Environmental influences on growth and survival during the ocean-nursery rearing of giant clams, Tridacna gigas (L.)

Aquaculture, 80 (1989) 45-61 Elsevier Science Publishers B.V., Amsterdam

45 -

Printed

in The Netherlands

Environmental Influences on Growth and Survival During the Ocean-Nursery Rearing of Giant Clams, Tridacna gigas (L.) J.S. LUCAS, W.J. NASH’, C.M. CRAWFORD’

and R.D. BRALEY

Zoology Department, James Cook University, Townsville, Queensland, 4811 (Australia) (Accepted 2 January

1989)

ABSTRACT Lucas, J.S., Nash, W.J., Crawford, C.M. and Braley, R.D., 1989. Environmental influences on growth and survival during the ocean-nursery rearing of giant clams, Tridacnagigas (L. ). Aquaculture, 80: 45-61. The effects of temperature, emersion, light and contrasting localities on growth and survival of juveniles of Tridacna gigas were studied at Orpheus Island, North Queensland, Australia. Shell lengths of ocean-nursery phase juveniles increased by an average of almost 10 mm per month over the 17-month study period. However, there was a strong seasonal component to growth rate, which varied from highest levels in late summer to almost zero in late winter when water temperatures were near 20°C. Juvenile clams tolerated up to 10 h per day mean emersion, but were completely stunted in growth. Periods of up to 3 h emersion during daytime, but not at night, had a positive effect on growth, suggesting that photosynthesis continues during emersion. Growth and survival were poorer at a more oceanic locality than at a more turbid, protected locality, apparently due to disturbance from turbulence at the oceanic site. Clams in 90% shade showed poor growth and survival compared to those in 50% shade and full sunlight. This species is an obligate phototroph. Thus, juveniles of T. gigas have conflicting environmental requirements in their need to be exposed to high light levels but concealed from predators. It is suggested that the complex structure of coral reefs provides juvenile clams with microhabitats where they are both cryptic and exposed to intense light, and this is the particular feature linking giant clams to coral reefs. In providing juvenile clams with protective cages, as in ocean-nursery culture, it should be possible to use environments other than coral reefs for their culture.

INTRODUCTION

Crawford et al. (1988) assessed four different niles of Triducna gigus during the ocean-nursery determine the optimum culture method in terms on growth and survival of clams. In this paper *Present address: Fisheries (Australia)

0044-8486/89/$03.50

Laboratories,

Department

methods for culturing juvephase. Their objective was to of practicability and effects we describe complementary

of Sea Fisheries,

0 1989 Elsevier Science Publishers

B.V.

Taroona, Tasmania,

7053

46 studies examining: (1) the effect of seasonal temperature variation on growth rate, (2) growth and survival at different levels in the intertidal zone, (3) growth and survival of juvenile clams at two quite different localities, and (4) growth and survival under different light regimes. It was evident from Crawford et al. (1988) that T. gigus juveniles are quite tolerant of some intertidal exposure. This study employed higher levels in the intertidal zone to see what level of exposure the clams can tolerate without adverse effects. The locality used by Crawford et al. (1988)) Pioneer Bay on the western side of Orpheus Island, north Queensland, Australia, often has highly turbid water. Although a convenient ocean-nursery site, it may be a poor environment for juvenile clams; hence a fringing reef at the northeast of Orpheus Island was chosen as a culture locality for comparison. Water clarity and, probably, water circulation are superior on the eastern side of Orpheus Island, which faces towards the offshore Great Barrier Reef. A further difference between these localities is that the northeast reef is exposed to the prevailing easterly winds and frequently experiences strong wave action, while Pioneer Bay, facing west towards the mainland, is rarely exposed to strong wave action. The different environmental conditions at these two localities are reflected in their hermatypic coral communities; with greater diversity, especially of acroporid and favid corals, on the northeast reef (J. Veron, personal communication, 1988). Light is obviously a major factor in the ecology of the phototrophic giant clams and the influence of constant light levels on photosynthesis/respiration (P/R) ratios in T. gigm juveniles has been studied (Fisher et al., 1985). The light regimes experienced by clams in the field, however, are complex: varying seasonally, diurnally and tidally. This study considers the long-term effects of natural light regimes on growth rates and survival of juvenile clams. METHODS The standard culture unit for this experiment was the same as in Crawford et al. (1988, Fig. 1): a set of four perforated plastic trays (freezer trays), 550 x 300 x 90 mm. Twenty juvenile T. gigas from a February 1985 spawning (the same cohort as used by Crawford et al., 1986) were placed in each tray onto a layer of 15-20-mm size gravel pieces, to which the clams subsequently attached their byssal threads. Each tray was covered with a 26-mm plastic mesh that could be lifted to handle the clams, and the tray sets were fixed to a substrate in close formation. The protocol for maintenance and measurements was essentially the same as in Crawford et al. (1988). Anterior-posterior shell length was the standard measure of size. The clams were measured initially and selected to achieve similar means and standard deviations for the four tray sets within each study, and to be of similar contemporary size to clams used in Crawford et al. (1988).

