Abstract
Juvenile individuals of the crab Neohelice granulata (Brachyura: Grapsoidea: Varunidae) were grown in laboratory using five artificial diets containing 3, 6, 12, 18 and 24 % of crude protein and 3 % of lipids during 7 months. Crabs belonged to three different populations. Two of them, San Antonio Oeste (SAO) and Riacho San Jose (RSJ) are close; they are not genetically differentiated but vary in morphological traits. The third, Mar Chiquita (MC), is far and genetically differentiated from the other two, but morphological traits of their individuals are intermediate. The growth (absolute and relative size change, final body mass), molting frequency, hepatosomatic index of individuals, as well as the Carbon and Nitrogen content and histological structure of their digestive glands, were analyzed. Mar Chiquita individuals differed from one or both of the other two populations. Time to first molt was significantly longer in MC, final size was significantly smaller in MC than RSJ and hepatosomatic index was significantly lower in MC compared to SAO and RSJ. San Antonio Oeste and RSJ individuals never differed. Furthermore, in all cases crabs were able to molt and growth with all diets, although growth in size and mass were higher, and intermolt periods shorter, when diets contained higher percentages of protein. It is concluded that species is well adapted to low quality food sources and it is suggested that size differences previously observed between SAO and RSJ crabs are due to phenotypic plasticity, while differences with MC population are governed by evolutionary adaptation to different food availability.
Highlights
Species with broad distributions are expected to show morphological, physiological or behavioral differences in response to local conditions that could be the result of phenotypic plasticity, the capacity of a single genotype to produce variable phenotypes in different environments when genetic differentiation is absent [22], or local adaptation if gene flow among populations is restricted [10]
The time until the first molt in laboratory varied among populations (Table 2), being significantly longer in Mar Chiquita (MC) than in the other two populations (t = 4.93, p < 0.05 and t = 3.16, p < 0.05; MC vs San Antonio Oeste (SAO) and MC vs Riacho San José (RSJ) respectively), and among diets (Table 2); time was shorter in diets enriched in protein, but only diets containing low CP (diet A) with D and E differed significantly (t = 3.45, p < 0.01 and t = 2.86, p < 0.01 respectively)
The first difference, time until first molt in laboratory indicates on one hand that some degree of synchronicity exists among individuals of each population in molting, probably associated to synchronic settlement events, and on the other, that SAO and RSJ molting are in phase
Summary
Species with broad distributions are expected to show morphological, physiological or behavioral differences in response to local conditions that could be the result of phenotypic plasticity, the capacity of a single genotype to produce variable phenotypes in different environments when genetic differentiation is absent [22], or local adaptation if gene flow among populations is restricted [10]. The genetic and morphometric pattern of the species was studied by Ituarte et al [8] in populations along its complete geographical distribution. They found strong genetic differentiation consistent with an isolation-by-distance pattern, with gene flow occurring mainly between neighbor populations.
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