No genetic differentiation of giant clam (Tridacna gigas) populations in the Great Barrier Reef,Australia

Six Tridacna gigas populations were sampled in 1990 from locations throughout the central and northern Great Barrier Reef (GBR). Despite separations in excess of 1000 km, mean Nei's unbiased genetic distances among the populations was 0.0007. The complete lack of spatial variation observed among populations did not results from lack of genetic variability. Genetic variation within populations was high, with mean heterozygosities from 0.221 to 0.250. Gene frequencies were consistent with expectations under conditions of Hardy-Weinberg equilibrium. These data suggest panmixis, or random mating, throughout the highly connected reef system of the central and northern GBR. The large gene exchange among the giant clam populations has important implications for conservation management of one of the few large populations of T. gigas in the world. Small local effects are likely to be overcome in time by inputs from other sources. However, large genetic perturbations, particularly from up-current sources, may spread rapidly through the population.Contribution No. 561 from the Australian Institute of Marine Science     

Nutrient enrichment and the ultrastructure of zooxanthellae from the giant clam Tridacna maxima

The separate and combined effects of ammonium (10M) and phosphate (2M) on the ultrastructure of zooxanthellae (Symbiodinium sp.) from giant clams, Tridacna maxima, were examined in the field. Nitrogen addition significantly changed the ultrastructure of the zooxanthellae inhabiting the clams. After 9 mo exposure, the cross-sectional area of zooxanthellae from N-treated clams was significantly lower than that from other treatments [N=39.3 m²; C=47.9 m²; P=43.2m²; N+P=44.5 m²; (P=0.001)]. There was also a significant decrease in the size of starch bodies, especially around the pyrenoid of the zooxanthellae from N and N+P treatments [N=1.2 m²; C=2.0 m²; P=1.8 m²; N+P=1.2 m²; (P=2.08E-11)]. This presumably occurs as a result of the mobilization of organic carbon stores in response to stimulated amino acid synthesis under enriched nutrient conditions. These data strongly suggest that the symbiotic zooxanthellae of clams are limited to some extent by the availability of inorganic nitrogen, and that relatively minor changes to the nutrient loading of the water column can have substantial effects on the biochemistry of symbioses such as that which exists between clams and zooxanthellae.     

High genetic variability in a population of Tridacna maxima from the Great Barrier Reef

A population of the bivalve mollusk Tridacna maxima (Röding) from Hron Island, Great Barrier Reef, Australia, was studied by gel electrophoresis, and proved to be highly variable genetically, with an average heterozygosity of about 22%. This compares closely with a population of T. maxima from Enewetak (Eniwetok) Atoll, with an average heterozygosity of about 20%, very high for marine organisms. Enewetak Atoll was the site of a series of nuclear tests. The Heron Island study verifies that the high variability is natural, and supports the hypothesis that species from trophically stable environments tend to be highly variable genetically.     

Phylogenetic relationships among giant clam species (Mollusca: Tridacnidae) determined by protein electrophoresis

Analyses of allozyme variation at 26 loci in eight species of giant clams, collected from the Western Pacific between 1989 and 1991, have confirmed the major groupings currently recognised within the family Tridacnidae on the basis of morphological characters: the two genera Hippopus and Tridacna, and the subgenera within Tridacna; i.e. Chametrachea (comprising T.squamosa, T.crocea and T.maxima), Tridacna sensu stricto (containing only T. gigas), and Persikima (comprising T. derasa and T. tevoroa). All these groupings were each supported by several shared characters. In contrast to previous views, Persikima was found to be intermediate between Chametrachea and T. gigas. Although statistical support for branch lengths linking Chametrachea and T. gigas was limited in maximum-likelihood analyses, the occurrence of several shared alleles between these groups, and none shared between only Chametrachea and Persikima, suggests a closer relationship between Chametrachea and T. gigas or, at least, an unresolved trichotomy. Morphological data traditionally assumed to support a closer association of Chametrachea and Persikima demonstrated that only one shared character linked subgenera, and this character supported an evolutionary association between Chametrachea and T. gigas. These findings, together with the number of shared characters supporting the subgenus Persikima, suggest that Persikima is the most basal offshoot of the Tridacna line, and that the group may deserve generic status. Received: 21 November 1997 / Accepted: 24 March 1998     

Calcification of the first larval shell of Tridacna squamosa (Tridacnidae: Bivalvia)

The transistional stage between the trochophore and the veliger larva of Tridacna squamosa Lamarck, including shell formation and initial organogenesis, is described. Calcification of the Prodissoconch I begins before formation of the organic component of the shell is completed, and proceeds in a pattern which reinforces the maximum stress areas of the shell, allowing the early onset of the larval escape behavior. Evidence is presented which suggests that the orientation of the mineral crystals in the larval shells is determined by an organic component of the shell and not by an active process of the clam during calcification.     