47

The trays were usually inspected twice weekly, meshes were cleaned if needed, and mortalities recorded and replaced with marked clams of similar size. The shell lengths of 20 clams per set of four trays were measured at 4-week intervals. The measured clams were selected haphazardly, five per tray, excluding the replacement clams. Seasonal growth Growth data were based on pooled measurements for all the groups of 80 clams cultured in the intertidal zone of Pioneer Bay, excluding the data for clams at the VI site (see below). These intertidal tray sets were dismantled in October 1986 and the clams transferred to 3-m long boxes in the intertidal zone at the NI site, along with the majority of juvenile clams from this spawning. Thereafter, growth data were obtained from samples of 80 clams from this total February 1985 cohort. Water temperature was continuously recorded on a chart-recorder in the station seawater system. The median daily value was used as the representative daily water temperature because the daily temperature range in this seawater system may be slightly greater than in the adjacent bay. Emersion study Sets of four trays were fixed at three intertidal levels in the central region of Pioneer Bay: MI (medium intertidal) at 0.72 m above datum, HI (high intertidal) at 0.78 m, and VI (very high intertidal) at 1.24 m. The intertidal sets of trays of Crawford et al. (1988) at three locations in Pioneer Bay were also used: Point (PI) at 0.59 m above datum, Central (CI) at 0.61 m, and North (NI) at 0.57 m. The trays at the central rack (CR) (subtidal) site of Crawford et al. (1988) were used as a control for the intertidal sites data, as explained in the results. Tidal height at the level of the tops of the clams within each tray set was determined using a surveyor’s level. Chart Datum for Lucinda port, on the adjacent mainland, was indicated at the southern point of Pioneer Bay from a previous study (Parnell, 1986). It was possible to calculate periods of exposure for each of the measured tidal heights from tide tables for Lucinda (Queensland Department of Harbours and Marine, 1986). The tides in this area are semidiurnal, mixed form with neaps down to 0.5-m range and springs up to 3.5-m range. The lowest tide was - 0.22 m below and the highest 3.37 m above datum during this experiment. The growth trials at PI, CI, NI and CR were initiated in January 1986, at MI and HI in March 1986, and at VI in April 1986. The study was extended progressively higher into the intertidal zone as the clams’ tolerances were evident. The number of clams in each tray was reduced to 10 after the Iate-Sep-

48 tember measurement because of crowding terminated at the end of October 1986.

