Deposit-feeding Sea Cucumbers Research Articles

Exploring Biofouling Control by the California Sea Cucumber (Apostichopus californicus) in Integrated Multi-Trophic Aquaculture (IMTA) with Organic Chinook Salmon (Oncorhynchus tshawytscha)

The growth of biofouling on aquaculture infrastructure is a universal challenge. Standard industry practices to remove biofouling in finfish aquaculture typically include in situ net cleaning via power washing. Since those cleaning practices can be potentially harmful to fish-gill health and expensive, development of other non-toxic biofouling controls is an industry priority. Deposit-feeding sea cucumbers are potentially well suited for biofouling control due to their feeding mechanism, but remain relatively untested in this capacity. We examined the use of California sea cucumbers (Apostichopus californicus) to control biofouling on cages containing adult Chinook salmon (Oncorhynchus tshawytscha) at a commercial farming operation. Four cage types were established: cages with salmon and sea cucumbers, cages with salmon only, cages with sea cucumbers only, and cages without either species. Results showed that the sea cucumbers actively fed on biofouling when salmon were absent (~16% cleaner on average) but preferred to consume uneaten feed/faeces at the bottom of the cages, neglecting the biofouling, when the salmon were present. It is hypothesized that biofouling control in cages with salmon may be possible with an increased density of sea cucumbers. This is the first study to examine the use of sea cucumbers as a direct net biofouling control agent with adult fish. Our results will be beneficial for industry to develop standard operating procedures for using California sea cucumbers as a biofouling control and could contribute to the development of a management framework for sea cucumber/salmon integrated multi-tropic aquaculture.

Evaluating sea cucumbers as extractive species for benthic bioremediation in mussel farms

Filter-feeding mussels blend suspended particles into faeces and pseudo-faeces enhancing organic matter flows between the water column and the bottom, and strengthening benthic-pelagic coupling. Inside operating farms, high bivalve densities in relatively confined areas result in an elevated rate of organic sinking to the seabed, which may cause a localized impact in the immediate surrounding. Deposit-feeding sea cucumbers are potentially optimal candidates to bioremediate mussel organic waste, due to their ability to process organic-enriched sediments impacted by aquaculture waste. However, although the feasibility of this polyculture has been investigated for a few Indo-Pacific species, little is known about Atlanto-Mediterranean species. Hence, for the first time, in the present study, we conducted a comparative investigation on the suitability of different Mediterranean sea cucumber species, to be reared in Integrated Multitrophic Aquaculture (IMTA) with mussels. A pilot-scale experiment was accomplished operating within a mussel farm where two sea cucumbers species, Holothuria tubulosa and Holothuria polii, were caged beneath the long-line mussel farm of Mytilus galloprovincialis. After four months, H. tubulosa showed high survivorship (94%) and positive somatic growth (6.07%); conversely H. polii showed negative growth (− 25.37%), although 92% of specimens survived. Furthermore, sea cucumber growth was size-dependent. In fact, smaller individuals, independently from the species, grew significantly faster than larger ones. These results evidenced a clear difference in the suitability of the two sea cucumber species for IMTA with M. galloprovincialis, probably due to their different trophic ecology (feeding specialization on different microhabitats, i.e. different sediment layers). Specifically, H. tubulosa seems to be an optimal candidate as extractive species both for polycultures production and waste bioremediation in M. galloprovincialis operating farms.

Appetite in captivity - feeding studies of the red sea cucumber Parastichopus tremulus

