The brooding process in the Chilean oyster, Ostrea chilensis (Philippi, 1845)

Chaparro, Oscar R. (1996) The brooding process in the Chilean oyster, Ostrea chilensis (Philippi, 1845). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Most bivalve molluscs are broadcast spawners, but several have evolved mechanisms for brooding the embryos and larvae. In the research described in this thesis, the Chilean oyster Ostrea chilensis is used as a model to allow us to understand more fully the efficiency of the brooding process and the role of this mode of reproduction as an adaptation to an estuarine environment. The reproductive biology of the adults, the brooding mechanism, and the physiology, behaviour, energetics and morphology of the brooded larvae were studied in oysters from the Quempillen estuary in southern Chile. -- In the Chilean oyster, reproduction occurs once per year, during spring and early summer. During the three years of the study, approximately 40% of the potential females in the population exhibited brooding behaviour. The brooding period was approximately seven to eight weeks, during which larvae lived in the infrabranchial chamber of the female, and grew until they reached approximately 500 jam shell length, after which they were released. Brooded embryos and larvae utilize protein and lipid as the principal energy sources. -- In vivo endoscopic observations showed that larvae were concentrated primarily in the mouth-palps region of the female, where they were constantly manipulated and kept in motion by the palps. However, larvae were also seen distributed along the gills, but with no physical connection to the gills. Larvae showed a clear circulation pattern in the mantle cavity, being transported from the palp region to the posterior region of the gills by the water current produced by the female, then returning to the oral region via the ventral and dorsal ciliated food grooves of the female. -- Larvae were observed to ingest exogenous particles, establishing that they are not obligatory lecithotrophs. Food ingestion began when larvae developed the ciliated velum at approximately 290-300 urn shell length. Observations with the scanning electron microscope revealed that the distribution of ciliary bands on the velum of the Chilean oyster larva is adapted to the unusual environmental conditions associated with brooding. The absence of cilia during all the embryonic stages and the early stages of the veliger, the absence of the apical tuft, and the apparent absence of the postoral cilia (POC) characteristic of pelagic veligers, demonstrate this adaptation. -- The brooding process had a considerable impact on some aspects of the physiology of the female, especially the filtration rate (FR), which greatly decreased in brooding oysters. Brooding females partially compensated by increasing absorption efficiency (AE) in comparison to non brooders. The scope for growth was much lower in brooding oysters than in non brooders, largely as a result of the reduced FR of the brooders. After larvae were liberated, the FR of the female increased, reaching values close to those that could be considered 'normal FR' in Chilean oysters. -- The brooding mechanism provides in the mantle cavity a protective environment in which larvae can develop until they reach a very advanced pediveliger stage before release. Brooding appears in this species to be an adaptation for producing and retaining offspring in the estuarine environment in which Chilean oyster populations occur. Whereas brooding female oysters undergo weight loss, deplete their biochemical reserves, and experience a negative scope for growth during the brooding period, their larvae are liberated at an advanced developmental stage, have a very short pelagic period, and settle immediately after release, implying that mortality is much less than in pelagic larvae.

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