Sinking behavior of gastropod larvae (Ilyanassa obsoleta) in turbulence

Abstract

Larvae of coastal gastropods sink in turbulence and may use nearshore turbulence as an initial settlement cue. Our objective was to quantify the relationship between turbulence and the proportion of sinking larvae for competent mud snail veligers (Ilyanassa obsoleta). We exposed larvae to a range of field-relevant turbulence conditions ( ϭ 8.1 ϫ 10 Ϫ3 to 2.7 ϫ 10 0 cm 2 s Ϫ3 ) in a grid-stirred tank, holding other factors constant. We used a video plankton recorder to record larval movements in still water and in turbulence. Larval trajectories and velocity measurements were extracted using video-image analysis. We also measured turbulent flow velocities independently, using laser Doppler velocimetry. To interpret empirical measurements in terms of larval behavior, we developed a threecomponent, normal mixture model for vertical velocity distributions of larvae in turbulence. The model was fitted to observed larval velocities by maximum likelihood, to estimate the proportions of sinking, hovering, and swimming larvae. Over the range of turbulence intensities found in typical coastal habitats, the proportion of sinking larvae increased exponentially (r 2 ϭ 0.89) with the log of the turbulence dissipation rate. The net mean behavioral velocity of the larvae shifted from positive to negative when the dissipation rate reached ϳ10 Ϫ1 cm 2 s Ϫ3 . By sinking when they enter turbulent, shallow water, competent larvae could improve their chances of settling in favorable coastal habitats.Very little is known about how larval behavior in the plankton affects patterns of larval supply and settlement of benthic invertebrates. Much work has been done to describe larval behavior during the exploration of substrates, when larvae can sometimes select settlement sites over small scales (millimeters to centimeters). Less progress has been made on understanding the behavioral contribution while larvae are transported through the water column to benthic habitats. Under some hydrodynamic conditions, larvae could settle more successfully if they responded to waterborne cues by sinking toward the benthos. If the swimming velocity and gravitational sinking velocity differ by a factor of two or more, behavioral changes can significantly affect larval sink-1 Corresponding author (hfuchs@whoi.edu). AcknowledgmentsY. Yamashita helped collect and culture the larvae. We are grateful to J. Sisson for assistance with LDV measurements and to J. H. Trowbridge for guidance on spectral analysis. B. Raubenheimer and E. A. Terray also gave advice on flow data analysis. We thank C. DiBacco for generously sharing his culturing expertise and supplies, V. R. Starczak for advising us on the experimental design, and S. P. McKenna for introducing us to the turbulence tank. S. M. Gallager provided video equipment, software, and advice on particle tracking.

Numerical simulations of larval transport into a rip‐channeled surf zone

“…We tested three types of w b : 0 m s 21 (neutrally buoyant), 210 23 m s 21 (negatively buoyant) and 4 3 10 23 m s 21 (positively buoyant) based on Fuchs et al (2004). No active horizontal swimming behavior was considered here.…”

“…According to Fuchs et al (2004), competent larvae sink at w s 5 210 22 m s 21 when the turbulent energy dissipation rate is e . 10 25 m 2 s 23 .…”

“…By regulating their depth, larvae of some species recruit onshore in surface waters, whereas other species recruit onshore near the bottom in upwelling regimes (Morgan et al 2009a). Furthermore, larvae of various invertebrate taxa sink under turbulent conditions (Fuchs et al 2004(Fuchs et al , 2013Roy et al 2012). We hypothesize that depth preferences and sinking behavior help larvae to avoid strong offshore currents or enter the surf zone in streaming currents.…”

Causes of decoupling between larval supply and settlement and consequences for understanding recruitment and population connectivity

“…At these finer spatial 18 scales, flow velocities and substrate characteristics, which determine the boundary layer, 19 turbulent flows, and shear stress, will affect whether larvae attach or are re-suspended from a 20 substrate (Koehl, 2007). For example, turbulence increases the settling of negatively buoyant 21 phytoplankton cells (Ruiz et al, 2004), and some larvae use sinking behavior to promote 22 settlement in turbulent flows (Fuchs et al, 2004). In fast flows with high turbulence, contact rate 1 of larvae increases, resulting in higher settlement than in still water or in low flow conditions, 2 whereas above certain flow speeds larvae are not able to make contact or adhere (e.g., Eckman et 3 al., 1990;Pawlik and Butman, 1993;Qian et al, 2000;Pernet et al, 2003).…”

Comparison of a sediment trap and plankton pump for time‐series sampling of larvae near deep‐sea hydrothermal vents

“…Johnson, unpublished observations). Larvae may exhibit different swimming behaviors in response to different flow conditions (Metaxas 2001), and gastropod larvae, in particular, may exhibit sinking behaviors in response to turbulence (Fuchs et al 2004). Strong swimmers (i.e., >1 cm s -1 ), such as copepods, might avoid or be attracted to the sediment trap (e.g., Forbes et al 1992), and the flux of copepods into the sediment trap was not correlated to abundance (r = -0.19) or horizontal fluxes (r = -0.12) at the pump moorings.…”