The deposit feeding sea cucumber Parastichopus tremulus is an underutilised resource in North Atlantic waters. Geographically it is distributed from the Barents Sea in the north to the Canary Islands in the south. At present performance of P. tremulus in aquaculture is largely unknown. Species and stage specific biological knowledge gaps need to be filled for a potential industry to develop, and feeds that support growth needs special attention. Particulate matter (sludge) from fish farms is an unutilised resource that has potential as ingredient in feeds for sea cucumbers, which would help to reduce the environmental footprint of P. tremulus aquaculture production. The suitability of salmon sludge as a feed ingredient is unknown. Feeds using dried salmon freshwater sludge (50% and 75% volume ratios) or seaweed powder (Sargassum spp. 25%, 50% and 75% volume ratios) were compared in this study. Feed mixes with different ratios of ingredients and sand (0.6-1 mm) were given in excess (50% wet weight/wet weight animal/week) to adult P. tremulus. Daily feed intake was estimated by measuring daily faeces production rate. Each animal was given all feeds sequentially, and faeces collected for a ten-day period. Absorption efficiencies were estimated based on analysis of organic matter content in feed and faeces. Large variations were found in feed intake, both between individuals and between days. Our results indicated that P. tremulus showed a higher intake of feeds containing seaweed, with a trend of higher intake with increasing seaweed content. Absorption efficiency estimates of seaweed-based feeds ranged from -337 to 73.7%. P. tremulus showed a preferential selection of organic particles in the feed with lowest content of seaweed. Absorption efficiency of feeds containing sludge (2.5 – 58.3%) was comparable to that of feeds containing seaweed, however, feed intake of sludge-based feeds was significantly lower than that of the seaweed-based feeds and resulted in large variation in estimates. The results suggest that salmon freshwater sludge could have a potential future use as an ingredient in sustainable feeds for P. tremulus, but that optimisation of feed formulations need to be studied further.

Carbon supplementation promotes assimilation of aquaculture waste by the sea cucumber Holothuria scabra: Evidence from stable isotope analysis

Cultivating high-value deposit feeders such as sea cucumbers on solid waste from intensive aquaculture is an innovative approach to turn potential nutrient pollution into a resource. Carbon supplementation to increase the carbon to nitrogen ratio (C:N) of the nitrogen-rich aquaculture waste has been demonstrated to support higher deposit feeder biomass, but commercially viable carbon sources remain to be tested. This experiment investigated the performance of laboratory grade cellulose and bagasse, an agricultural waste product, to improve growth and nutrient utilization from milkfish aquaculture waste in juveniles of the sea cucumber Holothuria scabra. Supplementation with bagasse supported a significantly higher final biomass density (205.71 ± 11.08 g m−2, mean ± SE) than waste without carbon supplementation (p = 0.027). For the first time, a species-specific trophic enrichment factor (Δ13Corg) was applied in stable isotope analysis of feed sources by H. scabra (Δ13Corg of 2.56 ± 0.96‰ (mean ± SD) for the internal organs and 4.31 ± 0.96‰ for the body wall). Results indicated that sea cucumbers were able to assimilate carbon directly from the cellulose. Bagasse supported higher uptake of carbon from aquaculture waste, thus identifying it as a viable resource to improve bioremediation by deposit feeding sea cucumbers in integrated aquaculture. While illustrating the importance of nutrient balances for bioremediation, questions remain about the roles of direct feeding by deposit feeders vs. the decomposition by microbial communities.

Current and future trophic interactions in tropical shallow-reef lagoon habitats

Calcium carbonate (CaCO3) sediments are the dominant form of CaCO3 on coral reefs accumulating in lagoon and inter-reefal areas. Owing to their mineralogy and a range of physical parameters, tropical CaCO3 sediments are predicted to be more sensitive to dissolution driven by ocean acidification than the skeleton of living reef organisms. How this scales up to impact infaunal organisms, which are an important food source for higher trophic levels, and thereby ecosystem functioning, is not well explored. We combined seasonal field surveys in a shallow-reef lagoon ecosystem on the Great Barrier Reef, Australia, with stable isotope analyses and a tank-based experiment to examine the potential top-down influence of the deposit-feeding sea cucumber, Stichopus herrmanni, on this infaunal community under current and future ocean pH. Densities of surface-sediment meiofauna were lowest in winter and spring, with harpacticoid copepods (38%) and nematodes (27%) the dominant taxa. Stable isotope analyses showed that S. herrmanni had a top-down influence on meiofauna and microphytes with a distinct δ13C and δ15N trophic position that was homogenous across seasons and locations. Tanks that mimicked sandy shallow-reef lagoon habitats were used to examine the effects of ocean acidification (elevated pCO2) on this trophic interaction. We used outdoor control (sediment only) and experimental (sediment plus S. herrmanni) tanks maintained at present-day and near-future pCO2 (+ 570 µatm) for 24 days, which fluctuated with the diel pCO2 cycle. In sediment-only tanks, copepods were > twofold more abundant at elevated pCO2, with no negative effects documented for any meiofauna group. When included in the community, top-down control by S. herrmanni counteracted the positive effects of low pH on meiofaunal abundance. We highlight a novel perspective in coral reef trophodynamics between surface-sediment meiofauna and deposit-feeding sea cucumbers, and posit that community shifts may occur in shallow-reef lagoon habitats in a future ocean with implications for the functioning of coral reefs from the bottom up.

