The three commercially important genera, Scylla, Portunus and Charybdis are brachyuran crabs belonging to

the family Portunidae. A number of species of these genera are widely distributed in the Indo-Pacific and is of significant

importance in the fisheries of most countries located in that region

Nair, K R Manmadhan

CMFRI Digital Repository

Course ManualCentral Marine  Fisheries Research Institute(Indian Council of Agricultural Research)P.B.No.1603, Marine Drive North Extension,Ernakulam  North ,P.O.Cochin, KERALA – INDIA - 682018Winter School  onRecent Advances in Breeding  and Larvicultureof Marine Finfish and Shellfish30.12.2008 -19.1.2009CMFRICompiled and  Edited byDr. K. Madhu, Senior Scientist  and  Director,Winter school&Dr. Rema Madhu, Senior  Scientist and Co-ordinatorCentral Marine Fisheries Research InstituteCMFRI - Winter School Course Manual on “Recent Advances in Breeding and Larviculture of Marine Finfish  and Shellfish”.30.12.2008 - 19.1.2009126CRAB SEED PRODUCTIONK.R.Manmadhan Nair,Research Centre of Central Marine Fisheries Research Institute,Calicut  - 673  005.E -mail : drkrmnair@yahoo.comIntroductionThe three commercially important genera, Scylla, Portunus and Charybdis are brachyuran crabs belonging tothe family Portunidae. A number of species of these genera are widely distributed in the Indo-Pacific and is of significantimportance in the fisheries of most countries located in that region.TaxonomyPhylum : ArthropodaClass : CrustaceaSubclass : MalacostracaOrder : DecapodaSuborder : PleocyemataInfraorder : BrachyuraFamily : PortunidaeSubfamily : PortuninaeGenus  (eg.) : ScyllaSpecies (eg.) : Scylla serrataS. tranquebaricaS. olivaceaS. paramamosainThe blue swimmer crab Portunus pelagicus and the mud crab/ mangrove crab Scylla serrata are the mostvaluable crabs contributing to the crab fishery and trade in India. Other species, though contributing substantially tolocal fisheries, are of lesser economic significance. The exploitation of marine crab resource has been increasingconsequent to the increased demand for crabs for export. Some empirical studies have shown that the crab fishery isclose to its biological optimum. Farming of crabs and searanching are two options available for augmenting the productionand sustain the fishery and wild population. Crab seed is not easily available from the wild; and if at all available, itshould not be collected from the wild for the sake of conservation.  To meet the requirement of crab seed for farmingand sea ranching, dedicated crab hatcheries  have been established or are being established in many countries.External morphology of crab: (Type – Scylla spp.)The body of crab is covered externally by a hard carapace which is chitinous in nature. In Scylla spp. There are 6 spines (serrations) between and 9 spines on either side of the eyes. The antennae and antennules are used for sensory perception. The crab has 4 pairs of legs, the first pair (chelate leg) has its dactylus developed as pincers or claws forgrasping prey . The second and third pair of legs are simple in structure with out claws and used mainly forcrawling. The fourth pair is paddle(oar) shaped and used for swimming. The abdomen is tucked under the ventral side of the carapace and having slender fused pleopods on it. Sexes are separate. In females the abdomen is triangular to semicircular depending on growth stages.Males have T – shaped abdomen and bigger chelipeds than females.CMFRI - Winter School Course Manual on “Recent Advances in Breeding and Larviculture of Marine Finfish  and Shellfish”.30.12.2008 - 19.1.2009127Courtship and matingThe male embraces the sexually mature female which is about to moult. After moulting, the male turns the femaleupside down so that their ventral surfaces meet. Then mating occurs and male transfers it spermatophores to thepaired seminal receptacle (spermatheca) of the female.SpawningThe eggs from ovary pass through a pair of short oviduct which opens in to the spermatheca. During the passagethrough spermatheca, eggs are fertilized by the sperm cells released from the spermatophores and gets attached tothe ovigerous setae (pleopod hairs) of the abdominal flap.  Although female portunid crabs sometimes become ovigerous(berried) in esturies, such individuals emigrate in to coastal marine waters, where they complete the incubation andhatching. The embryonic development of the fertilized egg is completed in the berry. There are nine developmentalstages of the egg before hatching.a. Precleavage stage – yolk fills entire egg.b. Cleavage stage – cleavage furrows can be observed on the surface of embryo.c. Blastula stage – dividing cells are of equal size, arranged spherically.d. Gastrula stage – ‘U’ shaped band with optic lobes visible, translucent embryonic body outline is formed.e. Naupliar stage – 3 pairs of appendages are visible.f. Metanaupliar stage – optic lobes(eyes) become largerg. Pigmented eye – eyes become pigmented.h. Heart beat – heart starts to beat.i. Prezoea – eye pigmentation is darker, abdomen and telson visible.Depending on the stage of development of embryo the orange coloured eggs turn greyish orange and finally todark grey during incubation on the pleopods. The incubation period may vary from 7 – 14 days depending on watertemperature. Normally hatching of zoeae occurs on the 9th or 10th day.Seed production.Most of the portunid crabs are amenable to culture. They can be made to mature, mate and get berried inspecialized broodstock rearing tanks.  