EVOLUTION ON FLEXIBLE HARD SUBSTRATES: METAZOAN ADAPTATIONS FOR LIFE ON SEAGRASSES

Seagrasses have provided more than 65 m.y. of opportunity for colonization and evolution of epiphytic organisms that can rise to the challenge of life on thin, flexible, living substrates that are in constant motion. Because seagrasses are seldom preserved in the fossil record, skeletal remains of seagrass associates are the primary means for identifying ancient seagrass habitats. Although seagrasses lack many of the properties of non-living hard substrates, their flat, continuous surfaces may provide on the order of 15x greater area for colonization than the surface of the underlying sediment. Both the blade itself and the microbial films and crusts on the blade may provide a correspondingly greater nutritional resource than the underlying sediment.

Sessile organisms are constrained spatially by blade form and by the rapid growth and high turnover rate of the blades to which they attach. They must evolve attachment mechanisms that differ from those used to attach to non-living surfaces. Motile organisms are constrained not only by the hydrodynamic challenge of negotiating and adhering to a substrate typically experiencing strong bidirectional flow, but also by the narrowness of the blade and the perils of reversing direction or moving from one blade to another. Fouling by sessile organisms also constrains motile organisms by eliminating access to food (predominantly periphyton films on the blade surface) or by making it impossible to negotiate the surface.

The most dramatic examples of motile epifauna with derived seagrass morphologies include several clades of patellogastropod limpets with narrow, elongate, parallel-sided shells, chitons that are narrowly elongate parallel to the blade, and streamlined trochoidean gastropods that have reduced the angle between the axis of coiling and the blade and detorted to bring the axis of coiling in line with the leaf axis. Serpulid polychaetes, ostracodes, larger foraminifera, and bryozoans are among the skeletonized taxa that have evolved seagrass morphologies. These morphologies may assist in identifying seagrass environments in the stratigraphic record.