J.S. Lucas

Quantitative studies of feeding and nutrition during larval development of the coral reef asteroid Acanthaster planci (L.)

Feeding and ingestion rates of Acanthaster planet (L.) larvae were measured over 24-h periods at five concentrations of a unicellular alga, Dunaliella primolecta Butcher, from 5 × 102 to 5 × 104 cells · ml−1. Bipinnaria and brachiolaria larval stages showed the usual inverse relationship between filtration rate and food concentration, although the detailed relationships for particular stages were complex. An unusual feature was that the attenuation of filtration rate with increasing food concentration was insufficient to cause a plateau or reduction in ingestion, except in the earliest larvae. Thus, ingestion rates generally increased with increasing food concentration to the highest concentration used in this study. To study the influence of food quantity on larval development of Acanthaster planci, groups of 120 larvae were reared from early bipinnaria to late brachiolaria at six concentrations of each unicellular alga, Dunaliella primolecta and Phaeodactylum tricornutum Bohlin, measuring rates of development and survival. Starved larvae (at 0 cells · ml−1) survived for several weeks and reached advanced bipinnaria stage. Further development, through the brachiolaria stages with production of a starfish primordium, depended on the environmental food regime. Food concentration for highest rates of larval development and survival was 5–10 × 103 cells · ml−1 for the above algae. Below 5 × 103 cells · ml−1, development was slow; above 5–10 × 103 cells · ml−1, larvae developed at irregular rates. Larvae at high food levels had their alimentary canals packed with cells, most of which passed through undigested, hence their poor performance at high food levels. This was a result of their inability to reduce ingestion rate. Similar rearing techniques were used to study the effects of food quality on larval development of Acanthaster planci. Five other species of unicellular algae were used as diets at 5 × 103 cells · ml−1 and, to take size differences into account, at the same packed cell volume per ml as 5 × 103 cells · ml−1 of Dunaliella primolecta. All these algae supported at least some survival through complete larval development, but there were marked differences in rates of development and survival on the different diets. Levels of phytoplankton numbers, plant pigment concentrations, and volumes of particulates reported for Great Barrier Reef waters are compared with equivalent data for diets in the laboratory studies. Field data are generally low or marginal for the nutritional requirements of Acanthaster planci larvae, suggesting that food is a major environmental influence on survival and development of A. planci larvae in these waters.

The biology, exploitation, and mariculture of giant clams (Tridacnidae)

Abstract Eight giant clam species have been overfished for meat and shells throughout their tropical Indo‐Pacific distributions. The 1960s through the 1980s were particularly severe for the two largest species, due in part to international poaching for their adductor muscle meat. Low densities, erratic recruitment, and a relatively long period to reach harvestable size make these populations prone to overfishing. Declining giant clam stocks and local extinctions were a major stimulus for research into mariculture methods. The life cycles of most species are well understood and a variety of mariculture methods have been developed. However, heavy mortality of the early juvenile stages remains a problem. Recent research has revealed new aspects of giant clam biology, such as the housing of their symbiotic zooxanthellae in a unique complex diverticulum of the stomach. Research has quantified dual modes of nutrition (photosynthates from zooxanthellae and effective filter feeding) that give giant clams a nutrit...

Genetic differences between geographic populations of the Crown-of-thorns starfish throughout the Pacific region

Ten population samples of the Crown-of-thorns starfish Acanthaster planci were collected between March 1982 and August 1983 from localities across the Pacific and were examined for variation at 14 enzyme loci using starch-gel electrophoresis. A sample of A. brevispinus was also examined for comparison. In contrast to the considerable genetic differentiation between A. brevispinus and A. planci populations (D=0.20±0.02), the genetic differences between geographic populations of A. planci were small (D=0.03±0.00; FST=0.07±0.02), in spite of the great distances separating them. A positive correlation was observed between degree of genetic differentiation and geographic scale, suggesting that the genetic homogeneity among A. planci populations is due to gene flow by planktonic larval dispersion. In view of such macrogeographic homogeneity, it is striking that significant allele frequency differences were observed between adjacent populations separated by approximately only 10 km. The Hawaiian population was most differentiated from other populations. Treating the morphologically-distinctive, eastern Pacific Acanthaster as a separate species, A. ellisii, is not supported by these data. The lack of unique alleles in these two central and eastern Pacific populations suggests that they were derived from those in the western Pacific.