Influence of field-based nutrient enrichment on the photobiology of the giant clam Tridacna maxima

Nutrients were added to 12 microatolls in One Tree Island lagoon every low tide for 13 mo to an initial concentration of 10 μM (ammonium, N) and 2 M (phosphate, P). These concentrations remained above background for 2 to 3 h after addition. The addition of ammonium (N and N+P but not P alone) significantly increased P _g (gross photosynthesis) P _n (net photosynthesis) and R (respiration) per unit wet-tissue weight and α (photosynthetic efficiency) in Tridacna maxima after 3 mo nutrient enrichment. These responses to small and transient changes in ammonium concentrations suggest that symbiotic clams are not nutrient-replete, and that even subtle changes in nutrients can have a measurable effect on photosynthesis. The same clams did not show significant differences in photosynthetic parameters 6 mo after the beginning of nutrient enrichment, suggesting that their previous responses had either been seasonal or that symbiotic clams such as T. maxima are able to adjust their photophysiology following external changes in nutrient concentrations. Received: 26 August 1997 / Accepted: 11 December 1998     

Stable isotopic investigation of physiological and environmental changes recorded in shell carbonate from the giant clam Tridacna maxima

The aragonitic shell of the photosymbiont-bearing bivalve Tridacna maxima contains a record of the physiological and environmental changes the organism has experienced during its lifetime. This record is preserved as chemical and microstructural variations throughout the shell. Stable isotopic analyses of oxygen (¹⁸O/¹⁶O) and carbon (¹³C/¹²C) in shell carbonate were combined with growth increment studies to interpret the shell record of specimens collected from the Rose Atoll (Lat. 14°31S; Long. 168°10W) in April 1982. The seasonal water temperature cycle is recorded in the oxygen isotopic signature of the clams, permitting the recognition of annual cycles in the ¹⁸O profile. The total number of these cycles corresponds to the age of a specimen, while the cycle length is a measure of the yearly growth rate. Large-amplitude cycles, reflecting year-round calcification, characterize the early portion of the growth record. With the onset of sexual maturity and slower growth at an age of approximately ten years, the cycles decrease in amplitude and become more erratic. During this later growth phase calcification is limited to the cooler months of the year, perhaps in response to a re-ordering of energy priorities between growth and gametogenesis. A growth curve developed from the ¹⁸O profile indicates rapid juvenile shell growth followed by slower growth thereafter producing a lifespan of several decades. Carbon isotopic analyses of T. maxima were compared to analyses of the symbiont-barren gastropod Terebra areolata collected from the same locality in April 1984. A 2 depletion in the ¹³C composition of T. maxima shell carbonate is attributed to a symbiontenhanced metabolic rate and an increased flow of isotopically light, respired CO₂ into the carbon pool used in calcification. Such a depletion may prove useful in identifying the presence of photosymbionts in extinct species of fossil mollusks.     

Genetic structure of giant clam (Tridacna derasa) populations from reefs in the Indo-Pacific

Large genetic differences were observed among the Great Barrier Reef (GBR), Fiji and Philippine populations of Tridacna derasa (Roding) sampled in 1989 and 1990 (Nei's unbiased genetic distance, D,=0.137 to 0.341). This result contrasted strongly with the low genetic distance (D=0.032) reported previously for the giant clam T. maxima over similar geographical scales. No significant genetic differentiation was observed among most populations from the GBR (mean D=0.007), consistent with the high gene exchange expected in this highly connected reef system. However, significant differentiation resulting from differences in the frequencies of less common alleles between the North-Central GBR and South GBR (Swain region) were observed. Historical isolation of blocks of the Central Indo-West Pacific from the GBR and present-day restrictions to gene exchange between the GBR, Fiji and the Philippines as a result of oceanographic current patterns, were thought to be responsible for the high degree of genetic differentiation of T. derasa populations. The relevance of these findings to clam mariculture and reef restocking are briefly discussed.Contribution No. 197 from the Marine Science Institute, University of the Philippines and Contribution No. 562 from the Australian Institute of Marine Science     

