Mollusks | Worldwide Animals

Mollusks

Sea snail is a common name for snails that normally live in saltwater; marine gastropod molluscs. (The taxonomic class Gastropoda also includes snails that live in other habitats, i.e. land snails and freshwater snails.)
Sea snails are marine gastropods that have shells. Those marine gastropods that have no shells, or have only internal shells, are variously known by other common names, including sea slug, sea hare, nudibranch, etc.
Many sea snails are edible and are exploited as food sources by humans. Some well-known kinds of edible sea snails are abalone, conch, limpets, whelks (such as the North American Busycon species and the North Atlantic Buccinum undatum) and periwinkles, including Littorina littorea.
There is enormous diversity within sea snails. Many very different clades of gastropods are either dominated by, or consist exclusively of, sea snails. Because of this great variability, it is not possible to generalize about the feeding, reproduction, habitat and so on of sea snails. Instead it is necessary to look at the articles about individual clades, families, genera or species.

Another name for a marine gastropod mollusk that lacks a shell as an adult and is usually brightly colored. Sea slugs, or nudibranchs, are distributed throughout the world, with the greatest numbers and the largest kinds found in tropical waters. They creep along the bottom or cling to submerged vegetation, usually in water just below the low tide line. Members of a few species swim on the surface in open ocean. Most sea slugs are under 1 in. (2.5 cm) long, although the largest, found in the Great Barrier Reef of Australia, reaches 12 in. (30 cm). Regarded by many people as the most beautiful of marine animals, sea slugs display a great array of solid colors and patterns. Many have feathery structures (ceratia) on the back, often in a contrasting color. Most sea slugs have two pairs of tentacles on the head, used for tactile and chemosensory reception, with a small eye at the base of each tentacle. Sea slugs graze on small sessile animals such as coelenterates, sponges, and bryozoans. Certain sea slugs that feed on corals and sea anemones ingest the stinging cells of their prey without discharging them; these then pass from the slug's digestive tract to the ceratia, where they are used by the slug for its own defense. Sea slugs are classified in the phylum Mollusca, class Gastropoda, subclass Opisthobranchia, order Nudibranchiata.

Sea cucumbers are echinoderms—like starfish and sea urchins. There are some 1,250 known species, and many of these animals are indeed shaped like soft-bodied cucumbers. All sea cucumbers are ocean dwellers, though some inhabit the shallows and others live in the deep ocean. They live on or near the ocean floor—sometimes partially buried beneath it.

Sea cucumbers feed on tiny particles like algae, minute aquatic animals, or waste materials, which they gather in with 8 to 30 tube feet that look like tentacles surrounding their mouths. The animals break down these particles into even smaller pieces, which become fodder for bacteria, and thus recycle them back into the ocean ecosystem. Earthworms perform a similar function in terrestrial ecosystems.

Sea cucumbers, particularly eggs and young larvae, are prey for fish and other marine animals. They are also enjoyed by humans, especially in Asia, and some species are farmed as delicacies.

When threatened, some sea cucumbers discharge sticky threads to ensnare their enemies. Others can mutilate their own bodies as a defense mechanism. They violently contract their muscles and jettison some of their internal organs out of their anus. The missing body parts are quickly regenerated.

Sea cucumbers can breed sexually or asexually. Sexual reproduction is more typical, but the process is not very intimate. The animals release both eggs and sperm into the water and fertilization occurs when they meet. There must be many individuals in a sea cucumber population for this reproductive method to be successful. Indeed, many parts of the deep ocean host large herds of these ancient animals, grazing on the microscopic bounty of marine waters.

The common octopus would be unique for its appearance alone, with its massive bulbous head, large eyes, and eight distinctive arms. But by far the most striking characteristic of the octopus is the wide array of techniques it uses to avoid or thwart attackers.

Its first—and most amazing—line of defense is its ability to hide in plain sight. Using a network of pigment cells and specialized muscles in its skin, the common octopus can almost instantaneously match the colors, patterns, and even textures of its surroundings. Predators such as sharks, eels, and dolphins swim by without even noticing it.

When discovered, an octopus will release a cloud of black ink to obscure its attacker's view, giving it time to swim away. The ink even contains a substance that dulls a predator's sense of smell, making the fleeing octopus harder to track. Fast swimmers, they can jet forward by expelling water through their mantles. And their soft bodies can squeeze into impossibly small cracks and crevices where predators can't follow.

If all else fails, an octopus can lose an arm to escape a predator's grasp and regrow it later with no permanent damage. They also have beaklike jaws that can deliver a nasty bite, and venomous saliva, used mainly for subduing prey.

Considered the most intelligent of all invertebrates, the common octopus is found in the tropical and temperate waters of the world’s oceans. They can grow to about 4.3 feet (1.3 meters) in length and weigh up to 22 pounds (10 kilograms), although averages are much smaller. They prey on crabs, crayfish, and mollusks, and will sometimes use their ink to disorient their victims before attacking.

The giant squid remains largely a mystery to scientists despite being the biggest invertebrate on Earth. The largest of these elusive giants ever found measured 59 feet (18 meters) in length and weighed nearly a ton (900 kilograms).

However, their inhospitable deep-sea habitat has made them uniquely difficult to study, and almost everything scientists know about them is from carcasses that have washed up on beaches or been hauled in by fishermen. Lately, however, the fortunes of scientists studying these elusive creatures have begun to turn. In 2004 researchers in Japan took the first images ever of a live giant squid. And in late 2006, scientists with Japan's National Science Museum caught and brought to the surface a live 24-foot (7-meter) female giant squid.

Giant squid, along with their cousin, the colossal squid, have the largest eyes in the animal kingdom, measuring some 10 inches (25 centimeters) in diameter. These massive organs allow them to detect objects in the lightless depths where most other animals would see nothing.

Like other squid species, they have eight arms and two longer feeding tentacles that help them bring food to their beak-like mouths. Their diet likely consists of fish, shrimp, and other squid, and some suggest they might even attack and eat small whales.

They maneuver their massive bodies with fins that seem diminutive for their size. They use their funnel as a propulsion system, drawing water into the mantle, or main part of the body, and forcing it out the back.

Scientists don't know enough about these beasts to say for sure what their range is, but giant squid carcasses have been found in all of the world's oceans.