Development of sessile marine assemblages on fixed versus moving substrata

Abstract

Epibiotic assemblages have been shown previously to differ between pontoons and rocky reefs. This may occur for a variety of reasons, one of which is that pontoons move, whereas reefs do not. Effects of movement of the substratum are particularly pertinent to studies involving settlement panels because these experimental units are often suspended from pontoons such that they can move up and down, or attached to ropes such that they can rotate. The development of epibiotic assemblages on panels that were fixed, moved up and down, or rotated, was studied to test hypotheses about the effects of movement on sessile organisms. The covers of barnacles, sponges and ascidians increased greatly with increasing movement and/or rotation of the substratum, whereas species of red and brown foliose algae and tubiculous polychaetes generally decreased with increasing movement and/or rotation of the substratum. In general, assemblages on panels that moved up and down were most similar to those on rotating panels. One of the most obvious differences was the 2 to 3-fold greater biomass on rotating panels compared to fixed panels, which was attributed to the greater abundance of barnacles. It was concluded that, although movement of a surface can influence the composition of assemblages, this factor on its own may explain only some of the differences reported between pontoons and reefs. Furthermore, the method of deployment of settlement panels was shown to have great effects on the types of assemblages that develop. Differences in water flow were proposed to explain some of the observed patterns, but this idea needs to be investigated further.

Why do floating structures create novel habitats for subtidal epibiota?

“…Care must be taken in extrapolating the results of these studies to assemblages on fixed structures or habitats. Our results warn that lessons learnt from work done in floating habitats may explain very little of the overall variance in distribution and abundance of epibiota on fixed structures (see also Glasby 2001). While understanding and accounting for patterns of abundances of epibiota within urban structures has provided considerable insights to marine ecology as a discipline (e.g.…”

“…Epibiotic assemblages developed in ways more consistent with differences in habitat than predation (Connell 2001b). The effects of oscillatory movement of surfaces due to currents and wave action caused different assemblages on fixed versus moving surfaces, but could not fully explain differences between fixed and floating structures (Glasby 2001).…”

Causes of decoupling between larval supply and settlement and consequences for understanding recruitment and population connectivity

“…Another approach has been to place settlement 7 substrates on mooring lines (e.g., Dobrestov and Miron, 2001;Rilov et al, 2008;Dudas et al, 8 2009). Comparison of hydrodynamics on fixed versus moving settlement substrates has not been 9 made although increased biomass of barnacles on rotating panels compared to fixed panels was 10 attributed to differences in water flow (Glasby, 2001). It seems possible that coupling or 11 decoupling of larval supply and settlement could be confounded with the effects of 12 hydrodynamics on collectors, but this has yet to be shown.…”

Mobile ‘reefs’ in the northeastern Gulf of California: aggregations of black murex snails Hexaplex nigritus as habitat for invertebrates

“…Many sessile animals and algae had settled on the shells, but the composition of epibiota may have been different. Species composition differences between artificial reefs and aggregations may illustrate another difference; black murex snail aggregations are alive and mobile, thereby generating a different microenvironment than our sessile artificial reefs (see Glasby 2001). Artificial reefs mimicked the structural attributes of aggregations (although without egg capsules), but could not recreate the biological and chemical attributes of aggregations.…”