Effect of nutrient enrichment in the field on the biomass, growth and calcification of the giant clam Tridacna maxima

Nutrients were added separately and combined to an initial concentration of 10 μM (ammonium) and/or 2 μM (phosphate) in a series of experiments carried out with the giant clam Tridacna maxima at 12 microatolls in One Tree Island lagoon, Great Barrier Reef, Australia (ENCORE Project). These nutrient concentrations remained for 2 to 3 h before returning to natural levels. The additions were made every low tide (twice per day) over 13 and 12 mo periods for the first and second phase of the experiment, respectively. The nutrients did not change the wet tissue weight of the clams, host C:N ratio, protein content of the mantle, calcification rates or growth rates. However, ammonium (N) enrichment alone significantly increased the total population density of the algal symbiont (Symbiodinium sp.: C = 3.6 · 10⁸ cell clam⁻¹, N = 6.6 · 10⁸ cell clam⁻¹, P = 5.7 · 10⁸ cell clam⁻¹, N + P = 5.7 · 10⁸ cell clam⁻¹; and C = 4.1 · 10⁸ cell clam⁻¹, N = 5.1 · 10⁸ cell clam⁻¹, P = 4.7 · 10⁸ cell clam⁻¹, N + P = 4.5 · 10⁸ cell clam⁻¹, at the end of the first and second phases of the experiment, respectively), although no differences in the mitotic index of these populations were detected. The total chlorophyll a (chl a) content per clam but not chlorophyll a per cell also increased with ammonium addition (C = 7.0 mg chl a clam⁻¹, N = 13.1 mg chl a clam⁻¹, P = 12.9 mg chl a clam⁻¹, N + P = 11.8 mg chl a clam⁻¹; and C = 8.8 mg chl a clam⁻¹, N = 12.8 mg chl a clam⁻¹; P = 11.2 mg chl a clam⁻¹, N + P = 11.3 mg chl a clam⁻¹, at the end of the first and second phases of the experiment, respectively). The response of clams to nutrient enrichment was quantitatively small, but indicated that small changes in inorganic nutrient levels affect the clam–zooxanthellae association. Received: 2 June 1997 / Accepted: 9 June 1997     

Sperm motility and longevity in the giant cuttlefish, Sepia apama (Mollusca: Cephalopoda)

The sperm kinetics and fertilisation literature in marine invertebrates is heavily biased toward free-spawning species. Nonetheless, many species (e.g. cephalopods) transfer and/or fertilise gametes in confined external spaces or internally, creating very different selective pressures on sperm storage, sperm longevity and hence sperm competition. Here we report the results of an investigation into the effects of sperm age, water temperature and sperm concentration on sperm motility in the giant cuttlefish (Sepia apama). Significant positive correlations were found between percent motility and sperm concentration, and between sperm motile speed and sperm concentration. Mean percent motility of cuttlefish sperm suspension was still 9% eight hours after being released from the spermatophore and diluted into filtered seawater at 12°C (ambient field temperature during the spawning season). Sperm resuspended from spermatangia taken from (mated) females in the field were motile for up to 100 hours. When spermatophores were stored at 4°C motility was still observed in resuspended sperm after two months. Our results show that spermatangia and spermatophores can retain and release live sperm for long periods. The observed longevity of sperm in S. apama greatly increases the potential for sperm competition in this species.     

