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Stichopus

Ecology
Stichopus is a family of aspidochirotid sea cucumber comprising of 14 species. Stichopodae species inhabit soft-bottom habitats such as coral reef lagoons (Stichopus chloronotus, Stichopus herrmanni, Stichopus noctivagus), inshore grass meadows (Stichopus ocellatus, Stichopus herrmanni), sandy and rubble bottoms (Stichopus regalis, Stichopus horrens, Stichopus mollis) and rocky intertidal areas (Parastichopus californicus ). Some species feed during the day while others emerge from crevices or caves at night (e.g. S. horrens, S. noctivagus). Stichopodae, like most aspidochirotids, gather organic detritus and sediments from the upper few millimetres of sediment surface with the help of their buccal tentacles. The main ecological role of aspidochirotids on coral reefs and nearby sandflats is assumed to be bioturbation of the sediment. Bioturbation activity plays an important role in redistribution of surface sediments.Stichopus species digest bacteria, cyanobacteria, decaying seagrass and algae, diatoms, foraminiferans and fungi. Defecated sediment is lower in organic matter than the sediments ingested. Consequently, cyanobacteria outbreaks are prevalent after the extraction of sea cucumbers. Stichopodae species increase local seawater alkalinity and dissolved inorganic carbon through their digestive processes and release of ammonia, facilitating calcification by associated organisms. Dissolution of carbonate sand in the sea cucumber digestive tract due to the low pH of gut fluids results in production of faecal casts higher in pH than the surrounding water.

Spawning
Stichopus species reproduce sexually, following a predictable annual spawning cycle with spawning during the warm season. The warm season is defined as the summer months of the respective hemisphere that the species is found. Lunar periodicity, salinity, photoperiod and food availability influences spawning, acting as endogenous cues Spawning times vary between species; spawning takes place at either dusk (e.g. Stichopus herrmanni, Stichopus chloronotus) or at deep night (e.g. S.horrens).

Pre-spawning behaviour includes rolling and twisting on the substrate. Both males and females are broadcast spawners. Males raise their anterior end prior to spawning and sway side to side  or in a circular motion as they release sperm.

Male stichopodids spawn about one to two hours before females. This is suggested to be a proximal signal for synchronising females to spawn. Males remain erect and can spawn for up to three hours from a single gonopore. Females hold the erect posture for a shorter, 15-30 minute time period before releasing short, powerful bursts of pale yellow/ pink eggs over 10-15 seconds.

Asexual reproduction
One Stichopodid, Stichopus chloronotus, reproduces both sexually by broadcast spawning and asexually by transverse fission Uthicke, 2001,. Asexual reproduction occurs outside the season for sexual reproduction in areas where sex ratios are very uneven. Populations with high rates of fission have lower mean weights, potentially due to the small size of fissiparous populations. Fission rates vary from reef to reef, with up to 60% in some areas. The morphology of the gonads consists of dichotomously dividing tubules located on the two sides of the dorsal mesentery. This morphology is different to solely sexually reproducing species, (e.g. Stichopus herrmanni, S.horrens), which consist of one distinct gonad rather than saccules, ,. Anterior parts of recently divided individuals are greater than the posterior parts,. Yet, the survival of anterior and posterior parts is even,. The process of fission is rapid, taking merely 5 minutes. Prior to dividing, S.chloronotus show a constriction slightly anterior to the middle with white connective tissue at the constriction, which becomes increasingly distinct during the division process,. Shortly, the anterior half of the animal moves forward while the posterior half of the animal remains stationary,. The body wall of fission site is nearly liquid, the two body parts separate without any effort, remaining connected only with a string of mucus for about 30 seconds. The liquid-like body wall morphology remains for about two hours until rapid healing takes place, the body wall having normal consistency and wounds nearly entirely closed after 24 hours,.

Embryonic and larval development
The development of Stichopus species is initiated by the elevation of the fertilisation envelope 20 minutes after fertilisation First cleavage occurs along the animal-vegetal axis dividing the cell into two equal hemispheric blastomeres. Cleavage is equal, radial and holoblastic. Blastula stage is reached by 3 hours. 6 hours after fertilisation, the blastula is covered in cilia and rotates within the fertilisation envelope. Embryos hatch from the fertilisation envelope about 14 hours after fertilisation forming the early gastrula. Gastrula swim with the aid of cilia. Auricularia larval stage is reached 35 hours after fertilisation. During this stage the buccal cavity, the mouth, oesophagus, intestine and the cloaca form. 7-8 days post-fertilisation, the auricularia reach 1.5mm before transforming to the doliolaria stage 9-10 days after fertilisation. During this process, larvae shrink to half their initial size. During the doliolaria stage, the cilia become aligned in five distinct bands along their cylindrical body, the digestive tract forms. At this stage, the larvae swim close to the substrate and successively go through swimming and settling phases.

Juvenile stage
Definitive settlement, generally occurs 12-15 days after fertilisation, depending on the species,. Settling in S. horrens occurs 4 days after most Stichopodid species as colder temperatures prolong developmental time. Settling juveniles are about 1-1.5 mm in length. Juveniles grow at a rate of 0.4 mm-1.0 mm per day, for the next 3-4 weeks. Growth is rapid, after approx. 60 days of settlement, juveniles are 4 cm long and 2cm wide. After reaching 20 cm, at around 210 days of development, growth slows and eventually plateaus in response to food availability, habitat and population size. During the juvenile stage predation risk is high, although survival rate of juveniles is unknown in the wild. In aquariums, survival rate is approximately 33%.

