Mussel larval responses to turbulence are unaltered by larval age or light conditions

Abstract

Larval responses to hydromechanical cues potentially have important effects on larval dispersal and settlement. This study examined the behavior of mussel larvae (Mytilus edulis) in laboratory-generated turbulence representative of nearshore currents. We video recorded the behavior of early-and late-stage veligers in a grid-stirred tank at five turbulence levels under light and dark conditions. Water velocities and kinetic energy dissipation rates were measured using particle image velocimetry and acoustic Doppler velocimetry. We characterized the vertical velocity distributions for sinking, hovering, and swimming modes in still water and calculated the average larval behavioral velocity in turbulence. In still water, young larvae had more positive (upward) velocities than old larvae, and both stages had more positive velocities in light than in dark. In turbulence, the mean larval vertical velocity varied from positive at low dissipation rates to negative at dissipation rates above a threshold of 8.3 £ 10 22 cm 2 s 23. At this threshold, the Kolmogorov length scale (h ¼ 590 mm) was two to three times the mean larval shell lengths (171 -256 mm), implying that turbulence is detectable even by larvae that are smaller than the smallest eddies. Responses to turbulence were unaffected by larval age or light conditions and contributed substantial behavioral variation. By sinking in strong turbulence, mussel larvae could increase their flux to the bed in energetic coastal flows, particularly over rough substrates like mussel beds. The response to turbulence by early-stage larvae will also affect their dispersal and may help larvae remain near coastal populations.

Transport of larvae and detritus across the surf zone of a steep reflective pocket beach

“…Morgan et al (2009), working in central California, observed higher recruitment of mussels to moored collectors on the bottom than to those near the surface. In a laboratory experiment, Fuchs & DiBacco (2011) observed late-stage mussel larvae swimming down in enhanced turbulence; a behavior that may direct them into near-bed streaming flow.…”

“…It has long been observed that as the larvae of nearshore invertebrates approach the completion of their development, many taxa exhibit behaviors that direct them toward the bottom (Thorson 1964). In addition, a variety of larval types swim down when they encounter strong turbulence (reviewed in Fuchs & DiBacco 2011). Clearly, these behaviors would aid competent larvae developing in the plankton to locate and settle on the benthos, but these behaviors may also assist larvae in their migration to shore.…”

Behavioral responses of invertebrate larvae to water column cues

“…mollusc veligers [26,28], vertical velocity of larval urchins did not increase with increased turbulence. However, these results are consistent with previous observations of other echinoderms through ontogeny: older larval sand dollars and larval green urchins have a lower vertical velocity under no flow or shear conditions compared to younger larvae [65,79].…”

“…Despite the prevalence of positive phototaxis in larvae [1] and observations of positive phototaxis in younger oyster larvae [6], light had no observable effect on vertical swimming direction of our late-stage oyster larvae, suggesting an ontogenic shift in phototaxis. Ontogenic changes in phototaxis have been widely documented in larvae of eels [144], polychaetes [138,145], crabs [146], mussels [26], nudibranchs [147], conch [148], and both larval and juvenile sole [149]. Competent larvae may cease to display positive phototaxic behavior because they no longer need to stay afloat in the water column.…”

“…When larvae are preparing to settle, vertical position can be especially important in permitting them to identify and investigate suitable settlement sites (e.g., [23,24,4]). Some examples of physical cues for the regulation of vertical position include turbulence (Chapters 2, 3, 4, [25,26,27,28,29]), sound [30], and light (Chapter 5, [31,32,33,34,35]). Cues incorporating biological and chemical signals include exudates from predators [36,37], prey or host species [38,39], and conspecific settlement sites (Chapter 6, [40,41]).…”