First evidence of gender-related differences in immune parameters of the clam Ruditapes philippinarum (Mollusca, Bivalvia)

For the first time, gender-related differences in some important functional parameters of haemocytes from the clam Ruditapes philippinarum have been studied. Clams were collected during the pre-spawning phase, when sex can be distinguished by microscopic observation of gonadal smears, in order to ascertain whether the two sexes reach the stressful spawning period with different degrees of immunosurveillance. Total haemocyte count (THC), haemocyte size frequency distribution, capability of haemocytes to assume the vital dye neutral red (NR, indicative of endocytotic activity), lysozyme-like and acid phosphatase activities in both haemocyte lysate (HL) and cell-free haemolymph (CFH), and superoxide dismutase (SOD) and catalase (CAT) activities in HL were all evaluated. No differences in THC values were found between the two sexes, but differing haemocyte size frequency distributions were observed: the fraction of larger haemocytes (7–9 μm diameter, 200 fl volume) was markedly increased in females and that of smaller haemocytes (<5 μm diameter, <200 fl volume) markedly increased in males. Interestingly, when haemocytes were observed under the light microscope, a significantly higher percentage of granulocytes was found in haemolymph from females, compared with that of males. Significantly increased NR uptake was recorded in female haemocytes. No significant variation in lysozyme-like activity was observed in HL, whereas in CFH, enzyme activity was significantly higher in females. HL acid phosphatase activity was significantly higher in males with respect to females; no significant variations in enzyme activity were observed in CFH. HL SOD and CAT activities were significantly higher in females than in males. Overall, on the basis of the cell parameters studied here, gender-related differences in immune parameters were found in R. philippinarum, indicating that females have more active haemocytes than males during the pre-spawning period.     

Gene flow among giant clam (Tridacna gigas) populations in Pacific does not parallel ocean circulation

Tridacna gigas (L.) populations were sampled from 13 locations throughout the west Pacific in 1990–1991 and screened for allozyme variation at seven polymorphic loci. Significant genetic differences were found between the few large stocks of T. gigas remaining in the world. Cluster analysis identified two major regional groups of populations: a Central Pacific group consisting of Kiribati and the Marshall Islands, and a West Pacific group comprising all other populations. Within the latter group, the Great Barrier Reef populations were significantly differentiated from those in the Solomon Islands. The main routes of gene flow did not run parallel with known major surface currents as might be expected, but crossed them. Gene exchange was limited east-west between the Central and West Pacific groups and the GBR and the Solomon Islands. The lack of correspondence between the major surface currents and the routes of gene flow suggests that the genetic structure of T. gigas reflects historical patterns of migration that no longer occur, rather than presentday dispersal. These findings raise fundamental questions concerning the origin and maintenance of genetic diversity in Pacific faunas, and have important implications for the mariculture and conservation of T. gigas.     

Coral-spawn slicks in the Great Barrier Reef: preliminary observations

Aerial surveys and surface samples in the spring of 1985, showed that slicks of coral eggs and embryos (coral spawn) formed in large numbers within and between reefs in the Central Great Barrier Reef Region. Coral-spawn slicks appeared on the days immediately following the annual mass multispecific spawning of reef corals. They were white or pink in colour, and were often highly elongate in form extending up to 5 km in length, and 10 m in width. Over 99% of each slick sampled during the surveys consisted of dead eggs, embryos and their breakdown products, which formed dense, highly viscous patches readily recognizable during aerial surveys. Despite the low proportion of live material, the slicks contained high concentrations of live embryos (15 to 230 per litre) which were over two orders of magnitude greater than concentrations in adjacent water masses. The distinctive colour, shape and texture of the coral-spawn slicks generally distinguished them from slicks formed by blooms of the blue-green alga Oscillatoria erythraea, which also occur commonly in the region. Many of the slicks were closely associated with surface oceanographic features, such as fronts between water parcels, and wakes and eddies behind reefs. Although more detailed sampling and surveys are required, these results suggest that it may be possible to track the movements of coral embryos and larvae directly for the first one or two days following mass spawning, and indirectly thereafter by monitoring other surface features.     

