Settlement of Chlamys varia (l.) in the hatchery.

ABSTRACT The employment of different types of spat collectors and their orientation were investigated, as well as the effect of light and depth on larval settlement, with the aim of optimizing hatchery production of C. varia spat. Competent larvae were seeded in tanks with and without collectors, and were maintained for 3 wk. The larvae showed preference for settling on the internal shaded surface of the collectors placed horizontally. Settlement was significantly higher in deeper than shallower situated collectors. The absence of light reduced the settlement significantly, The mean settlement on the walls of the tanks without collectors was 17.7 [+ or -] 0.59%, whereas settlement on the walls of the tanks with collectors was 13.7 [+ or -] 2.62% and increased an additional 18.6 [+ or -] 3.93 when collectors were in place (i.e., a final settlement of 32.3%). The results suggest that the presence of an attachment substrate improves the yield obtained during hatchery production of C. varia spat.

KEY WORDS: scallop, Chlamys varia, collector, settlement, orientation, photoperiod 

INTRODUCTION

The interaction of biotic factors (bacterial film, gregariousness, trophic preferences) and abiotic factors (light, gravity, temperature and composition of substrate) may explain the pattern of larval settlement in marine invertebrates (Loosanoff & Davis 1963, Meadows & Campbell 1972, Roberts et al. 1991). Settlement of competent bivalve mollusk larvae begins with a period of searching for a suitable substrate---during which the larvae swim and crawl on surfaces--and ends with settlement, after which metamorphosis takes place (Carriker 1986, Cragg & Crisp 1991, Rodriguez et al. 1993).

The settlement substrate must provide suitable conditions for successful larval metamorphosis (Bourne et al. 1989). The larval settlement on different substrates has been evaluated under hatchery conditions in various studies in which the following have been considered: the characteristics of the material, its position and the orientation (Pearce & Bourget 1996, Harvey et al. 1997, Taylor et al. 1998), the presence or absence of a bacterial film (Tritar et al. 1992, O' Foighil et al. 1990) and the effect of different conditions of temperature, water flow and depth (Hodgson & Bourne 1988, Pearce et al. 1994, Pearce et al. 1996, Manuel et al. 2000, Robert & Nicolas 2000).

Chlamys varia is a small sized scallop, currently under consideration for aquaculture in Spain. In Galicia, (NW Spain) settlement in collectors in natural environments has been found to be low (Roman, 1991) and therefore spat must be produced in hatcheries. Hatchery culture of C. varia has been successfully achieved in France, Ireland and Spain (Le Pennec 1978, Le Pennec & Diss-Mengus 1985, Burnell 1983, Acosta & Alvarez 1990, Roman 1991, Louro et al. 2003).

Rodhouse & Burnell (1979) have observed that under natural conditions, juvenile C. varia show preference for dark sites (e.g., cracks in rocks and the insides of empty shells). Burnell (1983) found that under hatchery conditions, the best settlement rates occurred on black surfaces in shade and in vertical or sloping positions. Louro et al. (2003) registered higher settlement rates on the walls of tanks than on the filaments usually used to settle pectinids (Pearce & Bourget 1996). The effects of external factors such as light and the characteristics and orientation of the substrate on the settlement process require further study.

In the present study we evaluated the use of different types of collectors and their orientation, as well as the effects of light and depth on settlement of C. varia larvae, with the aim of optimizing hatchery production of spat.

MATERIALS AND METHODS

Larvae were cultured in the experimental hatchery in the Center of Oceanography, A Coruna, Spain, after being obtained from C. varia reproducers conditioned between December and February, according to the techniques described by Louro et al. (2003).

Larval Settlement Experiments

Collectors were made from different materials (Table 1, Fig. 1). A commercial, Chinese hat type collector (area = 275.6 [cm.sup.2]) was used as the standard collector against which the others were compared. To standardize the attachment substrate, the collectors used in experiment 1 were constructed so that all had the same surface area. The settlement results were expressed as spat/[cm.sup.2].

