Larval Development

Pearl oysters release sperm and eggs into the water, and this is where fertilization takes place. The unfertilized eggs are irregular, becoming spherical when fertilized. The larval stage ranges from 16 to 30 days depending on temperature, nutrition and the availability of settlement substrates. D-stage veligers develop preliminary growth rings in the shell 1–2 days after fertilization and begin to develop opposing umbos, which arise dorsally above the hinge axis. Veligers swim by means of their ciliated velum and, being positively phototaxic, remain near the surface (Nayar and Mahadvan 1987). As the larvae approach settlement, a foot develops by which the larvae can crawl about the substrate while searching for a suitable place to settle. Larvae develop a pigmented spot, commonly called the “eye-spot”, on either side of the base of the foot and ctenidial ridges begin to develop. Larvae are able to control the settlement location by shortening or prolonging the planktonic and crawling preveliger stages. After settlement, some motility is retained and the foot can be used to crawl away from unfavorable conditions. Juveniles use byssal threads to attach themselves to the substrate (Allen 1906).

Early development follows the typical marine bivalve pattern of trochophore, D-stage veliger, umbo stage, eye-spot stage, pediveliger, metamorphosis and newly settled spat. This in total takes 3–4 weeks. Development rates are particularly influenced by food availability and stored fat is probably the primary energy reserve used during metamorphosis.

Pinctada maragritifera larvae exhibit an average daily growth rate of 3.7- 5μm (Tanaka et al. 1970). Growth rates are exponential and are likely to be influenced by genetics as well as endogenous and exogenous nutrition and conditions.