Effect of thermoclines and turbulence on depth of larval settlement and spat recruitment of the giant scallop Placopecten magellanicus in 9.5 m deep laboratory mesocosms

Abstract

An experiment was conducted from December 1992 to February 1993 in a 10.5 m deep, 3.7 m diameter tank to examine the effect of thermoclines and water column turbulence on the depth of larval settlement and spat recruitment of the giant scallop Placopecten magellanicus (Gmelin) Polyethylene tube mesocosms set up within the tank were used to enclose 9.5 m deep columns of seawater which were then used as experimental replicates. Five different treatments were established as follows: (1) no turbulence and a l.S°C thermocline, (2) no turbulence and no thermal gradient, ( 3 ) low level of turbulence with a 1.5"C thermocline, (4) medium level of turbulence with a 0.5OC thermocline. and (5) high level of turbulence with no thermal gradient. Various turbulence levels simulated turbulent energies ranging from open oceanic environments to near-shore and coastal conditions with vertical dissipation rates ranging from 10-' to 1 0 -~ cm2 s -~. Ropes with collectors positioned at every 1 m depth interval were suspended the length of the water column in each replicate tube to collect settled spat. Spat counts varied significantly with both depth and turbulence treatment and were dependent on the interaction between the 2 factors. Numbers of spat generally increased with increasing depth in the static and low turbulence treatments, but this relationship became less evident with increasing turbulence; spat recruitment in the high turbulence tubes appeared random with respect to depth. It is suggested that the trend of increasing recruitment with depth in the static and low turbulence tubes was driven primarily by larval behaviour at settlement. There was no indication of increased recruitment at or above the thermocline, in contrast to a prevlous mesocosm experiment with a stronger thermal gradient and a different population of larvae, suggesting that strahfication intensity may affect depth of larval settlement and spat recruitment. Settlement rate did not appear to be a strict function of larval encounter rate with the spat collectors. Higher spat counts in treatments with a 1.5"C thermocline than in other turbulence treatments, even when results were corrected for differences in competency among the various treatments, suggest that thermal gradients have a potential commercial importance in both the collection and hatchery production of scallop spat.

Behavioral responses of invertebrate larvae to water column cues

“…Various studies highlight that larvae actively respond to turbulence or components of turbulence, e.g., larval boat snails Crepidula fornicata increase upward swimming with increasing turbulence level [67], larval sea slugs Phestilla sibogae retract their vela when encountering turbulent filaments containing chemical cues [48], and larval eastern oysters Crassostrea virginica dive when experiencing high fluid acceleration over short time intervals [29]. Together with other modeling studies, these earlier works suggest turbulence enhances larval settlement [68,56] (but see [69], who suggested that an increase in turbulence reduces settlement in scallop larvae). Recently, deformation associated with horizontal shear has been suggested to induce competency in larval urchins [70] and to induce cloning in coral larvae [71].…”

Mortality of post‐settlement abaloneHaliotis iriscaused by conspecific adults and wave exposure

“…During the experiment however, waves were estimated to be 1.5 m in height at the exposed locations and less than 0.5 m at the sheltered locations. Gosselin & Chia (1995) similarly concluded that wave action might be an important contributor to early juvenile mortality in the intertidal snail Nucella emarginata, and under experimental conditions, Pearce et al (1998) believed that turbulence was a major factor causing the dislodgement of recruits of the scallop (Placopecten magellanicus). Recruits of abalone dislodged from hard substrata, especially in conditions of high wave activity, would certainly be exposed to increased physical damage, increased predation, or may be transported away from suitable settlement substrata.…”

Swimming patterns of larval Strongylocentrotus droebachiensis in turbulence in the laboratory

“…However, larval growth to competence and settlement success rate were significantly reduced at the highest levels (Pearce et al 1998). In contrast, settlement success of mussel larvae Mytilus edulis was enhanced with increased turbulence, up to a threshold level (ε not provided; Pernet et al 2003).…”

“…; Pearce et al 1998). However, larval growth to competence and settlement success rate were significantly reduced at the highest levels (Pearce et al 1998).…”

“…Larvae of a variety of species have been shown to respond to a number of cues both biological, such as food, predators and conspecifics (Young 1995, Metaxas & Young 1998, Metaxas & Burdett-Coutts 2006, Roy et al 2012, as well as physical, such as light, temperature, salinity and gravity, that they may encounter while in the water column (Welch et al 1999, Metaxas 2001, Welch & Forward 2001. Larval distribution can also be altered by the flow regime (Pearce et al 1998). Shear forces, caused by the presence of a salt wedge or simple laminar flow, have been shown to prevent the formation of aggregations of echinoderm larvae at the water surface.…”