[FIGURE 1 OMITTED]

Tanks of capacity 50 and 150 L were filled with filtered (1[micro]m) sea water. The collectors were suspended by lengths of nylon tied to a plastic bar propped against the edge of the tank (Fig. 1). Before use the collectors were submerged in unfiltered sea water for 7 days to favor growth of a bacterial film ("epifloral film"), which appears to stimulate larval settlement in various bivalve species (O'Foighil et al. 1990, Tritar et al. 1992, Pearce & Bourget 1996, Rupp 2004). Control tanks without collectors (CP) were used in all experiments.

For the settlement experiments, pediveliger larvae of between 15 and 20 days old were collected by retention on a 140-[micro]m mesh. The larvae were then counted, measured and seeded in the tanks at a density of 1 larva [mL.sup.-1], with light aeration. The experimental conditions were similar to those under which the larvae were cultured (Louro et al. 2003), except that 50,000 cells [mL.sup.-1] of Skeletonema costatum were added to the usual diet.

After 21 days, spat were detached from collectors and from the walls of the experimental (EP) and control tanks (CP) using a brush and pressurized seawater. The numbers of attached spat were estimated by placing them in a counting chamber (area = 145.3[cm.sup.2]) and counting the numbers present in 10 cells (each 1[cm.sup.2]) chosen at random. In experiment 5, the spat were maintained in the collectors for 60 days before being detached.

Collector Type

Experiment 1

This experiment was carried out to determine the rate of settlement in relation to the orientation of the surface provided: horizontal (H) or vertical (V). A total of 3 experimental tanks and one control tank were used. Two types of collectors were placed in a horizontal position--the standard, Chinese hat (H-hat) and the H-pipe--and 2 types in a vertical position: V-pipe and V-sheet.

Experiment 2

With the aim of optimizing settlement conditions, another experiment was carried out using the types of collectors in which the highest numbers of spat were captured in the previous experiment (i.e. the horizontally- placed collectors: H-hat; H-pipe; H-tile and H-hollow prism) (Table 1).

Depth Effect

Experiment 3

This experiment was carried out to compare the combined effect of substrate and depth on settlement. Collectors were installed at 3 different depths (subsurface: 15 cm; intermediate depth: 30 cm; bottom: 90 cm) in 150-L tanks (100-cm depth). Two experimental units and H-tile and H-hollow prism fixed collectors were used.

Light Effect

Experiment 4

This experiment was carried out to determine the effect of light on settlement in opaque, PVC (H-pipe) and transparent, perspex (H-pipe (t)) collectors of equal dimensions (Table 1). Three units of each type of collector were used with two different types of treatments: a photoperiod of 16 h light and 8 h darkness (16: 8), and total darkness (photoperiod 0:24). Each treatment was carried out quadruplicate and two controls without collectors were used in both cases.

Growth of Postlarvae

Experiment 5

In this experiment, with the same design as described for Experiment 4, the spat were maintained attached to the collectors for 8 wk, at the end of which the numbers of spat attached to each collector were counted and the heights of the shells of a subsample were measured (n = 50) (dorso-ventral distance). Two experimental units were used.

Data Analysis

The total number of spat attached to each substrate (collectors and tank walls) was recorded. The data were subject to logarithmic transformations so that they complied with the requirements of normality and homogeneity of variance (Bartlett test) (Steel & Torrie 1960), and were then compared using ANOVA. To compare growth of spat, the W Mann Whitney test was used. The Statgraphics Plus statistical package (Standard Edition, Manugistics, 1998) was used.

RESULTS

Collector Type

In experiment 1 (Fig. 2) we found significant differences in the number of attached spat, depending on the type of collector (ANOVA: GL = 3; F = 17.57; P = 0.0000). In the H-pipe and H-hat, horizontally-positioned collectors, the mean numbers of spat were 12.0 [+ or -] 0.7 and 2.0 [+ or -] 0.3 spat/[cm.sup.2] (mean [+ or -] std) respectively (i.e., significantly higher settlement than in the vertically-positioned collectors) V-pipe (0.40 [+ or -] 0.10) and V-sheet (0.04 [+ or -] 0.05).

