Intertidal Zone
Dunraven bay south wales, 2020
A photo study of the community of organisms present in the space between high and low tides—defined as the intertidal zone—at Dunraven Bay in South Wales. The term 'community' doesn't have an agreed biological definition, although it is in regular use by ecologists. Famed marine biologist Carl Georg Johannes Petersen used the term to describe groups of species which regularly occur together. These species range from crustaceans to shallow water fish, mollusks to plankton, and fauna to algae.
Neither the sea nor the coastline are static. It is an ever changing landscape. The moon and sun, though far away, play an intrinsic role in the organic life of the intertidal zone. Their gravitational pull plays a part in the rising and lowering of the tides.
There are thousands of species of seaweed, or macroalgae. They act as food sources, provide nursery habitats and a play a vital role in capturing carbon.
Honeycomb worms build reefs from sand and shell fragments, each hole houses a single worm which extends its feeding tentacles into the water when the tide is in to catch floating particles of plankton or detritus.
Coral Weed, or Corallina officials, is a seaweed whose name is inspired by the tropical coral reefs. Like its namesake, Corrallina undergoes a process of calcification. This adaptation provides protection from predation and a unique habitat for other species.
Barnacles, a close relation of the crab and lobster, have been described as masters of the intertidal zone. They have evolved to survive and thrive in areas where heavy wave exposure is common. The acorn barnacle (pictured here is the most common variant. British species have six shell plates, some of which can open and close at high tide for the barnacle to release eggs and catch passing food.
Coraline algae provides a habitat for other intertidal inhabitants.
Microbes, too small to see with the naked eye, surround us. In a drop of seawater, one can find 10 million virus, one million bacteria and about a thousand small protozoans.
Mussels feed by filtering algae, bacteria, phytoplankton and other small particles out of the water column. They are in turn preyed upon by fish, reptiles, amphibians, birds and mammals.
Rock-face seen at low tide reveal typical intertidal zonation.
Intertidal sea snails are only active during low tide. Unlike salt water sea snails — with their gills — the intertidal sea snail has a lung, and breathes the air.
Anemones belong to the phylum Cnidaria. The beadlet anemone is one of the most common anemones found on the shore. It has around 192 tentacles surrounding its mouth and can live more than three years.
Topographically, rocky shorelines are more variable than other coastal habitats. Organisms living on open rock surfaces are more exposed to the environment than other species. Rock offers a stable surface to which these species can attach, yet surface texture and weathering play a crucial role in attachment success.
Sea spray containing microorganisms can be swept high into the atmosphere and travel the globe by the current of the wind as aeroplankton, the atmospheric analogue to oceanic plankton.
There are three different groups of algal species that inhabit the intertidal zone: Chlorophyta (green algae), Rhotophyta (red algae), and Phaeophyta (the brown algae). They are a source of both food and refuge for other organisms.
Underside of a Patella vulgata specimen of Limpet.
Sand Goby are small elongated fish that camouflage well against the sand.
Crabs and lobsters belong to the Order Decapoda as they have ten legs. To enable them to grow, crabs must moult their old shells. The velvet swimming crab (pictured here) is fast moving and can defend itself aggressively with its powerful pincers.