Rocky Shorelines - Trace Fossils as Indicators of Sedimentary Environments - page 428

The main groups of organisms that dwell in recent rocky coasts are the following
(see Little and Kitching, 1996 for an exhaustive revision): (1) microorganisms
and algae, including cyanobacteria, fungi, foraminifers, diatoms, lichens, as well
as green (laminarians), brown, and coralline red algae; (2) plants, most important
in the highest, never submerged parts, where vegetation is usually dominated by
halophytes, but also present in the submerged habitats; (3) sponges, fouling and
boring forms dominant; (4) worms, mostly sabellid and spirorbid polychaetes;
(5) cnidarians, in particular, non-skeletal anthozoans such as anemones, but also
corals mostly in tropical seas; (6) arthropods, such as vagile decapod crabs, iso-
pods, and amphipods, as well as fouling acrothoracican, lepadomorph, and bala-
nomorph barnacles; (7) mollusks, including limpets, snails, chitons, bivalves
(mytilids, oysters, pectinids), and octopuses; (8) bryozoans, with forms encrust-
ing organic (shells, algae) and inorganic (rock) substrates; (9) echinoderms,
represented by vagile regular sea urchins, starfish, and sea cucumbers; and
(10) chordates, encompassing ascidians, fish, birds, and mammals (sirenids
and pinnipeda).
The distribution of benthic organisms is not random, and ecological zona-
tion is evident in virtually all modern rocky shores ( Little and Kitching, 1996 ).
Limits between zones are usually sharp and continuous, although it is difficult
to establish an archetypal zonation due to the high variability of physical and
biological constraints. There are two fundamental parameters affecting the dis-
tribution of the biota: (1) the ability to resist desiccation, particularly on coasts
with tides and (2) the competition among species. Overall, a lower (sublittoral)
fringe with a preponderance of green algae, accompanied by numerous groups
of invertebrates and fish, is generally present. Above it, the foreshore (midlit-
toral) zone is characterized by the growth of crustose coralline algae and the
frequent presence of byssate (mussels) and boring (lithophagous) bivalves, bar-
nacles (Balanidae), snails, chitons, crabs, sponges, sea urchins, starfish, etc. The
littoral zone is inhabited by species able to survive prolonged periods outside
the water (algae, limpets, barnacles, chitons), and some vagile arthropods.
Finally, the supralittoral fringe (or splash zone) is the highest one, wetted by
only a few tides in the year, or by waves or splash, and is dominated by lichens
and algae, periwinkle snails (littorinids), and barnacles (Chthamalidae).
Many species are obligate members of rocky-coast communities, unable to
live elsewhere in offshore environments, whereas others enjoy a wider range of
habitats, intertidal or subtidal. Many rocky-shore organisms show defensive
mechanisms that help them to survive in such a hostile physical environment.
Desiccation in supra- and intertidal areas is a major problem to solve. Epilitho-
bionts with protective skeletal structures (barnacles, gastropods, chitons) thrive
on the surfaces periodically exposed to the atmosphere. High-energy conditions
require fixation to the rock, either by cementing, as in coralline algae, vermetid
gastropods, oysters, or barnacles, or by byssal filaments, as in mussels. An
endolithic or semi-endolithic mode of life also provides protection against both,
the physical environment and the predators ( Fig. 3 ). This is the strategy used by