Branching dynamics of two species of arborescent demosponge: the effect of flow regime and bathymetry

A series of measurements were taken from 50 random examples of the arborescent sponges Raspailia ramosa and Stelligera stuposa at a number of sites and depths at Lough Hyne, Co. Cork, Ireland. Sites ranged from fast flowing habitats, experiencing little sediment settlement, to the converse where current flow was extremely slight and sediment settlement rates high. Depth intervals at which sponges were measured ranged from 6 to 24 m. Sponges of both species varied with respect to measurements taken and the growth proportions (ratios of measurements taken). The greatest morphological differences occurred between depth and current extremes. However, at the most sedimented site (24 m) branching complexity of both species actually decreased in opposition to the proposed current models. It is discussed that simple morphologies (such as singly dichotomized sponges) may be better suited to extremely sedimented regimes being more erect than larger, more complex specimens. Other biological factors that may limit branching complexity are considered and emphasis is placed upon the possible inadequacies of community or group level models of morphological adaptation.