The relationship between the feeding ecology of the protobranch bivalve Yoldia hyperborea and the seasonal changes in the deep-depositional zone in Conception Bay, Newfoundland

Stead, Robert A. (2001) The relationship between the feeding ecology of the protobranch bivalve Yoldia hyperborea and the seasonal changes in the deep-depositional zone in Conception Bay, Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The primary purpose of this study was to further the understanding of the deposit-feeding strategies through an overall comprehension of the environmental and physiological constraints and the feeding opportunities encountered by the obligate deposit feeding protobranch Yoldia hyperborea . -- Seasonal variation in the feeding response by Yoldia hyperborea from the deep-depositional zone of Conception Bay (∼250 m) was monitored through periodic quantification of morphological (digestive cell height) and biochemical changes (protein content, activity of acid protease and α-amylase) in the digestive gland. Results showed an increase of digestive cell height and protein content as soon as sinking organic material from the spring bloom fallout reached the benthic zone, suggesting storage of metabolic energy during spring and summer, this was followed by a decline of both variables in late summer, coinciding with gamete development. Sharp increases of digestive enzyme activity occurred only in early spring of each year, coinciding with the timing of the primary bloom fallout, and suggests activation of the formerly depressed lysosomal system after a prolonged period of low food availability ( i.e. late summer to autumn). Individuals exposed to laboratory simulated events of sedimenting algae showed an increase of digestive cell height to similar levels observed in the field, whereas digestive cell height in animals exposed to impoverished sediment remained at the low levels shown by animals within the inter-bloom period. -- Yoldia hyperborea is primarily a sub-surface deposit feeder, but switches to surface deposit-feeding when surface sediment is enriched with algae. Behavioural changes to the downward flux of sedimenting microalgae was displayed as it extended its siphons into the water column when suspended algae were in high concentration. This behaviour was followed by partial reemergence of individuals and extension of the palp proboscides over the sediment surface once suspended algal concentration decreased and algae accumulated on the sediment. Concurrently, orientation of the siphon changed from vertical to horizontal, thereby always keeping closer contact with the area of highest algal concentration. In contrast, activity of animals not exposed to algae was primarily restricted to strata below the sediment surface. -- Siphon extension into the water column and active ventilation during periods of high suspended algal concentration suggest active suspension-feeding during algal sedimentation events, although deposit-feeding would be resumed once suspended algae decreased. Despite the fact that feeding experiments demonstrated the feasibility of suspension-feeding behaviour in Yoldia hyperborea , ingestion rates were extremely low and individuals were not capable of meeting their metabolic energy demands, although absorption efficiency of organic carbon was high (50-72%). In contrast, high ingestion rates (4.358 mg sed. • ind⁻¹ • h ⁻¹ ) were observed during deposit-feeding on algae-enriched sediments, which together with high assimilation efficiency of algae-derived organic carbon (87.7-95%), supplied sufficient energy to meet metabolic demand and provide a positive scope for growth. -- The results of both field and laboratory studies suggest that the suite of behavioural changes displayed by Yoldia hyperborea during algal sedimentation events are linked to its nutritional dependence on this food source.

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