Diel migration and feeding patterns of the chaetognath, Sagitta friderici, off the west coast of South Africa
The vertical distribution patterns of adult and juvenile Sagitta friderici were investigated over a 48 h period, using samples collected at different depth strata with an RMT 1 × 6 net. Juvenile chaetognaths were generally found at depths of less than 50 m throughout the day and night, and exhibited limited diel migration patterns. Adults, on the other hand, migrated more extensively and were generally found below 50 m during the day and scattered throughout the water column or near the surface at night. Using the mean depths at each sampling time, the migration patterns of adult chaetognaths closely followed that of the large calanoid copepods. Juvenile chaetognaths, on the other hand, showed no significant correlation with any group of copepods. The greatest proportion of chaetognaths with food in their guts were not found at the depths of maximum copepod abundance, suggesting that the chaetognaths had not been feeding at the depths where they were caught. There was little evidence of a diel feeding rhythm, with no significant difference between the number of prey per chaetognath during the day or night. The majority of food items ingested were copepods (85.7%), while cannibalism on smaller chaetognaths occurred in 2.4% of the samples. The size of copepods consumed by adult chaetognaths was estimated from the width of the copepod mandibular blade, and was found to range from 0.16–1.96 mm prosome length. Although there was a large range of prey sizes which any given predator could consume, there was a clear trend of increasing prey size with increasing predator size. The energetic equivalent of ingesting a single copepod represented 47.4% of the body carbon of the juveniles, but only 16.7% for the adults. Consumption of two or more copepods increased this to 30.3% body carbon of the adults, but cannibalism on smaller chaetognaths represented an average of 46.8% of adult body carbon. Assuming equal handling costs, this implies that cannibalism is energetically much more advantageous for the adults than consuming copepods. Digestion times of copepod prey items were estimated from an exponential equation relating digestion time to temperature using values from the literature for six different chaetognath species. This yielded an ingestion rate of 0.5 copepods chaetognath–1 d–1, which resulted in 1.0–5.3% of the copepod standing stocks or 2.4–14.0% of the copepod production being consumed per day by the Sagitta friderici population.
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Document Type: Research Article
Publication date: August 1, 1991
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