in some trays, and the study was

Locality study Two sets of four trays were placed on a northeast reef of Orpheus Island (OC - oceanic site): one set subtidally, 3.7 m below chart datum, and one set intertidally, ca. 0.35 m above datum. The trays were scheduled to be inspected at weekly intervals, but it was not possible to keep strictly to the schedule because rough seas often limited access. This study was initiated in February 1986 and ran for 5 months. Growth and survival data for the clams at the northeast reef were compared with equivalent data for clams in the intertidal and subtidal zones of Pioneer Bay (Crawford et al., 1988). Sediment traps were placed at the northeast reef site and beside each set of trays in Pioneer Bay. These sediment traps were groups of two or three plastic tubes, 29.5 mm internal diameter and ca. 300 mm length, attached to a vertical post. Sediment collections over two periods were measured by weighing the dried sediment removed from each trap. Light regime study This study was conducted in an aquarium system at the Orpheus Island Research Station. Seawater was pumped in from the edge of the Pioneer Bay fringing reef, 450 m offshore, coarse-filtered through a pressure sand-filter and stored in reservoirs before supply to outdoor fibreglass tanks. A set of four trays with clams was placed at the inlet end of each of three raceways, 2000 1capacity. The inlet flow rate was approximately 250 1 h-l. One set of trays was sited beneath 90% synthetic shade cloth, i.e. excluding ca. 90% of light; another set was sited beneath 50% shade cloth; and the other set of trays was exposed to full sunlight. The trays were not covered with plastic mesh. The clams were inspected daily, but otherwise treated in the same way as the field clams. The study ran from February to September 1986. RESULTS The growth of juveniles of T. gigas from December 1985 to May 1987 is shown in Fig. 1. The discontinuity in the data from October to November 1986 is where the clams were transferred from trays to boxes and the measurements based on a different subset of clams. The clams were 10 months old at the commencement of this study and 27 months at the conclusion. Over this period their mean shell length increased from ca. 30 mm to ca. 180 mm. This repre-

1985

1986

1987

Fig. 1. Growth of T. gigas juveniles in Pioneer Bay from December 1985 to May 1987. The mean shell length values are for groups of clams in the intertidal zone, 0.5-0.8 m above tidal datum. The vertical bars are ?I2 SD.

sents about a 200-fold increase in whole wet weight since clam wet weight is approximately related to (shell length)3 (S.S. Mingoa, unpubl. data, 1988). Seasonal growth

Seasonal variation in growth rate is apparent in Fig. 1, with the growth curve tending to flatten during the winter months, June-August. This pattern is better displayed in Fig. 2, where growth increments are plotted with seasonal water temperature data. High growth rates in summer declining to low growth rates in late winter (August/September) and then increasing again are evident in both the absolute and relative growth increment data. Relative growth increments were higher when the clams were smaller in summer 1985/86; while absolute growth rates were higher when the clams were larger in summer 1986/ 87. Despite the reduction in relative growth as the clams increased in shell length, a linear regression analysis of % monthly growth increment ( Y) versus mean temperature over the period of increment (X) showed a correlation coefficient of 0.610 (PcO.02) for the period of study. The relationship was Y= 1.529X- 29.39. The intercept on the X-axis was 19.2”C, suggesting that this is about the minimum temperature for growth.

50 -.-..a.

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Fig. 2. Seasonal variation in growth increments of T. gigas juveniles and seawater temperature in Pioneer Bay from December 1985 to May 1987. Mean growth increments are based on the data for intertidal clams presented in Fig. 1.

Emersion effects Mean shell lengths and survival of groups of T. gigas juveniles in the intertidal zone are shown in Table 1. Mortality occurred only at the highest intertidal (VI) site. To compensate for seasonal and size-related growth variation (Fig. 2)) the monthly growth increments of a set of subtidal clams (CR - Central rack (see Crawford et al., 1988) ) were used as a control and the growth increments at each of the intertidal sites expressed as a proportion of the increment at CR over the same period. It is recognised that this method of compensation has shortcomings where groups of clams became dissimilar in size to CR, i.e. the VI clams, but there is no ready alternative. Also, the errors in estimating the mean growth increments based on subsamples of 20 out of 80 clams became relatively large in winter when the monthly growth increments were small. For this reason, growth increment calculations were based on overlapping 3-month periods to increase the growth increment values relative to the errors. Relative growth rates of intertidal clams versus mean period of aerial exposure are presented in Fig. 3. These data confirm the clams’ tolerance of emersion. Mean periods of up to 10 h emersion per 24-h period (Fig. 3A) were tolerated without major mortality (cf. low mortality in Table 1) . The deleterious effects of long emersion were, however, clearly evident in very low to