[FIGURE 2 OMITTED]

In experiment 2 (Fig. 3) all of the collectors were positioned horizontally, but there were significant differences in the number of spat settled on the different types of collectors (ANOVA: GL = 3; F = 11.21; P = 0.0204.). The highest settlement was recorded on H-tile collectors (3.0 [+ or -] 0.7). There were no significant differences between H-pipe (1.4 [+ or -] 0.6) and H-hat collectors (1.3 [+ or -] 0.5). The lowest settlement (0.2 [+ or -] 0.1), which was significantly lower than in the other collectors, corresponded to the H-hollow prism, In both experiments 1 and 2, the attachment occurred exclusively on the internal walls of the collectors (Fig. 2).

[FIGURE 3 OMITTED]

Depth Effect

The depth and type of collector had a significant effect on settlement (multifactor ANOVA). Settlement was significantly higher in H-Tile than in H-hollow-prism (GL = 1; F = 38.07; P = 0.0000). In both types of collectors, settlement was affected by depth (GL = 2; F = 12.96; P = 0.0066). In H-tile, attachment at 90 cm (8.6 [+ or -] 0.7 spat/[cm.sup.2]) was significantly higher than at 60 cm (3.1 [+ or -] 0.4), which in turn was significantly higher than at 30 cm (0.8 [+ or -] 0.2). In H-hollow prism, attachment was significantly higher at 90 cm (2.3 [+ or -] 0.5) and 60 cm (2.1 [+ or -] 0.5) than at 30 cm (1.2 [+ or -] 0.3) (Fig. 4).

[FIGURE 4 OMITTED]

Light Effect

The light regimen and the type of collector affected scallop settlement (multifactor ANOVA), (Fig. 5) and was significantly higher with the 16:8 photoperiod than the 0:24 photoperiod (GL = 1; F = 11.11; P = 0.0018) and in the opaque collectors than in the transparent collectors (GL = 1; F = 4.93; P = 0,0320). In opaque collectors the numbers registered were 8.7 [+ or -] 1.4 (photoperiod 16: 8) and 4.1 [+ or -] 1.1 spat [cm.sup.-2] (photoperiod 0:24) and in the transparent collectors 0.19 [+ or -] 0.02 (16:8) and 0.22 [+ or -] 0.04 (0:24) spat/[cm.sup.2].

[FIGURE 5 OMITTED]

Settlement on Tank Walls

Settlement registered in the experimental tanks with collectors (walls + collectors) and in the control tanks (without collectors) showed significant differences (ANOVA, df = 1; F = 7.65; P = 0.0096). The mean settlement in the experimental tanks was significantly higher than in the control tanks. However, if we only consider attachment on the tank wall, there were no differences between the two types of tanks. The difference between the two was due to supplementary attachment in the collectors (Table 2).

Spat Growth

There were significant differences in the growth of spat depending on the type of collector and the photoperiod (W Mann Whitney test: W = 54130.0; P = 0.000). In the transparent collectors the spat grew significantly more in the tanks subject to the 16:8 photoperiod (8.0 [+ or -] 0.1 mm) than in the tanks subject to the 0:24 photoperiod (6.0 [+ or -] 0.1 mm). In the opaque collectors the size did not differ significantly (W Mann Whitney test: W = 45154.0; P = 0.778) with the two photoperiod treatments (7.0 [+ or -] 0.1 mm and 7.0 [+ or -] 0.1 mm respectively).

DISCUSSION

Collector Type

The settlement rate of C. varia showed a clear tendency to negative phototropism, with preference for surfaces not exposed to the light in horizontally-positioned opaque collectors. This was clearly demonstrated by the present results because settlement took place preferably inside the H-tile, H-pipe and H-hat collectors (Figs. 2 and 3). Furthermore, collectors made of the same material but with different orientation (V-sheet and V-pipe) showed significantly lower spat settlement.

The settlement trend in C. varia was consistent with observations reported by other authors. Burnell (1983) registered a high percentage of settlement in the inner surface of substrates provided with a reduced level of light. Similar results were obtained by Louro et al. (2003) for the inner surface of scallop shells used as collectors. This settlement rate also corresponded with field descriptions. Juvenile C. varia of <2 mm have been found at low depths in the intertidal zone, fixed below rocks and shells (Rodhouse & Burnell 1979) and were especially abundant within empty shells in which both valves were intact, as well as below fragments of material and bottles (Michin 1976). Negative phototropism has been analyzed by von Buddenbrok & Moller-Racke (1953), who demonstrated that juvenile C. varia swim preferentially towards dark areas.

