In the states of Queensland and Western Australia, Australia, the wild fisheries of the saucer scallop, Amusium balloti, have fluctuated greatly in the last 20 years. Despite considerable interest in the cultivation of A. balloti, there have been many factors limiting the development of A. balloti aquaculture or stock-enhancement programs. One limitation is access to scallop juveniles, as wild spat collection is difficult for this species. To date, breeding of A. balloti using standard hatchery techniques has had limited success. The overarching objective of this research, therefore, is to understand the early life cycle biology of A. balloti and apply this knowledge to improve hatchery and culture methods and technology. The successful and efficient cultivation of A. balloti from fertilised egg through to juvenile should provide the foundation to develop future aquaculture and stock-enhancement programs.
Systematic experimental analysis of different diets and culture water temperatures allowed the identification of conditions that promoted high survival and growth rates of A. balloti larvae. Seven algal species were evaluated as mono-species diets or as combinations over a seven-day trial. As mono-species diets, golden-brown flagellates (lsochrysis aff. galbana (T-iso) and Pavlova lutheri) promoted the highest larval growth and survival compared with other algal diets and the unfed controls. Diatoms (Chaetoceros calcitrans, C. muelleri and Thalassiosira pseudonana) supported reasonable growth and survival, whereas a diatom (Skeletonema costatum) and a green alga (Tetraselmis chuii) were not suitable for A. balloti larvae. Multi-species diets were better than mono-species diets, which supported better larval growth and survival. Of the seven rearing water temperatures tested (14, 16, 18, 20, 22, 24 and 26°C), the optimum water temperature range for A. balloti larvae was between 18 and 20°C, where there was 50 to 95% survival and 14 to 23 µm growth in 11 days. These water temperatures were similar to ambient seawater temperatures of the Central Queensland waters during the peak spawning season for this species (July August).
To precisely define settlement competency in A. balloti larvae, and to determine the ideal period to induce settlement and metamorphosis, larval development and growth was characterized cytologically and morphologically. The nervous system development of larvae, postlarvae and newly-settled spat was assessed by detecting serotonin expression using an anti-serotonin antibody. Serotonin expression was first detected in the apical ganglia of two-day old larvae (48 h after fertilization). This expression appeared to be transient and ceased during metamorphosis. In the later (eyespot) larval stage all three major adult ganglia (cerebral, pedal and visceral) expressed serotonin. Based on immunocytochemical staining patterns, these ganglia appeared to progressively assemble throughout metamorphosis and postlarval development. There was no obvious change in the serotonergic nervous system at competency. The development of larvae and postlarvae and newly-settled spat musculature was assessed by phalloidin staining of myofibrillar F-actin. In early D-veligers, the muscular system was well developed, with the present of the anterior adductor, a number of velar retractors (VR) and posterior retractors (PR). With progressive larval growth, the myofibril mass and complexity of VR and PR increased significantly, and new muscles such as mantle retractors, anterior and posterior body retractors, posterior adductor, and foot gradually formed. After metamorphosis, A. balloti lost the VR and PR but retained anterior adductor and other muscles. The foot retractors were developed after metamorphosis. There were no apparent myogenesis events that correlated with competency. While no obvious change in the central nervous system or muscle architecture of A. balloti larvae was detected that correlated with acquisition of competence, it was observed that larval eyespot appearance was closely linked to competency. This might, therefore, be a useful marker to artificially determine when to induce metamorphosis in the hatchery.
To improve the speed, synchrony, and survival through metamorphosis, various cues, previously shown to be effective in inducing settlement and metamorphosis in a range of invertebrates, were tested both in terms of efficiency of inducing metamorphosis and for assessing the acquisition of larval competency. The results demonstrated that no settlement substrata tested in this study were effective inducers of settlement and metamorphosis. However, biofilms, 10 and 20 mM KCI-elevated seawater, 10-4 M epinephrine and 10-5 M L-DOPA were effective metamorphosis inducers. Potassium chloride was the most effective (15% to 37% higher than controls). However, higher concentration of KCI (e.g. 40 mM) was toxic to A. balloti larvae. These studies again confirmed that the presence of eyespots was the most reliable and prectical morphological indicator of larval competency in A. balloti.
Traditional scallop settlement and nursery systems, which use fibrous substrata to collect larvae, did not promote a high rate of settlement for A. balloti, possibly because of the lack of long-term byssal attachment in A. balloti spat. A downweller screen nursery system was evaluated for settlement of A. balloti larvae and culture of A. balloti spat. Compared to a traditional settlement and nursery system, the downweller screens greatly improved the rate of metamorphosis for A. balloti larvae and the survival of the spat. A mean yield of 57%) was achieved in the downweller screens, compared to 3% using a traditional settlement and nursery systems. Continuous aeration enhanced A. balloti larval metamorphosis in the downweller screen nursery system, with a mean spat yield of 50% achieved, compared to 10% using intermittent aeration and 15% with no aeration.
Previous reports indicated that A. balloti either lacks the ability to produce byssal threads, or could do so only briefly. As the byssal attachment is crucial to the settlement process in many other scallop species, this was examined in detail. Specifically, it was demonstrated that in the early spat stage, A. balloti does secrete a byssus. The first byssal attachment was detected only after A. balloti developed to spat (indicated by the presence of dissoconch shell). By contrast, the newly-metamorphosed postlarvae crawled over the substratum using their foot, and for immobile postlarvae, byssal attachment was not detected. Although spat remained byssally attached for most of the time, they commonly changed position, moving a mean of 17 mm/day. A gentle water jet and 40%0 hypersaline bath were effective in detaching spat, with the majority rapidly reattaching. Byssal attachment was shown to persist until the spat reached 4 - 5 mm, around the stage they first develop an ability to swim.
Large-scale post-hatchery nursery culture and growout could not be included in this study due to outdoor culture facilities being unavailable. However, a proof of concept trial was conducted to compare juvenile A. balloti growth and survival in suspended culture and bottom culture under laboratory conditions. The results showed that preventing juvenile A. balloti from recessing into the sand (suspended culture) may stress the animals and does affect their growth rate. Also, A. balloti is sensitive to high stocking density. Using photographic images can be a valid method to measure shell size in scallops. These results will provide valuable information to develop a post-hatchery nursery culture technique for A. balloti.
Together these data - effects of water temperature and diet on larval growth and survival, identification of the development of larval competency, understanding of external factors controlling settlement and metamorphosis, analysis of postlarval and spat development and behaviour, comparison of suspended cage culture and bottom culture of juveniles and a practical method for rapid, mass shell measurement - allow for the beginnings of an efficient cultivation system for A. balloti on a commercial scale. The application of this study to stock-enhancement and mariculture programs is discussed, particularly in regard to the further development of culturing A. balloti in Queensland and Western Australian.