Settlement and Early Post-Settlement Survival of Sessile Marine Invertebrates on Topographically Complex Surfaces: The Importance of Refuge Dimensions and Adult Morphology

We predicted that both refuge dimension and growth form would influence settlement and short-term post-setUement success (57 d) of sessile marine invertebrates that live attached to hard substrata in low energy environments. Individuals with unlimited attachment to the substrata should rapidly be protected by their growth form, thus decreasing their need to settle in refuges and limiting the length of time any locations on heterogeneous substrata act as refuges. Alternatively, organisms with limited attachment to the substrata should remain susceptible to the causes of mortality for a longer time, and as a result should settle in high quality refuges [sites that protect individuals from competitors, predators or physical disturbance events until either a size refuge or reproductive maturity is obtained). Results agreed with these predictions for 4 species of invertebrates examined on both the topographically complex surface of the solitary ascidian Styela plicata (hereafter Styela) and on settlement plates w ~ t h uniformly spaced roughness elements that mimicked the heights of roughness elements (2.0 and 5.0 rnm) found on Styela in Beaufort, North Carollna, USA. On all surfaces, the 2 species with limited attachment to the substrata, Balanus sp. (aclonal, solitary) and Bugula neritina (clonal, arborescent), settled almost exclusively in the location that provided individuals with the best refuge: the crevices formed where the bases of roughness elements intersect with the flat surfaces. Additionally, when roughness elements of various heights were present (Styela, range: 0 6 to 8.8 mm), intermediate size roughness elements (2.0 < X 1 5 . 0 mm) were picked over 72% of the time. Settlement locations and locations where survival were enhanced were less consistent for the 2 species with unlimited attachment to the substrata: a clonal, encrusting form (Schjzoporella errata) and a clonal stolon-mat form (Tubularia crocea). Fewer individuals of these 2 species settled on roughness elements on Styela and when they did, they were not restricted to the bases of the roughness elements. On the plate surfaces, most settlement did occur in crevices, but both species grew away from this location w~thin days and short-term survival was not consistently greater in this location. Additional trials were run on plates with pits of the same maximum dimensions as the tested roughness elements (2.0 and 5.0 mm depth) to see if crevices and pits provide refuges of equal quality for newly settled individuals. Only survival of Balanus sp. recruits was greatest in both crevlces and pits. Evidence for active choice of settlement location comes from consistent results in trials in which some larvae settled in greater numbers on specific size roughness elements on Styela and in areas of high erosion. Overall, these results show that one must be very cautious when generalizing about refuge quality on heterogeneous surfaces, and to determine if a location is a spatial refuge, i t is critical to consider: (1) the dimensions of the larva, (2) the relative dimensions of the individual and potential refuge location at any point in time from the moment settlement occurred, and (3) the growth form of the individual which is related to its need for protection from biotic and abiotic sources of mortality.