Abstract
Many marine invertebrates have planktonic larvae with cilia used for both propulsion and capturing of food particles. Hence, changes in ciliary activity have implications for larval nutrition and ability to navigate the water column, which in turn affect survival and dispersal. Using high-speed high-resolution microvideography, we examined the relationship between swimming speed, velar arrangements, and ciliary beat frequency of freely swimming veliger larvae of the gastropod Crepidula fornicata over the course of larval development. Average swimming speed was greatest 6 days post hatching, suggesting a reduction in swimming speed towards settlement. At a given age, veliger larvae have highly variable speeds (0.8–4 body lengths s−1) that are independent of shell size. Contrary to the hypothesis that an increase in ciliary beat frequency increases work done, and therefore speed, there was no significant correlation between swimming speed and ciliary beat frequency. Instead, there are significant correlations between swimming speed and visible area of the velar lobe, and distance between centroids of velum and larval shell. These observations suggest an alternative hypothesis that, instead of modifying ciliary beat frequency, larval C. fornicata modify swimming through adjustment of velum extension or orientation. The ability to adjust velum position could influence particle capture efficiency and fluid disturbance and help promote survival in the plankton.
Highlights
Many small planktonic organisms use cilia to propel themselves or to generate feeding currents, including larval stages of numerous marine invertebrates [1,2,3,4]
When controlled for age, there was no significant correlation between swimming speed and shell length, i.e., for any age, larger larvae did not swim faster (Fig. 2, Partial Correlation, r = 0.036, p = 0.727)
By modifying and applying high-speed high-resolution microvideography, we tracked the movement of freely swimming veliger larvae of Crepidula fornicata
Summary
Many small planktonic organisms use cilia to propel themselves or to generate feeding currents, including larval stages of numerous marine invertebrates [1,2,3,4]. Because of small size and relatively low swimming speeds, planktonic larvae, in general, operate in low Reynolds number environments in which viscous force dominates [7]. The thrust exerted by a larval body on the surrounding water is proportional to the length of the cilia, the frequency and synchrony of ciliary beat, and the total length of the ciliated bands relative to body size [2,8,9]. Veliger larvae of gastropods have long compound cilia on their velar lobes, long extensions used for both swimming and feeding [2]. These cilia are arranged in prototroch and metatroch bands surrounding a food groove. On the contrary, tethering, e.g., by weight of larval shell or a mucus strand, may impede motion, and could enhance filtering efficiency [15,16]
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