Composition of the near-reef zooplankton at Heron Reef,Great Barrier Reef

Using a light trap, zooplankton was sampled at three stations at Heron Reef, Great Barrier Reef: (a) a typical patch reef in the Heron lagoon, (b) a site in 8 m water on the southern slope of Heron reef, and (c) a station approximately 300 m south of (b), in the open water of the channel between Heron and Wistari reefs. Samples were taken at the surface and on the substratum at the lagoon and reef-slope stations, and at 3 to 6 m depth at the open-water station. A total of 114 taxa, many recognized as species, were distinguished in the samples. Pronounced differences existed in abundance, diversity, and taxonomic composition of the samples obtained at different stations. Less pronounced differences existed between surface and substratum samples from the same station. Near-reef samples were more similar to one another than to open-water samples. Decapod larvae, amphipods, and cumaceans were all abundant in near-reef samples and very rare in open-water samples. Forams, isopods, mysids and polychaetes were common in near-reef samples, and rare or absent in open-water samples. Copepods were abundant in all samples but the near-reef samples contained predominantly different species than did samples from the open water. The near-reef fauna included 66 taxa which did not occur in open-water samples. Many of these were epibenthic rather than strictly planktonic in behaviour.     

Zooplankton community structure and water flow in the lee of Helix Reef (Great Barrier Reef, Australia)

To investigate which physical processes contribute most in moulding zooplankton community structure in the waters close to coral reefs, light traps moored in a grid pattern were used to collect zooplankton from the sea surface at 16 stations on the downstream side of Helix Reef during three time periods (2100-2200, 2400-0100, and 0300-0400 hours) over three consecutive nights covering the new moon period in January 1992. Two distinct zooplankton communities were present: a community composed primarily of reef-resident, demersal plankton immediately to the south of the reef in an area of reduced flushing, and a community containing coastal and shelf-seas taxa at the more exposed sites in the open flow field. The fauna composition at a number of exposed stations was as rich as that at sheltered stations both in terms of number of taxa and diversity indices but was almost an order of magnitude less abundant. The reef-resident, demersal plankton community was dominated by gammarid amphipods, mysids, and polychaetes, whereas only transient, meroplanktonic forms such as echinoderm and echinopluteus larvae and shelf-seas, holoplanktonic forms such as doliolids and larvaceans were significantly more abundant in the exposed community. Zooplankton associations were apparently formed by a combination of hydrodynamic processes, spatial and temporal distribution patterns of individual taxa, specific behaviours of certain taxa, and the interactions among taxa at different trophic levels.     

Peribuccal organs ofPlacopecten magellanicus andChlamys varia (Mollusca: Bivalvia): Structure,ultrastructure and implications for feeding

In order to better understand the structure of bivalve peribuccal organs and relate this to existing functional paradigms of their role in feeding, the labial palps of two scallop species,Placopecten magellanicus from the Bay of Fundy, Canada (1985 and 1986), andChlamys varia from the Bay of Brest, France (1986), were examined using histological techniques and electron microscopy. The ridged palp surface displays a uniformly dense ciliation with relatively few mucocytes; these are essentially concentrated in the region of the secondary ledge and may, through their secretory activity, determine the fate of particle masses in this area. The mucus secretions of the ridged palp surface are qualitatively different from those of the smooth palp surface. Mucocytes are much more abundant on the smooth palp surface, where it is suggested that their homogeneous secretions attenuate the potentially adverse effects of anteriorly-directed cleansing and swimming currents. Two other cell types are found in the palp epithelia: ciliated cells, which are very numerous on the ridged surface and relatively rare on the smooth surface, and non-ciliated epithelial cells, which are very numerous on the smooth surface and rare on the ridged surface, where they are confined to the palp margin. In addition to the mechanical role of the ciliated cells and mucocytes, the ultrastructural characteristics of the ciliated and non-ciliated epithelial cells indicate a dichotomy of function between the ridged and smooth surfaces. The ridged surface epithelial cells present an ultrastructural specialization in the absorption of dissolved and colloidal matter, suggesting an accessory nutritive role, whereas the smooth surface simple epithelial cells show signs of active molecular synthesis. No specialised sensory cells were observed on the ridged surface; it is therefore not yet possible to conclude whether the labial palps are capable of selection based on individual particle characteristics.     

