Perhaps one of the most overlooked underwater species is sea cucumber. This tiny creature, with its flexible sausage-shaped body and tentacles surrounding its mouth and feet, has one of the most important contributions in the fisheries sector ecologically and economically speaking.
Considered as “earthworm of the sea floor”, sea cucumber feeds on deposits. As it filters the particles from water, it reduces the excess organic matter in the water. When it buries itself into the sea floor, it helps put the oxygen into the sediment so that other organisms can live well beneath the surface.
For people living in the coastal areas, sea cucumber is an important source of livelihood. One of its most common forms of processed product is known in the international market as trepangor beche-de-mer. These dried sea cucumbers are considered high-value export commodities. The Philippines, one of the major trepangexporters, caters to the demand of Asian countries including Taiwan, Hongkong, Korea, Japan, Singapore, and China. Traditionally, these countries use sea cucumbers in the preparation of various dishes such as soups and noodles. But due to nutritional and medicinal claims, sea cucumbers are now being consumed more for its tonic value rather than its taste. Dried sea cucumbers are rich in protein and are regarded to reduce arthritic pain, treat high blood pressure, and other illnesses.
Because of the increasing demand for sea cucumbers, yet lack of management measures that will ensure its conservation, the sea cucumber industry is facing the problem of overexploitation which prompts an impending decline in their population. Heavily fishing can result to their extinction which would impact the welfare not only of the people but of the environment as well.
A sought-after sea cucumber species
In the country, Stichopus horrens is considered as one of the sought-after species and is among the top five sea cucumber species that is providing income to the fisherfolk. Locally known as hanginan, S. horrens is a nocturnal species and remains hidden in the reefs during the day. In China and Malaysia, such species is used in the nutraceutical industry and as traditional medicine. Its coelomic fluid called gamatis believed to possess medicinal properties, thereby often processed into products such as gamatoil, gamatwater, balm, toothpaste, and soap. In Mindanao, hanginan is a popular delicacy among the Muslims prepared especially during the Ramadan.
Efforts to mass culture a commercially important species
Given the vast diversity of sea cucumbers, there is a need to develop culture techniques to explore more opportunities for other commercially important species such as S. horrens. Currently, only the Holothuria scabra has available technologies for mass culture which was initiated by the University of the Philippines Diliman-Marine Science Institute (UP-MSI) in 2000.
This prompted Dr. Marie Antonette Meñez of UP-MSI to lead the project on “Experimental Scale Culture of the Sea Cucumber Stichopus horrens (Selenka)”. It aimed to generate knowledge on the reproductive biology, as well as to develop an experimental scale culture for S. horrens.
Through the project, the reproductive biology of S. horrens was studied to determine the reproductive activities of the species, to characterize its spawning behavior, and to determine the rate of success of in situ fertilization that would provide valuable information on the appropriate management strategies for sea cucumbers.
There were 426 individuals of S. horrens collected in the coastal waters of Masinloc, Zambales for examination. Upon studying the structure of the gonads (glands in which sex cells are produced), it was revealed that the species are dioecious. This means that the S. horrens possess male and female reproductive organs. Through the use of an inverted microscope, the sexes were distinguished and described as follows: male gonads are thinner, with yellow-orange color, and the tubules are highly branching while the female gonads are thicker, with light yellow color, and the tubules are less branching. The stages of maturity were also classified as immature, developing, mature, and spent.
Based on gonadosomatic index (percent ratio of gonad weight to emptied body weight), reproductive activity of S. horrens was recorded to be higher during the southwest monsoon (i.e. from June to August 2011) whereas within a lunar cyle, it was higher during full moon. S. horrens, being nocturnal, were found to be active during the night. Spawning activity was observed to happen between 10 pm to 1 am, lasting for 3-4 hours that is 3-5 nights after the third quarter of the moon phase. When spawning, the species tend to stretch and sway the anterior part of their body (resembling a standing position). Through in situ fertilization, rate of success ranged from 54.64% to 72.54%. It was noted that females in close proximity with the males have higher fertilization success than the solitary ones. This may pose an implication towards maintaining an effective spawning population of sea cucumbers.
The project also embarked on the experimental scale culture of the S. horrens that will help in the development of optimal rearing strategies for the reproduction of the species. In the production of juvenile (young) S. horrens, eight spawning events were observed in the hatchery tanks at the Bolinao Marine Laboratory. Out of the eight, only four batches produced a total of 1, 604 early juveniles.
To determine other factors that can improve the rearing of larvae and production of juvenile, larvae were fed with the algae Isochrysis galbana (Ig) and Chaetoceros calcitrans (Cc). Higher survival of the larvae was recorded when fed with a 50-50 mixture. Mixed diets, according to previous studies, may provide better nutrition during stages of development. Larger growth was also observed when the species were fed with Cc and Ig-CC mixed diet. However, developmental stage was observed to be faster when fed with Cc alone.
Settlement induction was also carried out. Treatments utilizing the algae Sargassum and Spirulina as pastes were used as inducer. The number of larvae which have successfully settled and became juveniles was higher using the Sargassum treatment with 1.43% survival rate as compared with the Spirulina treatment with 0.62% survival rate.
Through bottom-set sea cages which served as an ocean nursery, the early juveniles were reared in the field. It was observed that an additional surface area may not necessarily affect and/or enhance the growth and survival of the juveniles.
The regeneration capability of the juvenile S. horrens through induced transverse fission (dividing across the mid-body of sea cucumbers) was also determined. After 43 days, it was observed that the species have completely recovered and survived the transverse fission. However, higher survival rate was observed for the bigger juveniles.
The behavior of the species was also observed in terms of their habitat preference and effect of light. It was revealed that early juveniles do not exhibit any preference towards any particular shelter, even when exposed to light.
Commercially important marine resources such as sea cucumbers that are thriving in the wild are the ones highly vulnerable to overexploitation and drastic species depletion. In order to protect and preserve them, regulations and management policies should be implemented that will monitor fishing activities. Backed up by strict enforcement, they will play an important role in the sustainability and survival of such invertebrates. Meanwhile, research and development efforts focused on technologies for the cultural management of sea cucumbers such as this project should also be strengthened as these are seen essential to reinforce conservation of the species and to increase production so as to meet the demands of the local and international markets.
The article is based on the project titled “Experimental Scale Culture of the Sea Cucumber Stichopus horrens (Selenka)”. For more information, you may contact Dr. Marie Antonette Meñez at (02) 922-3959 or through email: firstname.lastname@example.org
Choo, P.S. 2008. Population status, fisheries and trade of sea cucumbers in Asia. In V. Toral-Granda, A. Lovatelli and M. Vasconcellos (eds). Sea cucumbers. A global review of fisheries and trade. FAO Fisheries and Aquaculture Technical Paper. No. 516. Rome, FAO. pp. 81-118.
Choo, P.S. 2008. The Philippines: a hotspot of sea cucumber fisheries in Asia. In V. Toral-Granda, A. Lovatelli and M. Vasconcellos (eds). Sea cucumbers. A global review of fisheries and trade. FAO Fisheries and Aquaculture Technical Paper. No. 516. Rome, FAO. pp. 119–140.
By: Anne Camille B. Brion, BAR Digest, July-September 2013 Issue (Vol. 15 No. 3)