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Ecomorfologie van de mangroveoester Crassostrea cucullata (Born, 1778) = Ecomorphology of the mangrove oyster Crassostrea cucullata (Born, 1778)
Tack, J.F. (1990). Ecomorfologie van de mangroveoester Crassostrea cucullata (Born, 1778) = Ecomorphology of the mangrove oyster Crassostrea cucullata (Born, 1778). MSc Thesis. Vrije Universiteit Brussel: Brussel. 97 pp.

Thesis info:

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  • VLIZ: Archive VLIZ ARCHIVE A.THES8 [5802]
  • VLIZ: Non-open access 226594
Document type: Dissertation

Keywords
    Environmental factors; Saccostrea cucullata (Born, 1778) [WoRMS]; ISW, Kenya, Gazi Bay [Marine Regions]; Marine
Author keywords
    Ecomorphology

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Abstract
    In this study the influence of environmental factors on the size and form of Crassostrea cucullata (Born, 1778) is studied. In the first part the length of 1506 oysters growing along five transects in the mangrove creek in Gazi (Kenya) were measured. Length is defined as the distance from the centre of the hinge to the farest point on the opposite end. Measurements were made with calipers, to the nearest 0, 1 mm. For each of the measurements, a number of parameters were recorded: species of the mangrove tree and diameter of the branch serving as a substrate, approximate density of the oyster growth, height above the bottom, height above date, and orientation with respect to the main current. A regression analysis was performed to try to correlate length with each of these parameters separately. The species of mangrove tree, its diameter and the orientation appeared not to be of importance. For the density, the length was not influenced up to a cover of 70%. For densities higher than 70%, there was a fairly strong negative correlation (from r² = 0,356 to r² = 0,634). Length seemed uncorrelated with height above bottom for heights higher than 20 cm. If the height above the bottom is lower than 20 cm the oyster length is clearly lower than the lengths at higher levels above the bottom. The correlation with height above date was negative but very low. However, if all measurements of oysters closer than 20 cm to the bottom, and all from a density of more than 70% are deleted from the data set, the correlation increased dramatically, the slope still being negative (from r² = 0,357 to r² = 0,850). So height above date, and thus percentage of time immersed, seems to be the primary factor determining the length of the oyster. This relationship is obscured by the influence of crowding, and proximity of the bottom. In the second part, we tested the hypothesis that the form of the oyster is influenced by ecological factors. An image-analytic technique that determines the outlines for the 85 collected oysters is described. These outlines are numerically characterized using elliptic Fourier decomposition. Since the resulting Fourier coefficients can be normalized so that they are invariant to changes in magnification and rotation as well as other such transformations of the original silhouette, they quantify shape per se, and can be used as variables in multivariate analysis of form. Cluster analyses distinguishes distinct forms of the mangroveoyster Crassostrea cucullata. The vertical distribution, the form of the substrate and orientation with respect to the main current seems to be factors determining the form of the oyster Crassostrea cucullata.

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