51

TABLE 1 Mean shell lengths (mm) and final percent survival (in parentheses) for T. gigas juveniles in the intertidal zone, and for a control group (CR) in the subtidal zone, over the period January to October 1986. See text for explanations of the site codes Date

Sites and heights above datum (m) CR

15 Jan. 12 Feb. 12 Mar. 12 April 9 May 9 June 4 July 2 Aug. 30 Aug. 27 Sep. 26 Oct.


NI 0.57

PI 0.59

CI 0.61

MI 0.72

HI 0.78

VI 1.24

42.3 49.5 64.2 78.4 89.3 96.4 95.7 100.6 102.7 101.3 110.9 (88.8)

41.2 48.6 57.7 70.0 78.6 87.5 93.1 98.6 103.4 108.6 117.9 (100)

41.8 46.9 56.1 72.8 80.8 91.6 95.2 103.4 108.7 109.6 115.7 (100)

41.5 47.7 57.3 70.0 78.4 87.5 91.9 97.6 102.1 102.8 114.3 (100)

54.9 65.8 70.7 78.3 83.9 86.5 92.4 93.0 96.9 (100)

56.4 62.5 70.5 80.5 86.3 91.6 91.9 97.7 106.7 (100)

65.0 65.0 61.1 62.6 62.6 63.6 65.1 (97.5)

negative growth rates at VI. Negative growth occurred as some shell reabsorption occurred. Furthermore, the shells of clams at this VI site had marked discontinuities in shell deposition, evident as series of parallel edges. It appears that the mantle edge was not able to extend to the previous shell margin after disruption and commenced a new shell margin inside the existing shell (Fig. 4). There were significant to highly significant negative correlations between relative growth rates and mean periods of aerial exposure (broken fitted lines in Figs. 3A-C ) . Excluding the clearly deleterious results for very high intertidal exposure, however, produced quite different results (solid fitted lines in Figs. 3A-C). There was no significant correlation between relative growth rate and mean period of exposure per 24-h period (Fig. 3A); but there was a significant negative correlation between relative growth rate and night emersion (Fig. 3B), and a significant positive correlation with daylight emersion (Fig. 3C). The latter is most surprising in view of the fact that clams retract their mantles - containing zooxanthellae - during emersion. Thus, periods of emersion during the day should subtract from the period of photosynthesis by the clams’ zooxanthellae. Locality Growth and survival data for juvenile clams at intertidal and subtidal sites of the northeast reef at Orpheus Island are compared with pooled data for three

52

intertidal and three subtidal benthic sites (PI, CI and NI; PS, CS and NS ) in Pioneer Bay in Fig. 5. Poorer growth and survival rates were obtained for the clams at the northeast reef despite the more oceanic conditions prevailing there. In a two-way ANOVA the effects of locality, depth and the locality x depth interaction were all highly significant (P-c 0.001) for their effects on final shell length, i.e. on growth rate. The effect of locality was especially important. Whole wet weights were also measured for these groups of clams at the conclusion of 3’

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Fig. 3. Relative growth rates of T. gigas juveniles in the intertidal zone versus mean periods of aerial exposure per 24 h (A), per 12 h of night (B ) and per 12 h of daylight (C ) . Relative growth rates are based on growth increments relative to those of a subtidal group of clams over the same period (see text for further explanation). Two linear regression curves are fitted to each set of data: one (dotted) is fitted to all the data points; the other (solid) is fitted to the data excluding the values (squares) for clams at VI, the highest level of exposure.