Saponins in eggs and larvae of A canthaster planci (L.) (Asteroidea) as chemical defences against planktivorous fish

The suggestion that saponins in eggs and larvae of Acanthaster planci (L.) serve as chemical defences was tested by feeding groups of planktivorous pomacentrid fish with random series of gelatin food particles, some with and some without crude saponin extract from A. planci. The four fish species discriminated at statistically significant levels against food particles with crude saponin extract at 1× 10−3 and 1× 10−5 parts of wet weight. Three species also discriminated at significant levels against particles with 1× 10−7 parts crude saponin extract per wet weight. Degree of discrimination was strongly influenced by the state of hunger of the fish. Tastiness of particles containing saponins also influenced acceptability. The lower two concentrations of saponins used in these feeding trials were respectively two and four magnitudes less than in A. planci eggs and larvae. Thus, the saponins in eggs and larvae of A. planci are at levels detectable by, and unpalatable to, planktivorous fish and they account, at least in part, for the observed rejection of these early developmental stages by planktivorous fish.

Nutrient limitation in the giant clam-zooxanthellae symbiosis: effects of nutrient supplements on growth of the symbiotic partners

The effect of ammonium (5, 10 μM N) and phosphate (2, 5, 10 μM P) on the growth of the giant clam Tridacna gigas and its symbiotic dinoflagellate Symbiodinium sp. was examined. A 3 mo exposure to these nutritients significantly increased the N or P composition of the soft tissues, as reflected in a corresponding change in C:N:P ratio. Furthermore, exposure to N or N+P markedly increased the amount of soft tissue, but P alone did not, demonstrating that increased availability of inorganic nitrogen enhances tissue growth of the clam host. With addition of N, or N+P, there was a significant increase in the total number of zooxanthellae per clam, with a corresponding decrease in chlorophyll a (chl a) content per zooxanthella. However, only with N+P was there an increase in the zooxanthellae mitotic index. The inverse relationship between zooxanthellae number and chl a per zooxanthella is consistent with phytoplankton studies indicating conditions of nutrient-limitation. Furthermore, the unaffected C:N:P composition of the zooxanthellae and their relatively low specific-growth rates (4 to 10%) also suggest that they are nutrient-limited in vivo. In particular, their high mean C:N:P ratio of 303:52:1 indicates that, relative to C, they are much more depleted in P and less in N than are free-living phytoplankton. Furthermore, polyphosphates (phosphate reserves) were undetectable, and the activity levels of acid phosphatase in the zooxanthellae were relatively high and not influenced by the hosts exposure to increased P concentrations in the sea water, implicating the clam host in active regulation of P availability to its symbiotic algae. This is strong evidence that N-limitation of clam zooxanthellae is a function of the availability of ammonium to the symbiosis while, irrespective of nutrient levels in sea water, clam zooxanthellae still show characteristics of P-limitation.

Selective feeding by larvae of the crown-of-thorns starfish, Acanthaster planci (L.)

Abstract. The effect of phytoplankton size on feeding rates of planktonic larvae of the crown-of-thorns starfish Acanthaster planci (L.) was evaluated by examining their gut contents under an epifluorescence microscope. Concentrations of coccoid cyanobacteria in natural seawater ranged between 1.73 and 5.33×105 cells ml-1 and were three to four orders of magnitude greater than that of eukaryotes. Under these conditions, A. planci larvae ingested similar or smaller numbers of cyanobacteria than eukaryotes. Consequently, clearance rates of A. planci larvae on cyanobacteria were approximately three orders of magnitude lower than those on eukaryotes. Cyanobacteria and eukaryotes in the gut of A. planci larvae had mean equivalent spherical diameters (ESD) of 1–2 μm and 3.6–4.6 μm, respectively. Thus, the volume of cyanobacteria ingested was less than 10% of the volume of eukaryotes ingested. Acanthaster planci larvae were fed cultured phytoplankton Dunaliella tertiolecta and suspensions of three different sizes of plastic beads with fluorescence labelling. There was no significant difference in clearance rates on 6 and 20 μm plastic beads. Clearance rates on 1 μm plastic beads were, however, much lower than those on 6 and 20 μm plastic beads. Clearance rates of A. planci larvae on D. tertiolecta (ca. 5 μm ESD) were significantly higher than those on 6 and 20 μm plastic beads. Apart from particle size, this result shows that feeding of A. planci larvae is influenced by other properties of potential food particles.

Approaches to improve cultured pearl formation in Pinctada margaritifera through use of relaxation, antiseptic application and incision closure during bead insertion

Three treatments were tested on blacklip pearl oysters, Pinctada margaritifera, during the bead insertion process as part of research to improve the efficiency of round pearl culture. The oysters .100 to 150 mm shell height were maintained on a commercial farm at Manihiki atoll, Cook . Islands. The three treatments were: 1 immersion of the oysters in 2 mlrl propylene phenoxetol . for 15 min to relax them before operating; 2 disinfection of the operation site with an antiseptic . . 1:50 aqueous dilution of 10% povidone iodine solution ; and 3 closure of the surgical incision with a flexible cyanoacrylate adhesive. The three treatments were applied or not applied to oysters in a 3-factor arrangement to give eight treatment combinations. The treatment combinations were applied to two groups of oysters: Group I, consisting of 96 oysters 12 sets = 8 treatment .

