AbstractCilia are of particular importance to the planktonic veliger larva, fulfilling feeding, locomotory and sensory roles. This study concentrates on the ciliation of the larval mantle and velum. The mantle is an important adult organ yet one scarcely studied in the larval form. The larval velum is the characteristic swimming and feeding organ of the veliger larval form. Knowledge of the ciliary arrangement of larval bivalves, especially in relation to different life history strategies, provides important information for arguments over the phylogeny of such characteristics within the Mollusca. Anatomical work explored the possibility of ciliary sense organs being found on the mantle and further elucidates the pattern of distribution of cilia on the velum. Larval ciliary swimming in varying water temperatures has been investigated via the development of a filming methodology, revealing the thermal tolerance of swimming using velar ciliation and the ability of the larva to detect and respond to temperature stimuli.
Cilia were investigated throughout larval development from veliger stage to metamorphosis in two bivalve families, the Ostreidae (Crassostrea gigas and Ostrea edulis) and the Teredinidae (Lyrodus pedicellatus). The three larvae represented three developmental modes with C. gigas planktonic, O. edulis partly brooded and L. pedicellatus long-term brooded. Scanning electron microscopy (SEM) and light microscopy was used for anatomical investigations. Confocal laser scanning microscopy was used to locate catecholamines and serotonin in C. gigas larvae, with reference to the cilia locations identified by SEM. Swimming larvae were filmed in fixed and changing temperatures to identify behavioural or physiological reactions.
The mantle of the ostreid larvae had 13 different cilia groups which have been identified and photographed in C. gigas and O. edulis. Two of these groups appear to be sensory. One group is associated with the developing gill bud, and the other is located directly under the posterodorsal shell notch. The location of this group under the shell notch also was the location for several cells containing catecholamines. The ventral inner mantle rim of ostreid larvae has large tracts of cilia, probably primitive larval versions of adult rejection tracts. There were several flask-shaped cells with catecholamines and two long serotonin containing fibres in the mantle where these tracts were found. The mantle of L. pedicellatus had 2 distinct groups of cilia. One of these groups formed discrete clumps on the inner mantle fold and featured sensory structures. The velar ciliation of both the ostreid species larvae was almost identical, and both revealed a previously unreported band of cilia. This band may increase the efficiency of the opposed band particle capture system. The velum of L. pedicellatus was different from the ostreid larvae; its velar ciliature was similar to other long-term brooded larvae recorded in the literature with a large adoral tract and the loss of the post-oral band.
Larval swimming velocity increased with increasing water temperature. Acclimated larvae were able to swim in water temperatures where larval activity stopped during sudden temperature changes. Changes in larval swimming during sudden temperature changes are probably due to an interaction of three factors: water viscosity changes affecting cilia efficiency, the physiological effect of temperature and behavioural responses.
The additional ciliary band on the ostreid velum probably increases particle capture efficiency, and further investigations are needed to determine if this is a characteristic unique to the ostreids. The velar ciliation of the brooded L. pedicellatus is, by the pediveliger stage, similarly modified to the velum of other brooded larvae from widely separated taxonomic groups. Mantle ciliation is more extensive in ostreids than in teredinids, but both have sensory stereocilia on the mantle. The presence of catecholamines and serotonin suggests that the larva has a measure of control both of mantle and velar ciliary beat, seen in the variation in swimming patterns and particle rejection.
The filming and analytical techniques developed offer an accessible method for gathering laboratory data on larval behaviours. Analysis of swimming suggested larvae can sense changes in temperature and effect a behavioural response to it. Such responses may enable larvae to control their vertical position in the water column when encountering environmental discontinuities or heterogeneities such as thermoclines. Variation in larval swimming velocity between larval batches suggests this method could also be used as a quick assay to determine the fitness of different larval batches within a hatchery.
|Date of Award||Mar 2012|
|Supervisor||Simon Cragg (Supervisor) & Gordon Watson (Supervisor)|