Occurrence and characteristics of unusual protistan symbionts from surgeonfishes (Acanthuridae) of the Great Barrier Reef,Australia

The occurrence of unusual symbiotic microorganisms was examined in the intestines of a range of fish from the Great Barrier Reef, Australia. The fish taxa examined included 26 species of the family Acanthuridae, as well as representatives of phylogenetically related and herbivorous taxa. The microorganisms, referred to as protists, were only found in herbivorous and detritivorous members of the Acanthuridae. Protists were not found in planktivorous acanthurids, nor in any members of the families Kyphosidae, Pomacentridae, Scaridae, Zanclidae, Siganidae and Bleniidae we examined. In addition, protists were absent from the herbivorous acanthurids A. xanthopterus and A. nigricans. A range of protist forms, characterized by differences in size (8 to 417 m), shape and mode of cell division (daughter-cell production and binary fission), was observed. The occurrence of these forms appeared to be correlated with host feedingecology. Large forms (>100 m) of the protists were only found in acanthurids which fed over hard-reef substrata. Smaller forms were found in sand-grazing and detritivorous species. One of the protist forms appears identical to protists previously reported from Red Sea acanthurids.     

Genetic structure of fissiparous populations of Holothuria (Halodeima) atra on the Great Barrier Reef

Allozyme variation at five polymorphic loci was surveyed in a total of 311 individuals of the sea cucumber Holothuria (Halodeima) atra (Jäger, 1833) collected from two nearshore and two midshelf populations in the Great Barrier Reef in November 1996. Strong deviations in genotype frequencies from those expected under Hardy–Weinberg equilibrium, particularly a large number of heterozygote excesses, confirmed the occurrence of asexual reproduction. Females and males differed significantly in genotype frequencies as a result of differences in either the amount of fission in each sex or in the mortality of asexually produced recruits. The estimated maximum sexual input (number of sexually produced individuals: sample size = N*:N_i) to the two nearshore reefs (38 to 67%) was low relative to that to the two midshelf reefs (74 to 87%). The three ratios and G _o :G _e , N _go :N _i , N*:N _i , (where G _o = observed genotypic diversity, G _e = expected genotypic diversity, N _go = number of genotypes) considered to be indicators of the extent of asexual reproduction, showed a consistent trend in the degree of asexual reproduction similar to that derived from the number of regenerating individuals observed in the populations for which data were available. F-statistic analyses of clonal gene frequencies demonstrated that all populations received sexual recruits from the same gene pool. There was evidence of restricted sexual recruitment to the Fantome population, suggesting that asexual reproduction was dominant only in areas where sexual recruitment was limited by other factors.     

The distribution of herbivorous fishes on the Great Barrier Reef

The composition and functionality of ecologically important herbivorous fish assemblages were examined throughout much of Australia’s Great Barrier Reef (GBR). Diversity and abundance of surgeonfishes (Acanthuridae), parrotfishes (Labridae) and rabbitfishes (Siganidae) were strongly associated with position on the continental shelf, whilst effects of latitude were weaker and inconsistent. Species distributions varied considerably amongst taxonomic groups; parrotfishes were mostly widespread whilst distributions of surgeonfishes were often restricted. Most inshore environments supported depauperate herbivore assemblages dominated by different taxa and functional groups compared with assemblages in offshore environments. There were also strong cross-shelf transitions in the main taxa performing each functional role. Overall, this study show that the functional contributions of herbivorous fish assemblages to important ecosystem processes and the contributing taxa vary considerably amongst different GBR environments. Additionally, the two most numerically dominant herbivores actively select detritus, not algae, supporting increasing evidence for the importance of detritus in coral reef ecology.     

Molecular data from the 16S rRNA gene for the phylogeny of Veneridae (Mollusca: Bivalvia)

Veneridae, a cosmopolitan and ubiquitous family in world-wide littoral environments, lists more than 500 species, many of which are often subject to intensive commercial exploitation due to their numerical dominance in benthic communities. Historically, the family has been divided into 12 subfamilies by M. Keen and, even though this taxonomical arrangement was adopted for convenience and does not necessarily reflect genetic relationships, as suggested by Keen herself, this classification is still accepted. To contribute to clarify the systematics and phylogeny of Veneridae, the portion of the nucleotide sequence corresponding to domains IV and V of the gene encoding the large subunit of mitochondrial ribosomal RNA was analysed in 14 species representative of 10 genera belonging to 6 different subfamilies. The results obtained using the maximum-parsimony and neighbour-joining methods indicate that the current placement of the genera into subfamilies does not always reflect a natural subdivision. In addition, the six species of Tapetinae studied, though confirmed to be a monophyletic clade, do not exhibit a correct attribution at the genus level.