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Figure 3.Pubertal molt at first maturity
Left: Percentage of those females in each sequential 5 mm carapace width interval which had not undergone a pubertal moult and would thus not have mated and for those which had undergone a
pubertal moult and were therefore of mature size. The logistic curve was fitted to the percentage of crabs that had undergone a pubertal moult in order to determine the CW
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at first maturity. Right: For males, the logistic curve was fitted to the carapace widths of those crabs which, from the
relationships between the lengths of the propodus of the largest chela and the carapace widths, were capable of mating and were thus adults of mature size. Arrows denote CW
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for size at maturity of female and male crabs from Potter de Lestang 2000.
4.3 Fecundity
Ingles Braum 1989 measured fecundity of P. pelagicus as F = 972.75 W
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with F = fecundity number of eggs per female and W = body weight g. They found for the Philippines a fecundity of between 142,572 and 1,131,900 eggs per female. Fecundity increases
exponentially with body size of the female. Estimates of the number of egg batches produced in a spawning season ranged from one in small
crabs to three in large crabs de Lestang et al. 2003b. These data, together with the batch fecundities of different size crabs, indicate that the estimated number of eggs produced by P. pelagicusduring the
spawning season ranges from about 78,000 in small crabs CW = 80 mm to about 1,000,000 in large crabs CW = 180 mm. Exact number of ovipositions, however, is not known Hamasaki et al. 2006.
Campbell Fielder 1988 demonstrated that P. pelagicus moult 2–3 times after the puberty moult and can produce 3–5 batches of eggs during each intermoult period. It is reported that the number of
batches estimated in the field during the breeding season correlated with female body size in P. pelagicus de Lestang et al. 2003b.
4.4 Distribution of larvae and recruitment
Ingles Braum 1989 describe the larval ecology of P. pelagicus populations in the tropical waters of the central Philippines. Their main findings relevant here include:
• Zoeae are mostly found in the surface layers. They are rarely found near seagrass but abundant in
places near coral reefs.
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• P. pelagicusmegalopae usually are found in settling areas not far from the coastline and do not need a mechanism for long-distance transportation such as attachment to jellyfish as is the case in
Cancer magister. • Fast larval development of P. pelagicus means that there is no need for long distance transport.
• P. pelagicuszoeae larvae are positive phototactic and negative geotactic, older megalopae are positive geotactic
• Spatial distribution of P. pelagicus larvae is determined by 3 main factors: i small-scale patchiness as a result of schooling behaviour, ii local currents systems and iii behaviour,
particularly attaching behaviour, in older megalopae, explaining their relative abundance in seagrass beds and coral reefs.
• Megalopae swim to maintain themselves in the water column, not to purposely move actively in a horizontal manner
• Local currents including wind induced surface currents can have an important impact on the recruitment of P. pelagicus populations, e.g. by transporting larvae to deep areas where settling is
not possible, contributing the very high mortality during larval stages. Larvae can be transported by wind-induced currents for over two weeks at 6-7 km per day.
• Salinity tolerance of zoea of P. pelagicus ranges from 16.2-35 ppt. Diurnal tidal pattern salinity fluctuations in river mouths and irregular amounts of rainfall can cause very strong salinity
fluctuations within a very short time, which may render certain areas inhospitable for zoeae even when average salinity levels are within their tolerance range.
• Megalopae are believed to be more tolerant to low salinity levels. • Temperature range for normal development of P. pelagicus lies between 18 and 35 ˚C.
Megalopae of P. pelagicus, another Indo-Pacific portunid, have also been observed to be photopositive and more active when illuminated in offshore water Webley Connolly 2007
Juvenile recruitment success is dependent on factors such as i favourable ocean currents that transport eggs and larvae, and ii the availability of food and nursery habitats i.e. seagrass or other
suitable marine vegetation for juveniles N. 2006. The settling postlarvae of P. pelagicus, as distinct from benthic juveniles, were defined as those with a
carapace width CW of 20 mm or less. This definition was based on Potter et al. 1983 finding that 25 mm CW was the modal width for new recruits of P. pelagicus caught in the entrance channel of
the Peel-Harvey inlet in Western Australia. They found few individuals less than 30 mm CW in the estuary itself.
Settlement of the postlarvae of P. pelagicus on seagrass habitat ensures that they select a habitat with shelter and food. They have a preference for structured nursery habitats and recruit to suitable habitats
adjacent to established nursery habitat, a pattern similar to that of some other crustaceans. Crabs mate in coastal and estuarine waters. Estuary-based females then migrate to sea to spawn. Blue
Swimmer Crabs are short-lived, with a maximum age of about three years and maximum carapace width of 200 mm. Maturity is reached at about one year, and recruitment to lower fisheries occurs at
an age of about 18 months. Due to this life cycle, crab stocks have relatively fast rates of replacement and can recover quickly from depletion.
Despite the drift and movement of eggs and larvae of P. pelagicus, there is no general pattern of source-sink relations. Local current patterns and overall conditions determine the distribution and
settlement of the young crabs and their chances of survival. This is not to say that larval drift cannot be an issue as part of management of a crab fishery Ingles Braum 1989.
4.5 Habitats