Growth, maturation and effects of diet in Acanthasterplanci (L.) (Asteroidea) and hybrids reared in the laboratory

Abstract Specimens of Acanthaster planci (L.) and hybrids ( A. planci × A. brevispinus Fisher) were reared from in vitro fertilization to almost 8 yr of age in a closed-circuit sea-water system. Growth curves of both kinds of starfish were sigmoidal, and logistic growth curves were fitted as first approximations. However, the growth patterns were not strictly logistic because several different phases of growth were evident: (1) a post-metamorphic phase of exponential growth; (2) a phase of Von Bertalanffy-type growth, leading to sexual maturity at 2 yr of age; (3) slow growth after sexual maturity from 2 to 3 yr of age; (4) no growth and shrinkage after 3 yr of age, leading into a senile phase of gonad inactivity and then death at 5 yr of age and later. Phases 1–3 have been recognized in other asteroids. The pattern of growth in A. planci and hybrid starfish is clearly determinate, as has been found for other asteroids in field and laboratory studies. A. planci and hybrids commenced gametogenesis in the latter part of their second year of development; however, gonad development was much greater in the third and subsequent years. There was a seasonal pattern of gonad development and spawning in these laboratory animals, where water temperature was the only apparent cue for this seasonality. At times the starfish were affected by a disease of rapid spreading necrosis, which was controlled by antibiotic treatments. Juvenile A. planci maintained on coralline algae for 2 yr reached a maximum size of 18 mm diameter, about one twentieth the maximum diameter of siblings that made the normal transition to coral feeding. Scallop-fed A. planci reached a maximum diameter ≈ 60 mm smaller than coral-fed A. planci and scallop-fed hybrids. These data support previous observations of the high susceptibility of growth rate and maximum size to diet in asteroids. The influence of an intrinsic factor, appetite, which changes in nature and intensity during the life-cycle, was also evident.

Field measurement of feeding and movement rates of the crown-of-thorns starfish Acanthaster planci (L.)

The feeding and movement rates of Acanthaster planci (L.) were measured by monitoring tagged individuals on Davies and Little Broadhurst Reefs in the central Great Barrier Reef. Measurements were made on four occasions between October 1987 and October 1988. New samples (n = 20) of starfish were monitored each time. Feeding rates in summer (October samples) were about twice that in winter (June sample) but were significantly depressed following the summer spawning season (January sample). Large adult A. planci (⩾ 40 cm) killed a mean 161 cm2 of coral·d−1 in winter and 357–478 cm2·d−1 in summer. Smaller starfish (20–39 cm) killed 66 cm2·d−1 and 155–234 cm2·d−1 in winter and summer, respectively. Movement rates of A. planci averaged 2.8 m·d−1 at Davies Reef (low density starfish/high coral cover) and 10.3 m·d−1 at Little Broadhurst Reef (high density/low coral). Within the Davies Reef population movement rates were highest in areas of low coral cover and large starfish were more mobile than smaller ones. These accurate measurements of feeding rate of A. planci on the Great Barrier Reef for the first time have important implications for management of coral reefs. Thus, modelling starfish density, coral cover and feeding rate indicates that A. planci populations can fluctuate by several orders of magnitude without causing noticeable coral damage. A threshold level of 1000 A. planci · km−2 (10·ha−1)is proposed as the limit above which depletion of live coral cover on reefs will occur. Control of starfish to much below this level would be little benefit.

Spawning induction, and larval and juvenile rearing of the giant clam, Tridacna gigas

Abstract Specimens of Tridacna gigas (L.) with mature gonads were selected for broodstock using a biopsy technique and induced to spawn using intragonadal injections of serotonin. Some larvae were reared using conventional bivalve rearing techniques in 500-1 tanks, with up to 40% survival to settlement after 7–8 days. Other larvae were reared extensively from eggs added directly to raceways. There was heavy mortality at or soon after metamorphosis in all groups of juvenile clams and survival of juveniles from intensive larval rearing was less than 1% to harvesting at 5 months. Similar numbers of juvenile clams were harvested from raceways to which 4×105 intensively reared pediveligers or 2×107 eggs (extensive culture) were added initially, yielding a total of ca. 6000 juvenile clams. Some juvenile clams were fed cultured microalgae for several months to supplement the nutrients obtained from their symbiotic zooxanthellae. These clams showed poorer survival than unfed clams, apparently because they were more overgrown by benthic algae. The distribution of juveniles in the raceways at harvesting was not influenced by water turbulence or shading, but growth rates were reduced by shading. Growth rates and densities of juveniles were much higher on textured than smooth substrates. We have shown that the mass rearing methods used for bivalve mollusc larvae are appropriate to T. gigas larvae; however, the typical methods for bivalve nursery rearing are not readily applicable to giant clam juveniles. Further research is required on these methods and on factors affecting juvenile survival and growth.