Berried females collected from the wild or captive broodstock developed in thehatchery can be used for seed production. Healthy ovigerous females with characteristic orange coloured berry arecollected and brought to the hatchery in wide mouthed jerry cans with sea water. These crabs are kept in 1.5 tfiberglass tanks with seawater at 34 – 35 ppt salinity, 8.2 pH and  28 – 300 C  temperature, and provided with continuousaeration. The crabs are disinfected, after two days, in 25ppm formalin bath for 30 minutes. The spawners are maintainedin FRP tanks with cover, in filtered sea water and provided with 50% water exchange daily till the penultimate day ofhatching. The berried crabs do not feed and hence no feeding is  required  for the mother crabs till hatching.HatchingThe mother crabs are monitored daily. When the egg mass (berry) turns dark grey those crabs are transferred toseparate hatching tanks with known volume of water .Mild aeration is provided in the hatching tank.. The carapacewidth and weight of the crabs are recorded before transferring the crabs to the hatching tanks. Hatching generallytakes place during early morning hours. After full hatching, mother crab is removed from the tank and weighed again.This gives an idea of the weight of the egg mass (berry). Aeration in the hatching tank is stopped for a while to allowthe empty eggshells and unhatched eggs to settle at the bottom. The empty eggshell and unhatched eggs are siphonedout without disturbing the live Zoeae in the water column and surface.. The total number of zoeae estimated bydrawing representative samples. The number of zoeae produced per female in a single spawning ranges from 1- 4million for S. serrata (depends on size of animal and weight of berry) and after a single mating a female can spawn atleast three batches of eggs before next moult and mating.CRAB SEED PRODUCTIONCMFRI - Winter School Course Manual on “Recent Advances in Breeding and Larviculture of Marine Finfish  and Shellfish”.30.12.2008 - 19.1.2009128The number of stages and larval morphology of portunid crabs may differ slightly from species to species.However they all follow a basic pattern of development. The Scylla serrata  zoeae pass though five sub-stages beforemetamorphosing to the megalopa stage. Zoeae are with long dorsal spines and short lateral spines on the carapace.In Scylla serrata there are five zoeal stages.a. Zoea 1 – eyes not stalked (sessile), abdomen 5- segmented.b. Zoea 2 – eyes stalked.c. Zoea 3 – abdomen 6 segmented.d. Zoea 4 – pleopod buds appear.e. Zoea 5 – pleopod buds well developed.The Zoeae are stocked at a density of 50 nos./L in green water system in FRP tanks where a culture of unicellularalgae Marine chlorella/ Nannochloropsis (2 lakh cells/ ml) is maintained. The larvae are fed with marine rotifer (20 – 25/ ml)  and Artemia nauplii (5 – 10 / ml)   grown separately. Shrimp larval feed such as ‘Encap’ and ‘Friipak’ can also beused for feeding crab larvae especially during adverse cloudy or rainy weather when development of algae andconsequently rotifer culture are affected. About 30 – 50 % water from the larval rearing tank is replaced daily withgreen water  and live feed - rotifer and Artemia - added in the required density to ensure  adequate supply of feed atany given time.The duration of the zoeal stage is approximately 10 ± 2 days.Zoea 5 moults to megalopa stage.Megalopae are very similar to that of other portunids. Rostral spine  present. Eyes project as far as the lateralmargin of the carapace. Abdomen six segmented with dorsoventrally flattened telson. The megalopae  in another 4 –5 days metamorphose directly to the first crab instar. It resembles an adult crab in shape though in miniature form. Thetotal duration of the larval development from Z 1 -  to the 1st crab instar varies between 15 – 18 days. After severalmoults of progressively increasing interval the 1St crab instar grows in to juvenile and then adult crabs. The crab instarsare transferred to nurseries for further rearing at a lesser density. In the nursery, crab instars are fed at least twice(total 10% of body wt.) a day with finely chopped meat of clams, shrimp, fish or pelleted shrimp feed depending on therequirements and availability. Water exchange up to 30% or more is advisable at least twice a day before feeding.Crab instars are highly cannibalistic, and to ensure a fair percentage of survival, during rearing, it is essential toprovide maximum protection to the crablets. Hideouts are provided at the bottom of the tanks for this purpose.  Ten totwelve day old crab instars attain a size of  about  8mm to 10mm. In about a month they reach a size of 14- 19mm CW.Some baby crabs grow very fast – shooters, which should be harvested/segregated periodically to ensure maximumsurvival of the crablets.Mass culture of  live feedClean filtered sea water is pumped in to the tanks and fertilized with the following quantity of commercial fertilizersfor every ton of water.Ammonium sulphate – 100gUrea -                          - 5 gSuper phosphate         - 20 gMineral mix                  - 10 gFifty liters of Nannochloropsis at a cell concentration of 2 – 3 million cells/ml added as innoculum in approximately950 l of fertilized water and vigorous aeration provided. A bloom of Nannochloropsis develops by the 4th day. Thisgreen water is pumped  in to another tank and rotifer inoculated into it at a density of 25 no/ml. By the  8th day the rotiferdensity reaches around 200 – 250/ml. After stopping the aeration for some time the rotifers can be partially harvestedfor feeding the crab larvae. The collected rotifer is washed thoroughly before feeding. After harvesting a part of therotifer along with the culture water, an equal quantity of Nannochloropsis green water grown as above may be pumpedin to the rotifer tank to continue the batch culture for daily harvest. Such rotifer culture can be maintained for 3 – 4weeks. Marine chlorella can also be used in place of Nannochloropsis.CRAB SEED PRODUCTION

Crab seed production- Winter School on Recent Advances in Breeding and Larviculture of Marine Finfish and Shellfish

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Crab seed production- Winter School on Recent Advances in Breeding and Larviculture of Marine Finfish and Shellfish

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