The Veliger

Larval shell: The valves of the shell appear on either side of the body on the dorsal surface. As the valves increase in size, they grow together and meet along the hinge line. The fully formed larval shell or prodissoconch is equivalvular, each valve being discoidal with a straight hinge line and moderately convex externally. Distinct concentric growth lines are present, some of which are more prominent than others. The edges of the valves are entire and distinct and further apart ventrally. As the larva approaches the settling stage, the straight hinge becomes slightly and evenly curled and the lower left valve becomes deeper so that both valves are no longer equivalvular.

In the early stages of shell growth, the initial tendency is for the shell to elongate rather than to deepen. When the shell measures approximately 0.42mm × 0.28mm it deepens more rapidly so that when the larva is ready to settle, the shell measures approximately 0.47mm × 0.40mm. These measurements are in direct contrast to those given for C. virginica and O. edulis where the lengthening and deepening of the shell keep pace. The height-length relationship of the larval shell of O. lutaria, O. virginica and O. edulis is shown in Text-fig. 3.

The velum: The velum is very well developed, conspicuous and protrudes anteriorly from between the valves of the shell. With the development of the velum, velar retractor muscles become differentiated (Text-fig. 4, A). These muscles originate in the velum and are inserted onto the shell. The general outer surface of the velum is ciliated as was the prototroch. The marginal cilia are 100μ tall and are more powerful than the cilia covering the outer surface of the velum (Text-fig. 4, E and F).

The alimentary canal: Associated with the velum is the mouth which is ventral in position. The cilia of the velum become continuous with the cilia surrounding the mouth but at the same time become noticeably shorter until they are only 20μ in length (Text-fig. 4, C). The mouth opens into the oesophagus which passes back dorsally between the base of the velum and the foot. The stomach occupies the central mass of the body bounded anteriorly by the velum and posteriorly by the gill buds and the rectum. The right and left sides of the stomach are surrounded by the lobes of the digestive diverticula. In O. lutaria, the coiling of the intestine is first visible where it passes downwards and backwards as the rectum (Text-fig. 4, E). The rectum terminates in the anus which opens into the mantle cavity behind the posterior mantle suture.

The gills: The gills are first visible as a series of knobs extending from the mantle margin beneath the foot to a position near the posteroventral margin of the stomach. As the larva develops, the gill knobs become more distinct and segmented, the outermost knobs being smaller than the innermost knobs. In a fully developed larva there are approximately nine or ten gill filaments in the series. The filaments separate at the time of settling, and active cilia were noted on the surfaces of the larger filaments.

Adductor muscles: The adductor muscles are developed in the same manner as recorded for other oysters. The strength of the anterior and posterior adductor page 18

Text-fig. 4.—Fig. A—Trochosphere with velar retractor muscle visible. Fig. B—Trochosphere with shell developing. Fig. C—Trochosphere at later stage. Fig. D—Trochosphere with position of alimentary canal visible. Fig. E—Straight-hinged veliger. Fig. F—Straight-hinged veliger in swimming position.

Abbreviations: A., anus; a.a., anterior adductor muscle; d.d., digestive diverticulum; fo., foot; g., gill; l.v., left valve; m.e., mantle edge; mo., mouth; o.c., oral cilia; rec., rectum; r.m.e., right mantle edge; r.v., right valve; v.e., hinge region of valve; vel., velum; vel.r., velar retractor muscle; v.h., hinge.

page 19 muscles was particularly noticed when a 20% solution of cocain was added to the sea water containing the larvae. The solution caused the adductor muscles to close the shell so tightly that the velum was cut off.

The mantle: In the veliger the first region of the mantle to be observed was the margin (the mantle lobes are not easy to observe until the larva has become fully developed). In the early veliger, the mantle margin is yellow and the mantle lobes can be seen in the living larva moving backwards and forwards beneath the valves of the shell.

The foot: In the early veliger, the foot is represented by a small projection on the ventral surface, beneath the mouth (Text-fig. 4, A). As the veliger develops, the foot lengthens until finally it is as long as the valves. When not in use, the foot is withdrawn and contained entirely within the mantle cavity beneath the mouth. As the foot begins to protrude, a groove becomes apparent on the under surface. This groove is ciliated as is the entire outer surface of the foot. The two sides of the groove are capable of coming together and closing over the groove, thus forming a canal. As the foot protrudes even further, the groove becomes shallower. The fully extended foot is long, slender and strap-shaped. It is capable of moving anteriorly over the velum, posteriorly behind the gills and over the outer surfaces of the valves. Posteriorly behind the foot, is a heel. This heel is only observed when the foot is fully extended. Stafford (1913, p. 46) refers to this heel as the byssus-papilla and although the byssus gland was found in O. lutaria, the duct leading to the papilla was not (Text-fig. 5, B).

