosis and the effect of diet on growth and shell characters of silver mouth turban juveniles, raised in the laboratory. I studied the larval development, metamorphosis and juvenile growth of T. argyrostomus between August 1995 and July 1996. Adults collected from Komesu, southern Okinawa Island, were induced to spawn by packing and UV sterilised seawater treatment on 9 August 1995. The green eggs (180-190 ~km in diameter) hatched within 18 hours, underwent torsion a few hours later and metamorphosed in 3-4 days after fertilization in the presence of metamorphosis inducing cues. Forty-eight percent of 200 competent veligers spontaneously metamorphosed in the absence of metamorphosis inducing cues between 7 days and 21 days after fertilization without feeding. The protoconch, 170-185 ~krn diameter, was colorless and globose in shape with irregular articulations. The juveniles grew to a mean shell length of 3.7 mm in 5 months (mean growth rate of 0.7 mm per month) feeding on microalgae growing on coral rubble and the sides of the aquaria, and grew to 11.5 mm in 9 months (mean growth rate of 2.7 mm per month) feeding on macroalgae. Small juveniles of mean shell length 3.9 mm, reared on a small fleshy red algae, Gelidiella acerosa, grew approximately 2 times faster than those reared on the green algae, Ulva purtusa, over 18 weeks. The combined diet of these two algae species gave higher growth rates than either algae individually, but did not improve growth significantly in large juveniles with an initial shell length of 5.8 mm. Percentage survival after 18 weeks was higher for juveniles reared on red algae (92.2 %,) compared to green algae (81.1%). The shell color of the snails depended on the algal diet. The shells of juveniles collected from the reef were heavier, for the same size, than those reared in the laboratory. The juveniles reared on mixed algae growing on coral rubble, collected from the reef, and the green algae had higher mean shell weights than snails reared on red algae alone or on a combination of green and red algae.