Preference for darkness has also been found in other pectinid species. According to Dabinett et al. (1999) high attachment rates of Placopecten magellanicus can be obtained using horizontally-positioned glass fiber panels. Vera (1988) evaluated the effect of the angle of orientation of the substrate on attachment of Euvola ziczac and obtained the highest settlement rates on substrates placed at 135 and 180[degrees] to horizontal.

The physical characteristics of the surface and the contour of the substrate may or may not provide favorable conditions for settlement, so that the larvae may choose a substrate according to these characteristics (Roberts et al. 1991). On the other hand, according to Meadows & Campbell (1972) selection of a particular substrate may also depend on external, physicochemical (light, salinity and temperature) and biologic factors (gregariousness, food preferences), as well as factors inherent to the species itself.

Settlement of C. varia spat in the present study was successfully obtained in PVC (H-tile and H-pipe) and polythene collectors (H-hat). Preference for settlement in PVC and polythene collectors has been recorded in various species of scallops (Pearce & Bourget 1996) and in oyster species such as the Sydney rock oyster, Saccostrea, commercialis (Holliday 1996) and the mangrove oyster Crassostrea gasar (Ajana, 1979). The collectors used in experiment 2 were opaque and placed horizontally to guarantee a high settlement rate of spat, in accordance with the results obtained in experiment 1. Unlike the other collectors, H-hollow prisms (Fig. 1) were rectangular in shape with angled areas and smooth sides. Low settlement rates were recorded in spite of its horizontal position. Wheaton (1977) indicated that the water flow in structures with circular and rectangular surfaces was very different. Circular surfaces allow greater distribution of food and less accumulation of waste products, whereas rectangular surfaces generate dead areas where there is no water flow and a small circulation circuit, which may cause stress in the animal being cultivated and reduce the efficacy of physiologic processes (alimentation, respiration and growth). This situation, which is unfavorable for settlement may be produced in H-hollow prism collectors. We have observed that C. varia juveniles (2-4 mm) cultivated in H-hat collectors tend to move from the center of the collector, where settlement initially occurs, towards the distal region in search of higher water flow. We therefore believe that the form of the substrate may affect the water flow and thus determine the selective settlement of C. varia larvae.

The effects of substrate and water flow on larval settlement have been investigated in sessile invertebrates (Wethey 1986). In other species of bivalves, such as the silver-lip pearl oyster Pinctada maxima, the patterns of recruitment in horizontally and vertically-positioned PVC collectors appear to be affected by the contour of the substrate (Taylor et al., 1998). Harvey et al. (1997) reported that the differences in the response of various bivalve species to the settlement processes found in a field study indicated that the larvae are capable of perceiving a particular contour and/or texture of a substrate. Likewise, Hodgson & Bourne (1988) concluded that settlement and metamorphosis of Chlamys hastata larvae may be stimulated by the water flow and by the contour and form of the substrate.

Depth Effect

Greatest numbers of C. varia attached to the collectors situated closest to the tank bottom, and settlement decreased gradually towards the surface of the tank. Taylor et al.(1998) found that settlement of Pinctada maxima in PVC collectors placed at different depths did not differ significantly, although the number of spat that attached to the tank walls tended to be higher on or close to the tank bottom and decreased gradually towards the surface. The same author suggested that this phenomenon may be explained by the combined effects of light and gravity. Furthermore, Gonzalez-Ramos et al. (1997) found greater settlement of Lyropecten subnodosus spat on the basal mesh than on the walls of an inverted flow chamber, and they suggested that this was caused by the direction of the water flow. In the present study, the light aeration produced water flow in the opposite direction to that cited by the latter author, suggesting that the tendency for C. varia spat to settle on the tank bottom may be due to the effects of gravity or because of a preference for darkness. In the control tanks, in which settlement occurred in the absence of collectors, the larvae preferentially attached close to the bottom of the walls of the tanks where there was shade created by the exterior structure that sustained the tank (Fig. 2). Such preference for the bottom of the culture tank has been observed by O'Connor et al. (1994) in larvae of the doughboy scallop Chlamys (Mimachlamys) asperrimus.