Assimilation of shrimp farm sediment by Holothuria scabra: a coupled fatty acid and stable isotope approach

Deposit-feeding sea cucumbers are efficient nutrient recyclers and have the potential to contribute to the limitation of organic matter load in polyculture or integrated aquaculture systems. Assessing how they assimilate organic matter originating from other farmed species is therefore important for the development of such multi-species farming systems. Here, a coupled stable isotope − fatty acid approach was used to characterize the assimilation of organic matter from shrimp (Penaeus stylirostris) farming by Holothuria scabra in an experimental culture system. H. scabra were reared in mesocosms on shrimp farming-originating sediment with and without additional food sources (maize and fish meals). Although fatty acid results did indicate that shrimp-farming sediment was assimilated by holothurids, we found no evidence of maize waste and fish meal contribution to H. scabra organic carbon (no effect on δ13C, no accumulation of meal-specific fatty acids). However, a strong effect of fish meal on H. scabra δ15N was observed, suggesting that this additional food source could represent an alternative source of nitrogen for holothurids. Finally, this study supports the culture of H. scabra as a perspective to reduce sedimentary organic matter excess associated with shrimp farms, and suggest that the addition of selected food sources might contribute to increasing the content in some nitrogen organic compounds in holothurid tissues.

A deposit-feeder sea cucumber also ingests suspended particles through the mouth.

Although the sea cucumber Apostichopus japonicus has been characterised as a deposit feeder, nutrients sourced from the water column have been recorded in the intestines of this species. However, the mechanisms whereby nutrients in the water enter the intestinal tract of A. japonicus, and whether other suspended particles can be ingested via the mouth of A. japonicus adults, remain unknown. Here, we reveal how A. japonicus ingests suspended particles through the mouth. We used synthetic particles and video recording to confirm the suspension uptake by the sea cucumber. Apostichopus japonicus continued to ingest suspended particles (if present) over time, and the particle ingestion rate was positively correlated with the concentration of suspended particles (Pearson correlation: r=0.808). Additionally, clearance rates of the suspended particles ranged from 0.3 to 0.9lh-1 The findings of this study thus provide evidence of a previously undescribed particle uptake mechanism in a commercially important species.

Repeated selective enrichment process of sediment microbiota occurred in sea cucumber guts.

Deposit-feeding sea cucumbers repeat ingestion of sediments and excretion of faeces daily and consequently increase bacterial abundance in sediments and promote organic matter mineralization. Such ecological roles are expected to be collaborative activities of sea cucumbers and the gut microbiota. Here, we performed a spatiotemporally broad 16S rRNA gene analysis using 109 samples from sea cucumber faeces and habitat sediments to explore potential contribution of their gut microbiota to the ecological roles. Most operational taxonomic units (OTUs) observed in the faecal samples were shared with the sediment samples, nevertheless faecal and sediment microbiota differed from each other in UniFrac analysis. Lower bacterial diversity and increased relative abundance of specific OTUs in the faecal microbiota strongly suggest selective enrichment of ingested sediment microbiota in their guts. Interestingly, representative faecal OTUs were more abundant in sea cucumber-populated sediments than in un-inhabited sediments, indicating bacteria selectively enriched in the guts were spread on ambient sediments via faeces. Moreover, the predicted microbial community metabolic potential showed a higher abundance of genes related to carbohydrate and xenobiotics metabolisms in faeces than in sediments. Our study suggests the repeated selective enrichment transforms ambient sediment microbial communities and maintains the host's ecological roles by promoting organic matter mineralization.

