marine science oceanography facebook marine conservation society website home news london meetings courses contacts seasearch calendar marine based activities newsletter marine science education sitemap marine life world news
anemones tidal zones

Hydrothermal Vents

Hermit Crabs

Hermit crabs
Two families: Paguridae and Coenobitidae

Hermit crabs have no shell of their own so inhabit other structures, usually empty mollusc shells. Some also carry anemones, with which they form a symbiotic relationship.

There are several genera of hermit crab belonging to two families: Coenobitidae, which includes land hermit crabs, and Paguridae, right-handed hermit crabs.

They are found worldwide, except the Arctic and Antarctic.

They inhabit sandy or muddy-bottomed waters and occasionally on land and in trees. Some hermit crabs even live in plant stems. Several species are found in kelp forests.

Hermit crabs scavenge for plant and animal material on the sea floor.

The crabs use empty shells or other hollow objects to protect their bodies, moving to larger ones as they grow. This habit gives them their name. They compete with each other for the best shells. They move by dragging their shell along the ground and retreat into it when threatened.

The crabs use empty shells or other hollow objects to protect their bodies, moving to larger ones as they grow. This habit gives them their name. They compete with each other for the best shells. They move by dragging their shell along the ground and retreat into it when threatened.

Some hermit crabs develop mutualistic relationships with anemones. The anemone lives on the hermit crab's shell, protecting it from harm with stinging tentacles and also providing camouflage. In return, the anemone scavenges the crab's leftover food. Some crabs and anemones form such a close relationship that the crab will transplant the anemone when it moves shell.

Abdominal appendages on the left hand side of the female's body, called pleopods, carry the eggs until they hatch. The young look a bit like shrimps. They drift in the water until they find suitable shells.

Conservation status
Hermit crabs are often collected for the aquarium trade, but are not considered threatened.


Marine Ecology

Lower Eulittoral Zone

Middle Tide Zone: Also called the Lower Mid-littoral Zone (Lower Eulittoral) . This turbulent area is covered and uncovered twice a day with salt water from the tides. Organisms in this area include anemones, barnacles, chitons, crabs, green algae, isopods, limpets, mussels, sea lettuce, sea palms, sea stars, snails, sponges, and whelks.

The midlittoral or Eulittoral zone is covered and uncovered twice a day by the tides. Animals in this zone have adapted to being immersed in air and sea water. Anemones close when the tide is out, keeping in the moisture necessary for survival. Mussels close their shells tightly "clam up" for the same purpose, and open to feed as the tide brings in their food. Cockes ensure they are either back in their groove or in a nice niche when exposed.

There is less time of exposure to air as you get lower into the zone. These organisms while less tolerant of air are better adapted to wave exposure. Dense strands of kelp can occur here and all this provides living and hiding spaces for a variety of inverts. (hydroids, bryozoans, nudibranches, worms, crabs, tunicates. Some are more conspicuous like the sea anemones, sea urchins, and sea stars.

Some anemones have algae living with them but all capture food, sea urchins graze on the algae in this zone which increases species diversity. Exceptions are that often only one or two species of algae grow when the urchins are very active but then control of urchins by sea otters and sea birds, lobster and fish all return things to normal.

Some inverts. can BURROW or BORE into hard substrates: rock, coral, wood by either mechanical abrasion or chemical dissolution. Some mussels, date mussels, secrete acid and dissolve limestone, gribbles are small wood-boring isopods that simply chew into wood. Teredo (shipworm) are also filter feeders as well as eat wood.

Tide pools are depressions of varying size in the intertidal such as when the tide is out, standing water is left behind like an oasis for algae and animals. They are subjected to great fluctuations in regard to temp. salinity, acidity, dissolved oxygen content... The higher the pool is in the littoral zone, the longer the pool will be exposed, or isolated from the flush of the oceans waves. Depth in the pool is important, as is the overall size. If then tide is out at night, the release of CO2 from the respiring animals and plants will increase (NO Ps) and increase the acidity of the pool. During the day, PS will cause the pH to increase. The larger pools in the mid zones allow the inverts, seastars etc. to live higher up on the rocks and therefore be able to feed in the upper areas longer. In California. some kelps get started in these pools but once a series of spring tides in June arrive (esp. sunny days), the species living beyond their limits will be killed off.

Top of rock is High Tide Zone, middle of rock to the water is Mid Tide Zone, tide is about zero

Sea Star at water's edge, zero tide level. Not an indicator of the Mid Tide Zone, nor even commonly found there (at low tide), the sea star has a profound effect on the Mid Tide Zone due to its presence there at high tide.


Zonation & particle size.

Mudflats, sandflats and estuarine channels that are emersed at low tide are not treated as separate habitats in this classification but should be categorised on the basis of substratum type in the littoral section. Estuarine channels that retain water at low tide (freshwater or brackish) should be considered in the sublittoral section. Reedbeds in estuaries should be categorised as reed and large sedge swamps.

Table 1: Particle size ranges for loose rocky material and sediment

Particle Type Size range - diameter (mm)









Coarse sand


Medium sand


Fine sand


Mud (silt/clay fraction)


Littoral Rocky Habitats

Littoral rock includes rocky habitats of the littoral, or intertidal zone that extends from the upper limit of the supralittoral, or spray zone, to the MLWS tide mark. The extreme lower shore, or sublittoral fringe is excluded. Rock includes bedrock, stable accumulations of loose and mainly angular rock (ranging in size from boulders to pebbles), and intertidal peats. Shores with mixed substrata of rock and sediment are included in the littoral rock section. Accumulations of rounded and mobile rocky material, or shingle, should be considered under shingle and gravel shores.

Home  |   News  |   Meetings  | Courses  |  ContactsMarine Sciences  |  Things to do | Marine World News | Sitemap |