Influência da morfodinâmica praial na distribuição e variações morfométricas de mellita quinquiesperforata Leske, 1778 no litoral do Estado do Ceará

Abstract

The spatial distribution, occurrence and morphometry of populations of Mellita quinquiesperforata were studied in the Ceará coast, between August 2010 and April 2011. Samplings were performed in Redonda, Majorlândia, Prainha do Canto Verde, Diogo, Iguape, Futuro, Pecém, Paracuru, Flecheiras, Caetanos, Preá, Maceió and Bitupitá. In these sites, the height and period of the waves, granulometry of intertidal and suf zones and the tidal range were recorded. The beach profile, Dean’s dimensionless fall velocity ( ) and surf scaling parameter ( ), Beach Indexes (BI) and the Relative Tide Range (RTR) were obtained in order to characterize the beach morphodynamics. The specimens were sampled through trawls perpendicular to the coastline, divided in two sampling zones (0-30m and 30-60m, distant from the swash zone). For the studied zones, it were obtained the density, expressed in number of individuals per square meter (individuals/m2), and histograms relative to the length frequency, with class intervals of 6mm. Student’s t-tests compared the averages of density between the zones, and also of the age classes. The pattern of spatial distribution was estimated using the Morisita’s index. For the morphological analysis, thirty adult individuals, considering 33 parameters based on the carapace form, lunule positions and dimensions, and also on the food grooves. A size-free linear discriminant analysis was realized using the R Software. Redonda, Majorlândia, Caetanos and Bitupitá were classified as ultra-dissipative; Futuro and Pecém, dissipative; Diogo, Iguape, Paracuru, Flecheiras, Preá and Maceió, intermediate; and Prainha do Canto Verde, reflective. According the RTR index, these beaches were wave-modified; and only Bitupitá was tidal-modified. Considering the BI index, Bitupitá is also the only beach considered dissipative, while the others are intermediate. All M. quinquiesperforata populations have a mode indicating adult individuals, even Canto Verde, Iguape and Preá have also presented recruits. This species, mostly, was concentrated near the breaking waves. The greater density was observed in Canto Verde (7.1 ind/m2), with aggregate distribution. M. quinquiesperforata had different forms of spatial distribution in the analyzed beaches, which did not correspond to the beaches morphodynamics models. A reduction of specimens was observed in direction to the West, being absent from Preá Beach. According the Discriminant Analysis, with 83.4% of variance explained from the two first Axes, the test height and thickness, and also the posterior region, discriminated populations according the beaches morphodynamics. It is suggested a combination of Dean and RTR indexes to characterize the morphodynamics of the studied beaches. The variables grain size, wave height, slope of the beach profile and wave energy dissipation promoted the variations in density and distributions of the studied M. quinquiesperforata populations. The morphodynamics characteristics of beaches off Ceará contributed also for the morphometric variations exhibited by these populations, indicating morphological variations as a result of the different environmental conditions, confirming its phenotypical plasticity.