Report on a Centenary Research Grant for 2003

Ontogeny of Mantle Cilia in the Bivalve Larvae of Crassostrea gigas and Ostrea edulis

SAMUEL STANTON, Institute of Marine Sciences, School of Biological Sciences, University of Portsmouth, Ferry Road, Portsmouth, PO4 9LY, UK sam....@port.ac.uk

The anatomy of bivalve veligers is much less understood than that of the adults. The study of bivalve larval anatomy and its ontogeny brings challenges of methodology, requiring the modification of electron microscopy techniques. This study used critical point drying techniques to investigate mantle ciliation, because evaporation drying via HMDS leaves mucus deposits which obscure the cilia. To investigate the mantle folds and their associated cilia patterns, larvae were cracked open using fine glass needles. Some larvae were also decalcified in ascorbic acid.
I am currently studying Crassostrea gigas and Ostrea edulis, although ultimately a number of other species of economic significance such as teredinids and pectinids will be included to provide a comparative account of the development of these structures. At present there is no comprehensive review, with its inherent consistency of method, covering several species.

Previous studies have identified complex, organised and varied groupings of cilia on the mantle of Pecten maximus larvae, with five distinct cilia types, each specific to regions of the mantle (Cragg, 1991). The present study found a similar organization in C. gigas larvae, and has followed the modification of this ciliation through development. The mantle rim of veliger stage larvae has few cilia; but the ciliation can be mapped according to the different cilia groupings. Along the mantle rim a bud structure (fig. 1), probably the developing gill bud, appears to be the starting point of the cilia tracts on the inner mantle rim.

Fig. 1. Cilia on the developing gill bud and beginnings of the twin tract on the inner mantle rim.

Originating on the gill bud/mantle junction, cilia appear in small groups of approximately 5 or 6, but without obvious organisation. From this point a twin tract of cilia spreads along the anterior mantle rim, steadily changing to a single tract, before finally becoming a less coherent scattering of cilia (fig. 3a). Changes in the overall pattern throughout larval development have begun to be mapped as shown in fig. 3.The mantle ciliation increases in both density and complexity through the pediveliger stage (see fig. 3b). The twin tract of cilia extends around the ventral region forming a dense area of ciliation before becoming less distinct towards the anterior.

Fig. 2. Microvillus bases on cilia of posterodorsal notch.

These cilia have been observed beating in live larvae, especially in moribund larvae where they are not obscured by the preoral velum cilia. This beating suggests a distinct function, possibly mantle cavity aeration or interaction with velum cilia. The region of the gill bud, a starting point for this cilia development in the veliger, becomes densely ciliated; and the gill itself also features rows of cilia along the margins of the folds.
In both the veliger and pediveliger stage, ciliation in the region of the anus is comparable to the anal grouping previously identified in O. edulis by Waller (1981). This postanal tuft includes an interesting cilia group with specialised cilia bearing a corona of approximately nine microvilli around the base of each cilium (fig. 2), suggesting a mechanoreceptive function.

Fig. 3. Preliminary ciliation maps for (a) veliger and (b) pediveliger

Waller (1981) proposed that the beating of this cilia group assists excrement evacuation; however this function fails to account for the association of this group with the posterodorsal notch in the shell. This shell feature, just posterior from the hinge, produces a slight gap between the valves, too small for faeces to exit. While the post-anal tuft may be involved in faeces evacuation, the group of coronate cilia appear slightly separate from that group (fig. 2). These specialised cilia also occur in a small tuft below the gill bud (fig .3), and very occasionally scattered amongst the ventral mantle cilia. TEM will be used to examine the cells bearing these specialised cilia.
SEM images will be used to continue the development of cilia 'maps' -extensions of those found in figure 3 - illustrating cilia location, grouping, size, morphological characteristics etc. However, SEM images alone can not fully reveal the form and function of the bivalve larval ciliature. Future investigations of the mantle will use TEM to provide sectional diagrams for the areas of interest on the ciliation maps, and confocal laser scanning microscopy for fluorescence imaging of catecholamines within the larvae, to investigate the nerve network in relation to the presence, form and function of the mantle ciliation, expanding the work of Croll et al. (1997).
I would like to thank Dr Simon Cragg and Dr Gordon Watson for guidance and support with my research, and the Malacological Society of London for financial support (Centenary Grant 2003) and the opportunity to present my work to a wider audience through the Molluscan Forums in 2003 and 2004.

References
S. M. Cragg, D.J. Crisp (1991). In Scallops: Biology, Ecology, and Aquaculture. (Elsevier, New York) pp. 75-132.
R. P. Croll, D. L. Jackson, E. E. Voronezhskaya (1997). Biological Bulletin 193, 116-124.
T. R. Waller (1981) Smithsonian Contributions to Zoology, 328: 1-70.