The influence of environmental factors on juvenile Tridacna gigas

Mingoa, Sylvia Suzanne M. (1990) The influence of environmental factors on juvenile Tridacna gigas. PhD thesis, James Cook University of North Queensland.

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Abstract

Giant clams (F. Tridacnidae) have recently been investigated for mariculture, in order to replenish diminishing reef populations and to satisfy commercial demand for clam meat and shell. Development of juvenile-rearing techniques has necessitated a better understanding of the effects of environmental factors on juveniles. This thesis addresses this need by providing physiological bases for clam responses (survival, growth) to various environmental conditions, using Tridacna gigas juveniles cultured at the Orpheus Island Research Station. The following factors were studied separately or in combination: light, temperature, salinity, seawater flowrate, stocking density, nutrient supplements (microalgae, dissolved inorganic nutrients), cleaning, and emersion.

Measurement of rates of oxygen production (photosynthesis) and consumption (respiration) in clams acclimatized to high and low light regimes revealed that photoadaptation occurs in juvenile T. gigas. Zooxanthellar contribution to clam respiration (CZAR) was significantly greater for clams acclimatized to unshaded light conditions (CZAR = 92% at 32% Translocation, T), than those acclimatized to a 90% shaded light regime (CZAR = 72%, T = 32%). Appreciable clam growth was measured in suboptimal light intensities, although tissue condition indices (wet tissue weight/shell length; dry tissue weight/shell length) revealed reduced tissue growth in low light and negative growth in darkness. The irradiance level for maximum photosynthesis rate was significantly greater in high than in low light acclimatized clams (i.e. 206 and 150 uE.m⁻².s⁻¹, respectively).

Studies on upper temperature tolerance of juveniles using the direct transfer method showed better survival at 26, 28 and 29°C than at 33 and 37°C. Whole clam respiration measurements showed that respiration rate was detrimentally affected at temperatures below 19°C and beyond 33°C. Q10 values for temperature intervals between 12°C and 33°C ranged from 1.42 to 3.07, whereas for temperature intervals between 33°C and 40°C, Q₁₀ values were about 1.0.

Salinity tolerance studies also using the direct transfer method showed better survival at 26, 33 and 36 ppt than at salinities 22-0 ppt and 40-45 ppt. Studies on osmotic adaptation to varied salinities, based on comparisons of osmolal concentrations of their extracellular fluids and mantle cavity fluid against external salinities, indicate that Tridacna gigas is an osmoconformer, with its extracellular fluids slightly hyperosmotic to the ambient salinity. Maximum shell growth over a 6 week period occurred at about 35 ppt salinity. Studies on salinity-induced regulation of intracellular free amino acids by High Performance Liquid Chomatography analysis demonstrated that non-essential free amino acids are more important than essential free amino acids in intracellular osmotic pressure regulation.

The effects of seawater flowrate, stocking density, nutrient supplements and cleaning were separately assessed in terms of survival and growth. Studies on nutrient supplements and cleaning employed a 2-Factor experimental design. Clam survival was dependent on stocking density, whereas growth was related to flowrate, specifically flowrate per clam. Supplements and cleaning did not influence survival nor growth, due to confounding factors.

The effects of emersion on juvenile clams were investigated according to their needs for water and energy conservation, by measuring rates of water loss, respiration, excretion and photosynthesis during or after air exposure. Respiration and photosynthesis in air were both demonstrated in T. gigas, although at reduced rates. Respiration measurements on clams exposed to anoxic atmospheres indicated some tolerance of anoxia for an exposure period of 3 h; clams after a 9 h anoxic exposure manifested an oxygen debt 61% greater than that acquired after a 3 h exposure. Water loss after 27 h exposure to desiccating, oxygen-saturated atmospheres was minimal (5%). The rate of ammonia excretion, determined by the phenolhypochlorite method, was dependent on the duration of air exposure.

Overall, this study showed that a change in the environment of juvenile Tridacna gigas elicits corresponding physiological changes: 1) light modifies the photosynthetic capacity and efficiency of the algal endosymbionts, thus clam phototrophic capability, 2) temperature influences clam respiration rate, hence its metabolic rate, 3) salinity affects both extra- and intracellular osmotic balance in juvenile clams, and 4) emersion reduces rates of certain metabolic functions, i.e. respiration, excretion and photosynthesis. This study also showed that conditions of seawater flowrate and stocking density may be controlled to optimize clam growth and survival in culture. Juveniles must therefore be reared in environmental conditions that are non-lethal and not deleterious to physiological processes. Juvenile clams must be grown where light intensities are greater than 200 uE.m⁻².s⁻¹, in water temperatures from 26°C but not exceeding 32°C, in salinities near 35 ppt. Manipulation of clam stocking density and seawater flowrate may either increase water turnover time or flowrate per clam to enhance survival and growth. Juveniles can withstand periods of emersion of up to 27 h in oxygen-saturated atmospheres, and are capable of aerial photosynthesis.

Item ID: 33785
Item Type: Thesis (PhD)
Keywords: giant clams; Tridacna; juveniles; growth; Orpheus Island Research Station; salinity; light; temperature; stocking rates; diet supplements
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Mingoa, S. Suzanne M. (1988) Photoadaptation in juvenile Tridacna gigas. In: Copland, J.W., and Lucas, J.S., (eds.) Giant clams in Asia and the Pacific. ACIAR Monograph Series (9). Australian Centre for International Agricultural Research (ACIAR), Canberra, ACT, Australia, pp. 145-150.

Date Deposited: 21 Jul 2015 04:50
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
06 BIOLOGICAL SCIENCES > 0608 Zoology > 060806 Animal Physiological Ecology @ 50%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8301 Fisheries - Aquaculture > 830103 Aquaculture Molluscs (excl. Oysters) @ 100%
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