4.3 SCIENTIFIC BACKGROUND – POST-LARVAL DEVELOPMENT

Post-larval development has been well described for some scallop species (Sastry, 1965; Cragg and Crisp, 1991). Following the conspicuous appearance of the foot, the behaviour of the veliger larva changes and may be considered as the beginning of the process of metamorphosis and preparation for settlement.

The appearance of the post larval organs and the actual attachment take place in a relatively short time, but the process of metamorphosis is not an abrupt change from a pelagic larva to an attached post-larval life. In scallop larvae, as in other bivalve larvae, metamorphosis involves changes in the nature of shell secretion, loss of some organs, greater development and/
or relocation of others. The principal organs lost at metamorphosis are the velum, the velar retractor muscles and the anterior adductor. Accounts on the loss of the velum vary, but most state that the velum is lost at the end of the pediveliger stage. The veliger actively swims with the extended velum, but as the velum begins to reduce in size the swimming activity decreases. Prior to the conspicuous development of the foot, the larvae alternately swim and rest on the bottom. At this time, veligers are often collected from the bottom of larval tanks rather than the surface.
There is a general migration, relative to the axes of the shell of those organs which survive metamorphosis.

This results in the mouth migrating from its posterio-ventral larval location to the adult anterior-dorsal position, the foot becomes ventral rather than posterior and the posterior adductor migrates to the centre of the valve. After metamorphosis, the gill filaments increase in length and number. Outer filaments of the adult gill system appear after metamorphosis. The time at which the gill becomes capable of filter feeding is uncertain. Some studies have shown that efficient filter feeding does not occur for spat <460 µm (Bourne, pers. comm.). The shape of the foot gradually changes after metamorphosis, becoming narrow with cilia on the free end. The foot is projected to the outside of shell and acts as a locomotory organ. The glands present in the pediveliger foot become better developed and the byssus secreted changes abruptly to a more sticky form after metamorphosis. The statocysts and eyespots are retained.
Immediately after attachment, the shell grows rapidly. The thin and fragile post-larval shell (dissoconch) is sharply demarcated from the thicker and more homogeneous larval shell. The shell margins extend rapidly, resulting in a complete change in the outward form of the newly settled larval scallop. The most important feature of dissoconch growth is the formation of the byssal notch and the gradual change in the shell to adult form. The byssal notch appears as an indentation on the right valve below the hinge line. The byssal notch narrows to form a groove; teeth appear on the byssal groove and are retained even in the adult shell. Toward the end of dissoconch development, the shell margins are semicircular in outline; the colour appears as small dashes on the shell and spreads over the entire shell by the end of dissoconch stage. Both valves of post-larval and pre-adult scallops show pigmentation.
At the end of the dissoconch stage, the pigmentation has extended over the entire shell. The young spat can now be easily recognized as a scallop. The only difference in the external shell morphology from the adults is the absence of ribs and prominent ears. Furthermore, the plications (ribs) appear on the shell at an average size of 1.175 mm.
The true ears characteristic of adults make their appearance as indentations on the anterior and posterior margins below the hinge line. The only difference at the end of post-larval development with the adults is the absence of gonads.
A series of photos showing the development of Euvola ziczac spat is given in Figure 4.1.
Day 8 spat Day 23 spat 2-months old scallop

Figure 4.1: Development of sand scallop, E. ziczac, following settlement, showing dissoconch in Day-8 scallops, byssal notch formation and pigmentation in Day-23 scallops and similarity to adults in 2 months old scallops.