Research and farming techniques Larval rearing and spat production of the windowpane shell Placuna placenta S. Dharmaraj, K. Shanmugasundaram and C.P. Suja Central Marine Fisheries Research Institute, Tuticorin Research Centre, 115, N.K.Chetty street, Tuticorin-628001 (India), e-mail:[email protected] The windowpane shell, Placuna In Tuticorin Bay, the windowpane and antibiotics were not used during placenta is one among the pearl shell occurs in a 0.46 ha bed and its larval development. We observed the producing bivalves. It was identified as exploitation during the fishery is almost development under an inverted the second-priority mollusc species for total leading to the depletion of stock microscope using 10x10 magnification. research during the Second Conference in the bay. The natural population has After 24 hours straight-hinged larvae on Aquaculture Development in to be augmented in order to meet the were collected in 40µm mesh sieve. The Southeast Asia held in the Philippines needs of mariculture. We therefore larvae were reared in two 75 liter in July 1994 and is the basis for a undertook to develop the technology rectangular fiberglass tanks. Feeding ‘kapis’ (windowpane shell) fishery in for the production of seed of the was initiated at veliger stage on day the Philippines. Windowpane shell species in the hatchery and to sea two. The unicellular microalga provides fishermen an additional ranch them for the replenishment of Isochrysis galbana was given as food income-generating source through the wild stock. The report is the first to be to the larvae once in a day at a sale of its pearls and shells. The empty published on larval rearing and seed concentration of 5,000 cells larva -1 day - shells are used as raw material in production of P. placenta from India. 1 (10 cells µl -1) from day two; 10,000 making shell craft products and are cells day -1 (20 cells µl -1) from day five; exported. Techniques 15,000 cells day -1 (30 cells µl -1) from In India the distribution of the day seven; and 20,000 cells day -1 (40 species is confined to the Kakinada Bay The windowpane shells, collected from cells µl -1 from day ten. The microalga in Andhra Pradesh1,2,3,4; to the the Tuticorin Bay in the Gulf of was cultured in Conway medium15 and Okhamandal Coast in the Gulf of Mannar, were brought to the Shellfish harvested during its exponential phase. Kutch5,6; to the Nauxim Bay of Goa7 and Hatchery of the Tuticorin Research Algal density was assessed using to Tuticorin Bay8 and Vellapatti near Centre of the Central Marine Fisheries haemocytometer and proportionate Tuticorin9. The oysters were fished from Research Institute. Prior to the feed was given. Water change was these areas in considerable quantities experiment, these broodstock animals done once in two days by siphoning every year for pearls and shells causing were kept for 24 hours in aerated the larvae through a sieve with the concern about over exploitation of wild seawater held in a rectangular tank. mesh size smaller than the larval size. stocks. The development of techniques The oysters were treated for spawning Random sample of 50 larvae was on breeding, larval rearing and spat on the following day by thermal measured along the dorsoventral axis production of the species might stimulation at 37ºC at 1445 hours. (DVM) and anteroposterior axis (APM) eventually help to sustain the fishery. Males started to spawn at 1530 hours as a parameter to assess the growth. Hence efforts are made in different parts followed by females. When the The spat were allowed to settle free in of the world to breed the windowpane spawning was over the brood stock the tank itself, as they did not have shell in captivity and produce the seeds animals were removed and the eggs byssus thread or cement gland for for replenishment of wild stocks. allowed to fertilize. The eggs were attachment. No cultch material was Previous research has documented the yellow in colour. provided for settlement of spat. spawning and larval development of P. We collected the fertilized eggs by Aeration was given only after spat placenta10; documented the techniques gently siphoning through a 30µm sieve. setting. Unicellular microalga I. in induced spawning and early The material collected in the sieve galbana was continued to be supplied embryonic and larval development11; contained fertilized eggs, faecal as food for the spat up to 1.0mm and studied the effect of salinity on the matters, broken tissues, shell fragments was gradually replaced afterwards by embryonic development, larval growth and other waste materials. The mixed algal food. The mixed algae, and survival at metamorphosis12; contents of the sieve were passed chiefly containing Chaetoceros sp.and evaluated the effect of microalgal