Growth performance and survival of Holothuria theeli (holothurian) fed with feces of Tripneustes depressus (echinoid): A multi-trophic aquaculture approach

Integrate Multi-Trophic Aquaculture (IMTA) for deposit-feeding sea cucumbers and herbivorous sea urchin is currently a booming industry worldwide, including South America. Hence, it requires novel biotechnological protocols for culturing juvenile sea urchins and sea cucumbers with a high quality of natural food that meet organisms' nutritional needs. This study evaluated the effect on the growth performance (Specific Growth Rate, Feed Conversion Ratio, Feces Production) and survival of the sea urchin Tripneustes depressus (producer of feces) and the sea cucumber Holothuria theeli (consumer sea urchinʼs feces). Two independent experiments were conducted. For T. depressus, experiment #1: diet of seaweeds (D1: Padina durvillaei, D2: Sargassum ecuadoreanum, D3: Kappaphycus alvarezii, D4: mix of three seaweeds) and size categories (small, S: 15.0 ± 1.1 mm test diameter, TD and large, L: 25.0 ± 1.1 mm TD) were considered. For H. theeli, experiment #2: feces of diets of experiment #1 and sediment (with, without sediment) were considered. Experiment #1, SGR: small and large sea urchins fed P. durvillaei grew and gained weight faster, respectively (S: 3.0 ± 0.05 cm TD, 7.9 ± 0.2 g; L: 2.1 ± 0.1 cm TD, 5.3 ± 0.1 g). Sea urchins fed P. durvillaei and those of small size had lower FCR (diet: 7.2 ± 0.6 g; size: 13.1 ± 2.4 g, P < .05) and large and small sea urchins fed P. durvillaei (L: 31.1 ± 2.7 g d−1; S: 11.1 ± 1.2 g d−1) and mixture of seaweeds produced higher amount of feces per day, respectively (L: 28.9 ± 1.9 g d−1; S: 10.9 ± 1.2 g d−1, P < .05). Experiment #2, sea cucumbers fed diets plus sediment grew and gained weight faster (1.69 ± 0.31 cm, 4.1 ± 0.4 g, P < .05) than those diets without sediment (0.96 ± 0.25 cm, 3.3 ± 0.4 g). Sea urchins kept in the starved condition massively died at day 80, but sea cucumbers in the starved condition without sediment reduced their length and weight, by 85%, but did not die. This study highlighted the importance of feces of T. depressus on the growth of the sea cucumber H. theeli. This is a perfectible protocol for production of juveniles of T. depressus and H. theeli at IMTAs in Ecuador.

Composition of phytodetrital food resources affects reproductive success in the deposit-feeding sea cucumber, Parastichopus californicus (Stimpson 1857)

Phytoplankton are important sources of dietary fatty acids (FAs) for higher trophic levels, yet FA composition varies among phytoplankton taxa, and under different growth conditions. Changes in the environment that lead to shifts in the composition of phytoplankton communities may thus alter FA supply in marine food chains. Reproductive processes in free-spawning marine invertebrates, which provision eggs with lipid to fuel early embryonic development, are particularly impacted by FA composition of the diet. In order to explore the effects of taxonomic changes in the source of phytodetrital food resources on invertebrate consumers, we conducted captive feeding experiments to examine differences in reproductive fitness of deposit-feeding sea cucumbers (Parastichopus californicus) fed two different algal diets. Body- and egg-condition variables were measured in females that were fed detritus of either the green alga Tetraselmis sp. or the diatom Thalassiosira sp., which differ in nutritional and FA composition. Subsequent timing of development and survival were recorded for pre- and post-feeding larval stages. FA analyses were conducted on feeds, spawned gonads, and eggs to identify specific FAs allocated to reproduction. Females that were fed Tetraselmis sp. had higher fecundity, but showed reduced larval survival relative to females that were fed the diatom Thalassiosira sp. Similar rates of larval development were recorded in both treatments. Significant differences were observed in the abundance of 20:5ɷ3 (EPA), 20:4ɷ4 (ARA), 22:3ɷ6 (DHA), 12:0, 16:0, and 18:0 FAs in eggs and gonads from females fed the two diets. Diet-related variation in fecundity and egg quality could directly affect recruitment success in P. californicus, and suggests that reproductive strategies may be altered under different environmental conditions. If feeding conditions are favorable to planktotrophic larvae, then producing a larger number of eggs with lower energy density (such as under Tetraselmis-fed treatments) may be acceptable, whereas different conditions may favor maternal investment in a smaller number of better-provisioned eggs (such as in Thalassiosira-fed treatments).