Cragg & Crisp (1991) analyzed the results obtained by other authors and suggested that pectinid settlement is positively geotactic and negatively phototropic.

Light Effect

The importance of a low level of light as a favorable condition for settlement of C. varia has already been discussed here. However, the results of experiment 4 showed that the total absence of light may reduce settlement. In a similar experiment in which transparent and opaque collectors were used, Cole & Knight Jones (1939) found that settlement of Ostrea edulis larvae was 3 times higher during daylight hours than in the hours of darkness and suggested that in the absence of light, swimming activity and the search for substrate become reduced. We do not discount the possibility that the quality of the material may have affected the results because the opaque collectors were made of PVC and the transparent collectors of perspex. In both photoperiods low settlement was registered in the transparent collectors, independently of the light regimen.

Gregarism and Early Growth

Although the distribution of spat in the collectors was not recorded, a clear tendency for gregariousness was observed on certain attachment substrates. The massive settlement in opaque collectors took place on the inner surface, and tended towards the center, so that the spat were attached one on top of another.. This was similar to the distribution of juvenile and adult C. varia in natural environments; according to Hily & Le Foll (1990) groups of individuals can be found underneath or attached to empty shells. In many species of benthic invertebrates the pattern of gregariousness may occur in larval stages, as a response to mechanisms of chemical attraction when the first larvae select a suitable substrate (Meadows & Campbell 1972). As a consequence of the high concentration of spat recorded in opaque collectors, growth was negatively affected when compared with the growth of the scarce spat settled on the transparent collectors.

Settlement on Tank Walls

The results obtained indicated that the provision of additional substrate in experimental tanks increased the rate of C. varia larval settlement, reaching double the settlement that in the control tanks (Table 2). Louro et al. (2003) recorded 12.7% settlement of C. varia on tank walls and 19.7% when counts included the substrates provided. This indicates that the availability of attachment substrates apparently increases the likelihood of settlement of C. varia larvae. According to Hily & Le Foll (1990), the density of C. varia is directly correlated to the density of potential substrates under natural conditions, so that a greater abundance of spat in areas with a higher concentration of substrates is to be expected. In a laboratory study, Ambrose & Lin (1991) found that the preference of Argopecten irradians larvae for attachment depends on which other attachment substrates are present.

We conclude that in the settlement of C. varia larvae is determined by factors such as light intensity, as well as the orientation, type and availability of substrates. The best settlement rates can be obtained by providing large areas of substrate in conditions of low light (not total darkness), using collectors with a concave inner surface, made from PVC or polythene and placed in a horizontal position. Juvenile C. varia show gregariousness.

ACKNOWLEDGMENTS

The authors thank J. Fernandez Feijoo and M. Carmen Vazquez for their help in hatchery work. This project, ACU01-022-C2-1, was financed by government of Spain (INIA) and J.P. De la Roche was in receipt of a grant from CYTED.

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J. P. DE LA ROCHE, A. LOURO AND G. ROMAN *

Instituto Espanol de Oceanografia, Ministerio de Ciencia y Tecnologia, Centro Oceanografico de A Coruna, P.O. Box 130, 15080, A Coruna, Spain

* Corresponding author: E-mail: [e-mail]

TABLE 1.
Characteristics of the substratesused as collectors
in the experiments.

Collector
Name (Code)        Orientation   Material    Surface

H-hat              Horizontal    Polythene   Opaque rough concave
                                             disc. Reddish colour

H-pipe             Horizontal    PVC         Opaque smooth cylindrical
                                             pipe. White

V-pipe             Vertical      PVC         Idem H-pipe

V-sheet            Vertical      Polythene   Clear rough sheet

H-tile             Horizontal    PVC         Opaque smooth concave
                                             tile. Grey

H - pipe (t)       Horizontal    Perspex     Clear smooth pipe

H - hollow prism   Horizontal    PVC         Opaque smooth hollow
                                             prism. White

TABLE 2.
Percentage of spat attached to collectors and walls of
experimental and cointrol tanks.

Tanks                 Walls         Collectors           Total

Control        17.7 [+ or -] 0.59                        17.7
Experimental   13.7 [+ or -] 2.62   18.6 [+ or -] 3.93   32.3

Value = mean [+ or -] s.e. (n).