Description and characteristics
The curryfish, Stichopus herrmanni (previously Stichopus variegatus), is a sea cucumber from the class Holothurodiea and family Stichopodae. Stichopus herrmanni is abundant in the Indo-Pacific, inhabiting sandy lagoons, seagrass and coral habitats around 0-30m depth. Curryfish are yellow/brown, covered with small dark spots. It is a medium to large-sized species (300- 55 mm, about 1.8 kg), thick, with low papillae. Size is linked to food availability and population size,.

Diet
Stichopus herrmanni are deposit feeders, playing a critical role in sediment breakdown and nutrient cycling, encouraging seagrass and algal productivity and calcium carbonate cycling. The breakdown of calcium carbonate in sea cucumber gut as a result of digestive processes is suggested to increase local alkalinity and dissolved inorganic carbon, potentially buffering the negative effects of ocean acidification and thereby assisting calcification on coral reefs.

Symbiosis
Stichopus herrmanni hosts many different epibiont species, including mainly echinoderms, such as synaptid sea cucumbers (Synaptula media) ophiuroid seastars as well as crabs, gastropods, shrimps, polychaetes. Both the ophiuroid and synaptid holothuriods are small compared to the hosts. These epibiont species do not affect the diurnal movement or feeding rates of ‘‘Stichopus herrmanni’’.

Reproduction
Curryfish are dioecious, with separate male and female animals. S.herrmanni has an annual reproductive cycle. Gametes develop from late winter to summer. As typical of aspidochirotids, S.herrmanni adopts an erect posture while spawning facilitate dispersal of gametes and fertilisation. Males raise the anterior end prior to spawning and often sway from side to side, or in a circular motion as they release sperm from a single anterior gonopore. Male release of sperm occurs a few hours before females. Male gamete release is spaced over a long period compared to short, dramatic egg release in females. Eggs are fertilised externally which develop into free-swimming larvae. After the spawning period, remaining gametes appear to be absorbed and the gonads enter a resting phase for about 6 months. In temperate environments, a change in temperature, salinity, photoperiod and food availability may initiate this aestivation-like response in reproductive activity. After this resting period, in late winter, gametes start developing, correlating with increasing day length.

Development
Embryos develop into planktotonic larvae (auricularia) then into doliolaria (barrel-shaped stage) which later metamorphose into juvenile sea cucumbers. After a larval stage, sea cucumbers are thought to settle in shallow intertidal areas on coral and crustose coralline algae. The smallest recorded individuals (9-12 cm length) have been recorded in such habitats of 0.2-1.5m depth. Estimated size of sexual maturity is 22 cm, studies on the Australia’s Great Barrier Reef (GBR) have struggled to find individuals below 20 cm length. Stichopodids are known to exhibit rapid growth from juveniles to adults, body length plateauing in larger adults. Juveniles around 10-12 cm in length are estimated to be 3-4 months old. Growth rates of S.herrmanni in situ are unknown, it has been suggested that growth plateaus at larger sizes in adulthood based on the theory of phenotypic plasticity. Growth trajectory is influenced by food availability and habitat. S.herrmanni migrate to deeper waters as they mature. This migration has several ecological benefits: i) increased availability of productive lagoon sediments, suggesting increasing feeding potential (ii) reduced exposure to wave energy, which causes displacement for large adults with a greater surface area : body size ratio and/or (iii) increased reproductive success with increased density of mature adult . Largest animals of this species are 50 cm and are observed at 3-6m depth on Heron Reef . Adult migration occurs at mesoscale as a response to habitat properties, recruitment processes such as the presence of suitable substrate, and carrying capacity of habitats.

Fishing
Sea cucumbers particularly vulnerable to over-fishing due to their ease of collection, simple low-cost processing methods and high commercial value,. Bêche-de-mer (the dried body wall of sea cucumbers) are one of the most conspicuous products traded in Asia’s top-end market for dried seafood. S. herrmanni is a mid- to low-value species that is somewhat difficult to process in comparison to other species. Subsequent to the depletion of the high value black teatfish, curryfish catches, mainly S.herrmanni but also small quantities of Stichopus ocellata and Stichopus vastus, increased rapidly with an average annual pace of 200% from 2007-11. Curryfish populations have been heavily depleted in South East Asia. and parts of the South Pacific (India, Viet Nam, Madagascar, Philippines, Indonesia, Papua New, New Caledonia). In 2009-10, on Australia’s Great Barrier Reef alone, a total of 61000 individuals were caught equating to a total weight of 31.68 tonnes. Comprising of 3% of the 10000 tonnes of dried sea cucumber traded internationally per annum,.

Currently, the fishery operating on the GBR has a minimum size restriction of 35cm, based on the size of first reproduction. There are no constraints on fishing throughout the year. Shallow waters are more heavily impacted. Owing to such high fishing activities, S.herrmanni populations have decreased by 60-90% over 50% of its global range. Recently, S.herrmanni, along with fifteen other sea cucumber species has been listed as Vulnerable by the IUCN. Recovery of populations post-harvest can take decades, depending on Allee effects e.g. low spawner density.