diets through an 80µm sieve and the other diatoms, were developed in and rearing condition on gonad maturity, unwanted materials discarded. The outdoor tanks. During larval rearing the fecundity and embryonic fertilized eggs that passed through water temperature ranged from 28ºC to development13; and reported on 80µm sieve were allowed to develop in 30ºC; salinity 34.6 – 35.2 ppt. and pH hatchery management techniques14. the tank and the larval development 7.91-8.09. was studied. No aeration was provided 20 aquaculture Asia Research and farming techniques Window-pane shell Placuna placenta. Fertilized eggs with polar body, size 50µm. 4-celled, 8-celled and 32-celled stages. Straight-hinge larvae, average size 79.9 x 65.2µm. Typical umbo stage, size 140 x 130 µm. Spat with its transparent shell. Larval development The spawned eggs are spherical in shape measuring 50µm. Fertilization was immediate and the first polar body was released 15 minutes after fertilization. First cleavage started after another 15minutes leading to the two-celled stage. The two-celled stage had a smaller micromere and a larger macromere along with a polar body at the furrow of cleavage. The trefoil stage, containing three micromeres and one macromere was obtained four minutes. After the two- celled stage. The cell wall of the 3 micromeres slowly disappeared and resulted in the four-celled stage 65 minutes after fertilization. The macromere did not take part in further Hatchery reared juveniles of P. placenta, average size 26.4 x 10.6 cell divisions. The 8-celled, 16-celled and 32-celled stages mm on day 135. are obtained in 80 minutes, 100 minutes and 155 minutes respectively after fertilization. Morula stage was reached in April-June 2004 (Vol. IX No. 2) 21 Research and farming techniques four hours and 45 minutes. The embryo conspicuous granules. The time 200µm on day five, pediveliger at 215 x began to move at the morula stage. As sequence of early embryonic 205µm on day seven, and plantigrade at the larvae move in water column, the development of larvae of P. placenta is 235 x 210µm on day eight. On day nine morula is transformed to a blastula given in Table 1 and the time series the umbo larvae constituted 14%, stage by the development of a growth data of the species from veliger eyespot 28%, pediveliger 34% and blastocoel, which opens to the exterior to spat is presented in Table 2. plantigrade 24% of the population. The through blastopore. Gastrulation spat had grown to 340 x 300µm on day commenced at this stage by Larval growth ten. On day thirteen the pediveliger convolution of cells to interior through formed 14%, plantigrade 36% and spat blastopore. After completion of The relationship between the larval 50% of the population. In view of such gastrulation, the dermal layers namely shell length (APM) and shell height heterogeneity in growth, the average ectoderm, mesoderm and endoderm are (DVM) is linear and is described by the size of larvae at different stages was formed along with an archenteron. By equation: taken into account in working out the the formation of single flagellum at the Y=17.982 + 0.9595X with r = 0.9981 growth rate. The average shell height apical end, the embryo reached Where Y represents APM and X on day one is 65.2µm; 81.6 µm on day trochophore stage in 5 hours and 41 represents DVM in µm. three; 121.6 µm on day six; 205.8 µm on minutes. The ectodermal cells secrete On day three all larvae were at day nine and 300 µm on day thirteen. embryonic shell material veliger stage and subsequently the D- The average daily rate of growth of the (prodissoconch 1) and formed a D- shaped veliger became globular at larvae is 23.0 µm from day 0 to 13. shaped veliger 18 hours and 45 minutes 110µm APM x 100µm DVM resulting in post fertilization and measured an the disappearance of straight hinge Spat setting and spat average of 65.2µm in DVM or shell line. On day four the typical umbo production height and 79.9µm in APM or shell stage constituted 90% of the length. The shell valves of the veliger population measuring 140 x 130µm. The The larvae set as spat on day 7-8. The are equivalve and transparent with late umbo stage is reached at 210 x spat has neither byssus nor cement gland for attachment and hence they Table 1. Time sequence of early embryonic development of larvae of Placuna were allowed to settle on tank surface. placenta The spat has an exceptionally long foot which would be of much use in Stage Time after fertilization burrowing. The shell is highly Madrones-Ladja (1997) Present study transparent with concentric growth Egg 0 0 line. The initial larval population in all First polar body 15 min 15 min the culture tanks was 1.5 x 105 in a total Second polar body - 20 min volume of 150 litres of seawater.