Virtual Galápagos: Wildlife

Wildlife - Coastal Zone - Intertidal Life

Introduction

Intertidal Life

Sea Turtles

Land Iguanas

Marine Iguanas

Sea Lions

Fur Seals

Birds

Though the intertidal zone is but a strip around all the islands, the total area has been estimated as about 40 square kilometers, or two thirds the size of Española Island. Organisms here must adapt to spending part of their time submerged and part above water, resisting the incessant action of waves. Only a few of the intertidal creatures, like the marine iguana and Sally Lightfoot crab, will be familiar to most visitors, but if you take a close look at life on the tidal rocks and in the tide pools, you will discover an ecosystem every bit as complex as that on land.

Most of the creatures that live in the intertidal zones are invertebrates and there are many hundreds of species. Among them are sponges (porifera), the natural ancestor of the bathroom sponge that can be found under rocks in the intertidal zone. These amorphous, variously colored blobs are really animals that filter out minute plants and animals from the water.

You'll also find sea anemones, related to the corals, hydroids and jellyfish. These "animal-flowers" are sedentary predators -- they have numerous stinging tentacles which they use to immobilize and kill their prey, including small fish and invertebrates.

Starfishes, brittle stars and sunstars are a familiar group. Most species are predatory, eating mollusks and the like. Starfish are often brightly colored and can regrow their limbs, if lost. Two species of sunstar, Heliaster spp., are common in intertidal areas. These look like many-armed "starfish."

Sea cucumbers (Holothurians) are soft bodied relatives of the starfish which live on the sea bed and feed on debris and small creatures that they take in with the help of tentacles around the mouth. Just as starfish can eject their stomach to start the external digestion of a mollusk, so sea cucumbers can eject and abandon their lower intestine and other entrails when disturbed. Like the lizard's tail, these regrow. Due to recent marine harvesting by Asian entrepreneurs, sea cucumbers are extremely endangered.

Sea urchins are common in the Galápagos. One species, Eucidaris thouarsii, the pencil-spined urchin, is covered with thick pencil-like spines that disintegrate only slowly and often cover beaches. This species feeds on corals and other organisms. The endemic green sea urchin, Lytechinus semituberculatus, is also common. Like sea cucumbers and starfish, the sea urchins move about slowly with many small "tube-feet." Sand dollars and sea biscuits are also echinoderms.

Mollusks

Mollusks are soft-bodied animals, most of which have some type of shell. The shells that we find tossed up on the beaches, and that we associate so closely with the seaside, are the discarded "homes" left after their inhabitants have died. The variety is tremendous, even in the Galápagos, where many species are endemic.

Chitons are a primitive type of mollusk whose carapace consists usually of eight angled plates which overlap like shingles to form an effective armor. Chitons are common in the intertidal zone. They feed by rasping off microscopic algae from the rocks.

Bivalves, or pelecypods, are a familiar and abundant group that includes the clams, cockles, mussels, oysters, and scallops. Most species filter feed on microscopic plant life. They have a muscular "foot" which they use to move around. Some can "swim" by ejecting a forceful jet of water.

The gastropods are the largest group of mollusks and include limpets, sea slugs, cowries, cones, whelks, and many other types, a great variety of which are found in the Galápagos.

Crabs and Barnacles

There are over a hundred crab species in the Galápagos but one stands out from the rest. The Sally lightfoot, or red lava crab, Grapsus grapsus, is so abundant and so brightly colored with red above and blue below that no visitor could miss them. Their name comes from their habit of skipping across short stretches of water. The young of this species are black and even faster runners than the adults.

The large pincers on this crab can give a nasty pinch but the spoon-shaped tips are adapted to grazing on algae and scavenging. This bright denizen of the lava rocks follows the tides in and out, day and night, picking up tiny fragments of algae and detritus. Sally lightfoots are very alert to moving objects and are not easily approached. However, if you remain still they will walk over you, as if you were a marine iguana. When disturbed, they often eject one or two squirts of water rather like a water pistol. They fall prey to herons, moray eels, and occasionally hawkfish.

Ghost crabs, Ocypode, are common on beaches where they dig burrows in the sand and prey on other beach creatures. I have seen them taking turtle hatchlings into their burrows. they also leave characteristic traces of "sand-balls" on the beach which are pellets of sand that they have sorted through for micro-organisms. Their eyes are unusual in that they can be raised vertically on their eyestalks or lowered into grooves of their carapace.

Fiddler crabs, too, live in sandy areas. These small crabs gain their name from the huge pincer that the males have. When courting, the males wave these huge "flags" rather like a violinist bowing his instrument. Hermit crabs are another interesting group. They have a soft abdomen which they protect by living in abandoned gastropod shells. As they increase in size, they discard small shells for large ones. They may switch "houses" some four or five times before reaching adult size.

Crabs are among the most active of the intertidal organisms and barnacles the least so, yet they are closely related. Both share the characteristics of jointed limbs and tough external skeleton. Barnacles, though sedentary in the adult stage, have six pairs of feathery limbs which they extrude when covered by water and catch small organisms for food. Across the middle of the intertidal zone is usually a wide band of small white Rock barnacles, Tetraclita sp. These are resistant to drying and endure a considerable portion of the day out of the water when they are closed up. In the lowest intertidal and subtidal zones is the large Acorn barnacle, Megabalanus galapaganus, which grown 5 cm or more high. When dead and washed ashore the "shells" are white and pink.