Sea cucumber (Apostichopus japonicus) eukaryotic food source composition determined by 18s rDNA barcoding

Food source determination of deposit-feeding sea cucumbers is difficult because the majority of organic debris in gut cannot be reliably or accurately identified by conventional microscopic examination. Advanced DNA barcoding is an effective and rapid technique to determine dietary composition of such feeders. In this study, the composition of eukaryotic food sources of the sea cucumber, Apostichopus japonicus, harvested in two bays of an offshore island in winter and spring was surveyed by high-throughput DNA sequencing. The 18s rDNA v4 loci sequences were amplified to provide a total of 616,184 optimized reads and 823 operational taxonomic units. Winter group sequences were assigned to 27.20 +/- 3.63 and 28.40 +/- 2.88 phyla, and those in spring to 25.80 +/- 3.90 and 24.20 +/- 3.11 phyla. Eukaryote richness was higher in winter than in spring, with Dinoflagellata, Bacillariophyceae, Arthropoda, Cercozoa, and Mollusca the dominant phyla in decreasing order of importance. Our results indicated the guts of A. japonicus contained more organic material sourced from the water column than from within the sediments. The profiling table and cluster analysis revealed winter and spring samples clustered separately, indicating significant seasonal differences in eukaryotic composition. Main winter dietary components were Bacillariophyceae, Dinoflagellata, and Streptophyta, whereas those in spring were Bacillariophyceae, Arthropoda, and Mollusca. In any one season, gut contents did not differ significantly between sampling locations. Our study indicates that DNA sequencing can greatly improve the accuracy of diet studies on sea cucumbers.

Growth performance of the sea cucumber Holothuria scabra and the seaweed Eucheuma denticulatum: integrated mariculture and effects on sediment organic characteristics

Deposit-feeding sea cucumbers play a key role in marine ecosystems through biotur- bation, burrowing and feeding on organic matter in marine sediments. Many deposit-feeding holothurians have therefore been recommended for integrated multitrophic aquaculture systems (IMTA). We set up an integrated mariculture system of sea cucumber Holothuria scabra and sea- weed Eucheuma denticulatum in Bweleo, Unguja Island of Zanzibar, Tanzania, to investigate the effect of stocking density on the growth and survival of culture species, total organic matter (TOM) and total organic carbon (TOC) content in the sediment. Treatments that included a fixed stocking density (500 g, ca. 200 g m �2 ) of E. denticulatum and 4 sea cucumber stocking densities (monoculture, low, medium and high density; 0, 150 ± 5, 236 ± 24, 345 ± 48 g m �2 , mean ± SD) of medium-sized H. scabra (114 ± 37 g) were established. Stocking density of H. scabra did not influ- ence survival of either species. Seaweed cultured under high stocking density of H. scabra had a higher specific growth rate of 2.33% d �1 than that cultured at the medium or low densities or with- out sea cucumbers. Sea cucumbers cultured at low stocking density had a higher mean growth rate of 0.80 g d �1 compared to those cultured at medium or high densities. TOM and TOC in sed- iments decreased over the experimental period at medium sea cucumber stocking density, while at low and high stocking densities, organic matter accumulated. The study demonstrates that the integration of E. denticulatum and H. scabra at 200 g m �2 enhances seaweed growth and can reduce organic matter content in the sediments.

Redox stratification drives enhanced growth in a deposit-feeding invertebrate: implications for aquaculture bioremediation

Effective and affordable treatment of waste solids is a key sustainability challenge for the aquaculture industry. Here, we investigated the potential for a deposit-feeding sea cucumber, Holothuria scabra, to provide a remediation service whilst concurrently yielding a high-value secondary product in a land-based recirculating aquaculture system (RAS). The effect of sediment depth, particle size and redox regime were examined in relation to changes in the behaviour, growth and biochemical composition of juvenile sea cucumbers cultured for 81 d in manipulated sediment systems, describing either fully oxic or stratified (oxic-anoxic) redox regimes. The redox regime was the principal factor affecting growth, biochemical composition and behaviour, while substrate depth and particle size did not significantly affect growth rate or biomass production. Animals cultured under fully oxic conditions exhibited negative growth and had higher lipid and carbohydrate contents, potentially due to compensatory feeding in response to higher microphytobenthic production. In contrast, animals in the stratified treatments spent more time feeding, generated faster growth and produced significantly higher biomass yields (626.89 ± 35.44 g m-2 versus 449.22 ± 14.24 g m-2; mean ± SE). Further, unlike in oxic treatments, growth in the stratified treatments did not reach maximum biomass carrying capacity, indicating that stratified sediment is more suitable for culturing sea cucumbers. However, the stratified sediments may exhibit reduced bioremediation ability relative to the oxic sediment, signifying a trade-off between remediation efficiency and exploitable biomass yield.

