The escape behavior of marine copepods in response to a quantifiable fluid mechanical disturbance

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(-1) for 50% of the animals tested to elicit an escape reaction (S-50) Acartia tonsa and Euchaeta rimana, in contrast, were behaviorally the most sensitive requiring an S-50 of only 1.5 and 4.1 s(-1), respectively, to initiate an escape reaction. Pleuromamma xiphias and Oithona required intermediate shear values with an S-50 of 7.2 and 8.1 s(-1). 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 developmental stage. Naupliar stages of A. tonsa required greater than six times the S-50 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 threshold needed to elicit an escape reaction. This study supports the hypothesis that selective feeding patterns exhibited by predators of copepods may be the result of the differential behavioral sensitivities of different species and developmental stages of copepods.