Fig. 4. Specimen of T. gigas, ca. 65 mm shell length, from the VI site. Note the disrupted of shell deposition characteristic of high intertidal exposure.

pattern

NORTHEAST REEF

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Fig. 5. Growth (A) and survival (B) of T. gigas juveniles in the intertidal and subtidal zones of two contrasting localities, northeast reef and Pioneer Bay. The vertical bars are f 2 SE.

this study and, as for shell length, there were highly significant effects (P < 0.001) of locality, depth and the interaction, with the effect of locality being especially strong. These differences between localities are at least partly explained by general observations at the northeast reef. The clams were usually pushed against the inshore ends of the trays and their shells were sometimes chipped. These effects were particularly pronounced in the intertidal zone. Clearly, wave action

55 TABLE 2 Mean sedimentation rates and sediment size ranges recorded from sedimentation traps in Pioneer Bay and a northeast reef, Orpheus Island, February to July, 1986. Numbers in brackets are standard deviations Pioneer Bay

Northeast Beef

Intertidal Bate (mg cm-* dd’) Percentage -C63 pm Percentage > 63 pm

2.25 (1.06) 100% 0%

107.2 (58.3 ) 1.6% 98.4%

Subtidal Bate (mg cm-’ dd’) Percentage < 63 pm Percentage > 63 pm

2.85 (1.21) 100% 0%

104.0 (91.2) 9.7% 90.3%

during strong seas was moving the clams about within the trays and this apparently had a disruptive effect on growth. The difference in water turbulence between the two localities was shown by the sediment trap results (Table 2 ) . Water turbidity is often high in Pioneer Bay, yet sedimentation rates were more than a magnitude higher at the northeast reef than in Pioneer Bay. Most of the sediment at the reef locality consisted of particles greater than 63 pm and there was much large grain and shell fragment material; while all the sediment particles collected in Pioneer Bay were of the finest fraction, < 63 pm. The sediment trap contents at the northeast locality result from the periods of heavy wave action when large particles are suspended in the water column. The sediment trap contents in Pioneer Bay reflect turbidity. Light Growth and survival of juvenile clams under three light regimes in the seawater system are shown in Fig. 6. Survival was poor in the 90% shade-cloth treatment and this treatment was terminated after 3 months when only 16% of the initial clams remained (Fig. 6B). There was some mortality in the full sunlight and 50% shade-cloth treatments in the latter part of the study. This was apparently from infestations of ectoparasitic gastropods (Family Pyramidellidae), which recruited onto the clams from larvae in the seawater supply and then proliferated in the absence of predators (see Cumming, 1988). The clams under 90% shade-cloth showed very poor growth compared to those experiencing the higher light regimes (Fig. 6A). There was a small but significant difference between growth in the full sunlight and 50% shade-cloth treat-

56 100

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1986

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Fig. 6. Growth (A) and survival (B) of 2’. gigas juveniles at three light regimes in a seawater system. The vertical bars are zk2 SE.

ments; their final mean shell lengths were 92.8 mm and 88.3 mm, respectively (t-test, PCO.01). Incident radiation levels were measured at noon on 29 June 1986, at the level of the clams in the aquarium tanks, using a quantum radiometer photometer and underwater sensor. The levels were 1684,732 and 298 PE mm2 s-l under the full sunlight, and 50% and 90% shade-cloths, respectively.

