Indirect method for the assessment of population and size distributions and activity pattern of the ghost crab Ocypode gaudichaudii (Crustacea, Decapoda, Ocypodidae) in Tacna, 2010 - 2013

Abstract

In this paper, the spatial distribution and activity pattern of Ocypode gaudichaudii is analyzed by means of an indirect method through the count of the number of burrows, diameter, depth and distances between random points and burrows in order to determine different types of spatial dispersion and measure the state of the intertidal environments of sandy substrate. In the field, at least 2 grids of 20 m2 (20x10 m) were established and grilled every 4 m2 (2x2 m) which were distributed between the dune and undertow zones. In each grid, 15 quadrants (c) and 15 vertex (v) were selected at random and the dispersion index (IBR) of Byth-Ripley was estimated, based on the calculation of distances for independent samples. In the 2010-2013 period, 96 sampling stations were carried out that allowed us to perform 729 similar spatial distribution simulations (grouped, uniform and random), and dispersion variograms were constructed based on density and burrow distribution. A comparison was made between beaches according to the dispersion values that reflected the same trend between the simulated distribution and the observed distribution in the field. The Byth-Ripley index was efficient to differentiate the spatial distribution types described. The variograms allowed to analyze the distribution of the variables and determine the spatial behavior of O. gaudichaudii. The distribution of the species was of the uniform type in both beaches, except in summer when it was random for Santa Rosa (occupying any available space) and grouped for Rancho Chico (in response to possible anthropic or environmental impacts). The periods of maximum activity were distributed between 5 and 10 am and between 5 and 8 pm, especially for adult specimens, and between periods for juvenile specimens.