The threshold shear values needed to elicit the escape reaction to a quantifiable fluid mechanical disturbance were compared between five free-swimming oceanic copepod species. The results indicate a significant difference in the threshold for different species of copepods and between different age groups within a single species. In general, animals captured from more energetic regimes required a higher threshold than those captured from more pacific locations. Labidocera madurae required the highest shear values with 51.5 s~' for 50% of the animals tested to elicit an escape reac- tion (5jo). Acartia tonsa and Euchaeta rimana, in contrast, were behaviorally the most sensitive requir- ing an 550 of only 1.5 and 4.1 s~', respectively, to initiate an escape reaction. Pleuromamma xiphias and Oithona required intermediate shear values with an 550 of 7.2 and 8.1 s~'. When compared to literature values, the threshold needed to elicit an escape reaction was consistently higher than average environmental shear values. Threshold shear values also varied significantly with develop- mental stage. Naupliar stages of A.tonsa required greater than six times the 550 value required by adults of the same species. This suggests that the higher vulnerability to predation of naupliar stages of copepods may not only reflect inferior escape strength, but may also result from the higher thresh- old needed to elicit an escape reaction. This study supports the hypothesis that selective feeding pat- terns exhibited by predators of copepods may be the result of the differential behavioral sensitivities of different species and developmental stages of copepods.
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