51 DISCUSSION

Intertidal and temperature effects

The mean growth rate found here for T. gigas juveniles in intertidal culture in Pioneer Bay (Fig. l), 8.8 mm month-’ shell length increment, is the highest growth rate reported for juveniles of this species and all other giant clam species. It appears that they were growing under very favourable environmental conditions. Other data on mean shell length increment for comparable sized T. gigas juveniles are: 4.9 mm month-’ between ages 24 to 36 months in raceway culture at MMDC (Heslinga et al., 1984); 2.0 and 7.0 mm month-’ in raceway and subtidal culture, respectively, at Bolinao, Philippines (Gomez and Belda, 1988); and 5.7 and 6.7 mm month-’ for wild juveniles about 130 and 170 mm shell length, respectively, based on mark and remeasurement data for Michaelmas Reef, GBR (Pearson and Munro, in press). The work of Crawford et al. (1988) on growth and survival rates of T. gigas juveniles in the intertidal zone is extended by this study. It reveals a threshold somewhere between 5 h and 10 h daily emersion beyond which growth is severely retarded (Fig. 3A); however, survival is not severely affected even at 10 h daily exposure for clams about 65 mm shell length (Table 1). With the VI data excluded, it is apparent that the nonsignificant correlation between mean emersion time per 24 h and growth rate (Fig. 3A) is the net effect of a significant negative correlation for night emersion (Fig. 3B) and a significant positive correlation for daytime emersion (Fig. 3C). Another study of emersion effects on T. gigas juveniles has confirmed these results of negative correlation of night emersion and positive correlation of daytime emersion with growth at Orpheus Island (Nash, 1988). Nash attributed this result to the seasonal differences in day and night emersion periods. In this region extreme low tides occur during the day in winter months, AprilSeptember, while extreme low tides occur at night during summer, JanuaryMarch. Nash suggested that air and water temperatures are lowest when daytime emersion is longest and that this is less stressful. This suggestion does not, however, consider the strong effect of temperature on growth (Fig. 2). In winter the clams will be exposed to both air temperature higher than water temperature and to heating by radiation. In summer the clams will be exposed to night air temperature lower than water temperature. Thus, the disparate effects of day and night emersion on growth rate may reflect these seasonal differences between air and water temperature. This explanation of the positive effects on growth of winter daytime emersion should only be valid if clams are able to exploit the higher temperature during emersion by continuing to photosynthesise in air. A period of higher body temperature and metabolic rate would only increase the metabolic cost of emersion if energy and nutrient uptake are suspended. It has been observed

58

that 2’. gigas juveniles do not close their valves tightly during emersion (note the typical gape in the exposed clam shown in Fig. 4), and it is highly likely that at least some photosynthesis continues in air due to very high incident light levels on the withdrawn mantle tissue. Thus, unlike normal filter-feeding bivalve molluscs for which periods of emersion are lost feeding time, it is possible that giant clams continue to gain energy and nutrients during emersion. Even “normal” bivalves differ in their physiological responses to emersion, e.g. see Widdows and Shick (1985). Measurements of photosynthesis versus respiration (P/R) rates in exposed T. gigas are needed to elucidate the metabolic gains and losses during emersion in daylight and at night for this species. Nash (1988) found that year-old clams were much less tolerant of prolonged emersion than two-year-old clams, so there is an age/size effect on tolerance. Possible seasonal effects (see previous discussion) also mean that these results should not be taken as universally applicable, i.e. that in the intertidal zone at any locality clam growth will be optimal at about 3 h daylight exposure. These results, however, indicate the extent of emersion that T. gigas juveniles will tolerate and that some levels of daytime intertidal exposure can be beneficial to growth. Enhanced growth rate is a further advantage of intertidal culture beyond the various technical advantages, such as ease of construction and maintenance (Lucas, 1987; Crawford et al., 1988). The strong effect of temperature on growth of T. gigas juveniles within the 20-30” C range (Fig. 2) means that annual water temperature range is an important aspect in selecting localities for mariculture. A locality where water temperature is substantially above 20’ C for much of the year is recommended. It is possible that other environmental factors vary in a seasonal pattern and not all the season growth variation (Fig. 1) is due to temperature. Food availability is potentially another major factor in seasonal variation; however, this is not likely to be of great importance for giant clams because of their endogenous source of nutrition. Light and locality effects The lethal and growth inhibiting effects of reduced light (Fig. 6) demonstrate that T. gigas is an obligate phototroph. Although various other authors have concluded that zooxanthellae are very important in the nutrition of this species (e.g. Fisher et al., 1985; Munro and Gwyther, 1981), in no previous paper is it explicitly stated that the symbiosis is obligate for the clam. This experiment was conducted in an aquarium system with coarse-filtered seawater and it could be argued that the supply of suspended organic particulates was unnaturally low; however, the same phenomenon was seen in the field where there was heavy mortality of juvenile T. gigas held subtidally in boxes with heavily fouled lids. The combined light-reducing effects of ca. 5 m depth,