Deposit-Feeding Sea Cucumbers Enhance Mineralization and Nutrient Cycling in Organically-Enriched Coastal Sediments

BackgroundBioturbators affect multiple biogeochemical interactions and have been suggested as suitable candidates to mitigate organic matter loading in marine sediments. However, predicting the effects of bioturbators at an ecosystem level can be difficult due to their complex positive and negative interactions with the microbial community.Methodology/Principal FindingsWe quantified the effects of deposit-feeding sea cucumbers on benthic algal biomass (microphytobenthos, MPB), bacterial abundance, and the sediment–seawater exchange of dissolved oxygen and nutrients. The sea cucumbers increased the efflux of inorganic nitrogen (ammonium, NH4 +) from organically enriched sediments, which stimulated algal productivity. Grazing by the sea cucumbers on MPB (evidenced by pheopigments), however, caused a net negative effect on primary producer biomass and total oxygen production. Further, there was an increased abundance of bacteria in sediment with sea cucumbers, suggesting facilitation. The sea cucumbers increased the ratio of oxygen consumption to production in surface sediment by shifting the microbial balance from producers to decomposers. This shift explains the increased efflux of inorganic nitrogen and concordant reduction in organic matter content in sediment with bioturbators.Conclusions/SignificanceOur study demonstrates the functional role and potential of sea cucumbers to ameliorate some of the adverse effects of organic matter enrichment in coastal ecosystems.

Organically selective movement and deposit-feeding in juvenile sea cucumber, Australostichopus mollis determined in situ and in the laboratory

Highly accurate methods were used to investigate varying types of feeding selectivity in juvenile sea cucumber, Australostichopus mollis both in situ and in laboratory experiments. Juvenile A. mollis exhibited significant levels of positive selectivity for organic particles. Nitrogen enrichment in fine grain foregut samples exceeded 100% compared to diet sediments, whilst carbon enrichment was as high as 145%. Foregut organic enrichment was, however, less than that reported for other aspidochirote holothurians. In laboratory behavioural experiments significantly more juveniles inhabited areas where sediment total organic matter (TOM) was higher. However, the ability of juveniles to distinguish between different TOM levels of sediments became limited once TOM in treatments exceeded approximately 3%. Comparative analyses of grain size spectra in foregut and diet sediments, however, showed feeding is non-selective for particle size. The ability of juvenile A. mollis to effectively select organic particles from sediments underlines the importance of deposit-feeding sea cucumbers in nutrient recycling and regulation in coastal sediments.

Feeding, selection, digestion and absorption of the organic matter from mussel waste by juveniles of the deposit-feeding sea cucumber, Australostichopus mollis

An understanding of the feeding and digestive capabilities of juvenile sea cucumbers is an important step for their aquaculture development. The feeding behaviour, food selectivity, digestion and absorption of total organic matter (TOM) were determined for juveniles (35–40 g wet weight) of the Australasian brown sea cucumber, Australostichopus mollis, kept in circular plastic tanks (9 l; 6 replicate tanks per feed treatment). Sea cucumbers were fed with feeds containing different levels of TOM (1, 4, 12 and 20%) prepared from sand mixed with the biodeposits of aquacultured green-lipped mussel, Perna canaliculus. No significant diurnal patterns in digestion were detected in all feed treatments. As the TOM level in the feed increased, the selection of organic particles decreased and most importantly overall nutrient absorption increased, which is likely to lead to improved growth rates. However, absorption efficiency declined as organic matter content of the feed increased. This study suggests that palatable-artificial diets rich in organic matter can be used to increase nutrient uptake and potentially facilitate increased stocking densities of sea cucumbers under controlled conditions.