Otocysts: Stafford (1913, p. 50) observed about a dozen small otoconia in each cyst. The right and left cysts are situated at the base of the foot near the surface of the first gill filament. On a few occasions when the veliger foot was fully extended, structures that could have been the otocysts were observed, but confirmation of the presence or absence of otocysts awaits further investigation.

Nerve ganglia: The only nerve ganglia observed in O. lutaria were the pedal and cephalic ganglia. The former are situated in the proximal region of the foot and are only visible when the foot is fully extended. The cephalic ganglia are associated with the apical sensory organ which is situated in the central region of the velum (Text-fig. 5, A and B).

Pigment spots: The pigment spots are sometimes called eyespots but are better known as the former since their photosensitivity has not been confirmed. The pigment spots are paired, and lie on the lateral walls of the right and left mantle lobes just anterior to the proximal region of the gill buds. They are almost black in colour, irregular in outline and measure approximately 12μ in diameter (Text-fig. 5, A, B and C).

Duration of the incubation period.

Because artificial fertilisation was unsuccessful with O. lutaria, the duration of the incubation period was studied by regular examination and measurement of larvae liberated by oysters held in the laboratory.

A larva takes approximately five days from fertilisation to develop into an early veliger. The time of development between an early veliger and a middle veliger without pigment spots is approximately three days. The shell measurements of early veligers are 0.22mm × 0.18mm (length × height) and total measurements are between 0.32mm × 0.29mm and 0.33mm × 0.32mm. The shell measurements of the middle veliger are between 0.30mm × 0.22mm and 0.31mm × 0.19mm and total measurements are between 0.40mm × 0.29 and 0.35mm × 0.32mm. The page 20

Text-fig. 5.—Fig. A—Veliger with foot fully extended. Fig. B—Veliger with velum and foot in swimming position. Fig. C—Veliger with velum and foot withdrawn. Fig. D—Spat settled for approximately 72 hours.

Abbreviations: A., anus; a.a., anterior adductor muscle; an.s., anterior suture; a.s., apical sensory organ; b.g., byssus gland; diss.s., dissoconch shell; e.s., eyespot; fo., foot; g.p., gill plate; h., hinge; he., heart; int., intestine; lab.p., labial palp; m.e.m., mantle edge; mo., mouth; p.a., posterior adductor muscle; p.g., pedal ganglion; prod., prodissoconch; p.s., posterior suture; rec, rectum; r.v., radial vessel; vel., velum; vis.m., visceral mass.

page 21 total time taken for an early veliger to develop into a veliger with pigment spots is unknown, but observations made on oysters held in the laboratory in sea water between 18.0°C. and 20.0°C., suggest that it is five to eight days. A fully developed veliger measuring 0.47mm × 0.35mm is liberated after about a further six days. Thus the total time of development within the mantle chamber appears to be about 21 days.

The stage of development reached by the larva under natural conditions before it is liberated is discussed in the section on the free swimming larva, but briefly if O. lutaria follows other incubatory species of oysters, the larvae will be liberated when they have attained a size of about 0.32mm × 0.26mm. Larvae of O. lutaria with these measurements were liberated in the laboratory, but more fully developed larvae complete with foot and pigment spots were also observed being liberated from the parent indicating that if conditions are favourable the parent oyster will incubate the larvae through all the stages of development until the latter are ready to settle.

Liberation: In O. lutaria the developing larvae are liberated by violent contraction of the posterior adductor muscle of the parent. This fast closure of the shell forces the larvae out through a gap in the inner pallial fold as in other incubatory species.

Liberation of the larvae in some cases was completed within an hour; in other instances the liberation took as many as four days to complete. The only regular feature of liberation that was observed was that the majority of larvae were liberated in the first two or three rapid closures of the shell and that subsequent liberations yielded very much lower number of larvae. Towards the end of liberation, larvae appeared to "spill" out of the shell and over the left valve to the dish beneath. The last few larvae were always held in strands of mucus that was probably secreted by the gills during the process of feeding.