59

turbidity and fouling were sufficient to reduce the incident light to lethal levels for the juvenile clams (Lucas, 1987). The reduction to 50% light caused a significant decrease in growth compared to full sunlight and this is in accord with the observations of photosynthesis versus irradiance (P-I) in T. gigas juveniles by Fisher et al. (1985). They found that juveniles in the 40-100 mm shell length range did not show light saturation of photosynthesis even up to 2000 ,uE rnw2 s-l. With midday full sunlight readings up to about 1800 PE mm2 s-l, any marked reduction in light reaching the clams will cause significant reductions in growth rate. Although our experimental design does not allow an unequivocal answer, it appears probable that the severely retarded growth rates at the northeast reef were caused by the turbulent conditions there (Fig. 5A). The effect was more pronounced at the intertidal than the subtidal site, in contrast to the results for Pioneer Bay. Crawford et al. (1988) attributed the poor growth of clams in subsurface trays to regular disturbance from being suspended from the surface. It is clearly advisable to grow clams in sheltered conditions with little water turbulence. There is no evidence from this study that turbidity per se is detrimental to growth of T. gigas juveniles. Growth rates in Pioneer Bay were higher than at the other two, less turbid, locations tested; the northeast reef and aquarium. Growth rates at these three locations reflect differences in a range of environmental factors, not just turbidity; however, if turbidity was strongly detrimental to growth, it is very unlikely that the best growth would be obtained in Pioneer Bay. This discussion relates to the direct effect of turbidity on filterfeeding and respiration in T. gigus juveniles. At deeper sites the effect of turbidity on the light/depth profile, and hence the light regime experienced by the clams, will be important. Coral reefs The environmental conditions of intense sunlight, water temperature greater than 20°C and lack of turbulence, shown by this study to be required by Tridacna gigs juveniles, are not limited to coral reefs. These conditions occur in other shallow marine environments in the tropics. We suggest that giant clams are invariably associated with coral reefs because of two conflicting environmental requirements of juvenile clams. They must be exposed to high light levels, but they must also be concealed from predators. If well hidden, they die from insufficient light; if well exposed, they are readily detected by predators. The surfaces of coral reefs are highly complex, three-dimensional structures and there are microhabitats where juvenile giant clams can be both cryptic and exposed to light. Braley (1987) suggested that the clumped distributions he observed for T. gigas and T. derasa on the Great Barrier Reef resulted from larval settlement and juvenile survival be-

60

neath thickets of branching coral, through which they ultimately grew. In Fijian waters, T. derma adults are found on plain sandy bottoms, but the juveniles are found on adjacent coral pinnacles from which they later detach and fall to the lagoon floor (Adams et al., 1988). We suggest that complex structure is the particular feature of coral reefs required for juvenile clam survival. Thus, if provision is made for attachment and for protecting the juveniles from predation, it should be possible to culture giant clams in other than coral, reef environments. This is already recognised to an extent at the Micronesian Mariculture Demonstration Center, where giant clams are cultured on a sandy lagoon floor (Heslinga and Watson, 1985)) and in this study, where an intertidal rubble zone was used; however, both these habitats are adjacent to hard coral communities. We suggest that proximity of coral reef need not be a criterion for selecting a suitable site for the ocean phases of giant clam mariculture. ACKNOWLEDGMENTS

This research was part of an international project funded by the Australian Centre for International Agricultural Research. We wish to thank a number of people who assisted with the research at Orpheus Island, especially Stephane Westmore, Emre Turak, Ray Giddins, Geoff Charles, Harry Brown, and Janet Estacion. Dr. John Collins wrote the computer programme for analysis of emersion periods and Dr. Ross Alford assisted with the statistical analyses.

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