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 influenc e settlement and short-term post-settlement success (less than or equa l to 7 d) of sessile marine invertebrates that live attached to hard s ubstrata in low energy environments. Individuals with unlimited attach ment to the substrata should rapidly be protected by their growth form , thus decreasing their need to settle in refuges and limiting the len gth of time any locations on heterogeneous substrata act as refuges. A lternatively, organisms with limited attachment to the substrata shoul d 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). Res ults agreed with these predictions for 4 species of invertebrates exam ined on both the topographically complex surface of the solitary ascid ian Styela plicata (hereafter Styela) and on settlement plates with un iformly spaced roughness elements that mimicked the heights of roughne ss elements (2.0 and 5.0 mm) found on Styela in Beaufort, North Caroli na, USA. On all surfaces, the 2 species with limited attachment to the substrata, Balanus sp. (aclonal, solitary) and Bugula neritina (clona l, arborescent), settled almost exclusively in the location that provi ded individuals with the best refuge: the crevices formed where the ba ses of roughness elements intersect with the flat surfaces. Additional ly, when roughness elements of various heights were present (Styela, r ange: 0.6 to 8.8 mm), intermediate size roughness elements (2.0 < x le ss than or equal to 5.0 mm) were picked over 72% of the time. Settleme nt locations and locations where survival were enhanced were less cons istent for the 2 species with unlimited attachment to the substrata: a clonal, encrusting form (Schizoporella errata) and a clonal stolen-ma t form (Tubularia crocea). Fewer individuals of these 2 species settle d on roughness elements on Styela and when they did, they were not res tricted to the bases of the roughness elements. On the plate surfaces, most settlement did occur in crevices, but both species grew away fro m this location within days and short-term survival was not consistent ly 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 crevices and pits. Evidence for acti ve choice of settlement location comes from consistent results in tria ls in which some larvae settled in greater numbers on specific size ro ughness elements on Styela and in areas of high erosion. Overall, thes e results show that one must be very cautious when generalizing about refuge quality on heterogeneous surfaces, and to determine if a locati on is a spatial refuge, it is critical to consider: (1) the dimensions oi the larva, (2) the relative dimensions of the individual and poten tial refuge:location at any point in time from the moment settlement o ccurred, and (3) the growth form of the individual which is related to its need for protection from biotic and abiotic sources of mortality.