Highly localised distribution patterns of juvenile sea cucumber Australostichopus mollis

Information on the environmental characteristics of the juvenile habitat of many deposit-feeding sea cucumber species is limited, despite most fished species exhibiting rapid localised depletion. The current study combined large and small scale surveying techniques within a New Zealand harbour to identify areas with high densities of juvenile Australostichopus mollis, a commercially valuable aspidochirote holothurian. Data from detailed surveys were used to relate densities of juveniles and adults with measures of physical habitat characteristics including depth, sediment facies type, grain size range, as well as measures of chlorophyll-a, phaeopigment, carbon and nitrogen content of surface sediment. Results revealed a highly localised distribution of juvenile A. mollis focused on one site associated with an area of high adult density. Sites of high juvenile A. mollis density were characterised by sediment qualities favouring epibenthic detritivorous deposit feeding, including high nitrogen content, high phaeopigment:chlorophyll-a ratio and small grain size. The high-density juvenile site had facies that were further characterised by the presence of large shell fragments (>10 cm length) of the horse mussel (Atrina zelandica), which may provide a unique settlement microhabitat for early juveniles. Unlike some other sea cucumber species, juvenile A. mollis shows no distinct spatial separation from adult sea cucumbers, no association with dense macroalgae and no clear preference for shallower depths than adults. Overall, the results illustrate the highly localised pattern of recruitment of this species to a widely distributed adult population, which may help to explain the lack of previous observations of juveniles in this species. These results indicate the importance of identifying and protecting what appear to be very specific juvenile habitats in deposit-feeding sea cucumbers to ensure continuing recruitment to exploited populations.

Do benthic sediment characteristics explain the distribution of juveniles of the deposit-feeding sea cucumber Australostichopus mollis?

Despite the economic importance of many deposit-feeding sea cucumbers, the ecology of their juveniles is poorly understood and factors influencing juvenile habitat selection remain largely unexplained. We investigated the importance of the characteristics of the available sediment in determining the highly localised distribution of juveniles of the deposit-feeding Australasian sea cucumber Australostichopus mollis. Wild-caught juveniles were displaced to non-juvenile habitats with surface sediments characterised by lower total organic content (TOM) and nitrogen content, higher chlorophyll- a content and coarser grain size profiles compared to juvenile sites. The growth of displaced individual animals was monitored over 9 months and compared to control animals caged in the juvenile habitats. Displaced juvenile sea cucumbers had high survival rates that did not differ significantly from juvenile habitats. Displaced juveniles exhibited significantly higher specific growth rate (SGR) than those at juvenile sites ( p < 0.001), although the growth of individuals was highly variable within individual cages and among sites. The lower TOM and nitrogen content, and coarser grain size profiles at non-juvenile sites did not result in reductions in juvenile survival or growth. Higher microphytobenthic activity may have resulted in the higher growth rates observed at shallow non-juvenile sites. The SGR of juveniles over the first 6 months of the experiment ranged between 0.45% d − 1 and 0.74% d − 1 for all sites. This was followed by marked growth limitation between 6 and 9 months either as a result of increasing juvenile biomass in cages or seasonal growth limitation. A subsequent reduction in juvenile density resulted in markedly increased growth over the following 3 month period. Juvenile A. mollis show an ability to exploit a variety of benthic sediment food sources, indicating that their highly localised distribution is not due to differences in the food quality of sediments, but is more likely to be due to other localised effects, such as larval settlement or predation.

Preliminary study of management of red tide water by the filter feeder Mytilus edulis galloprovincialis

An examination was made to determine whether red tide water masses could be controlled by the filter-feeding bivalve, Mytilus edulis galloprovincialis L., cultured on rope collectors. The mussels effectively retained food particles larger than about 4 μm, and their retention rate increased with increase in plankton density between 10 5 and 5×10 6 cells ml −1, this being equal to the density of plankton in red tide water masses. Thus, plankton in red tide water masses should be removed rapidly from seawater by mussels through their retention and excretion, since the faeces excreted by mussels that had been fed on plankton were deposited speedily. Furthermore, the faeces excreted by mussels which contain a large amount of organic matter unabsorbed by the mussels, were eaten by an edible deposit-feeding sea cucumber, Stichopus japonicus. Thus, certain species of plankton concentrating in red tide water masses may serve not only as food sources for filter feeders and deposit feeders, but can also be removed from the seawater.