Sea slugs — from small pools to the deep sea

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The phylum Mollusca is one of the largest of all animal groups.  It includes such animals as snails, clams, oysters, squids and octopuses. 

Sea slugs are a very close relative of snails that have evolved a shell-free adult life. They are found creeping and occasionally swimming from small pools to the deep sea. There are more than 4,700 known species of sea slugs, and they are found in both tropical and temperate waters as well as in the Arctic and Antarctic. Sea slugs come in an amazing variety of colors and shapes.

Some species are hermaphroditic and can function as a male, a female or both depending upon the circumstances. After mating, sea slugs lay eggs. Some species lay a few at a time while other species can lay thousands or millions. The eggs will hatch in a few days or weeks.

Some species produce mini sea slugs, while many other species produce swimming larvae (veligers) which possess a tiny coiled shell. The larvae will swim around until stimulated to abandon their shell, settle onto the seabed and mature into adult sea slugs. 

Lacking an external shell for protection, the fleshy, soft-bodied sea slugs appear to be rather defenseless against predators. However, many species have evolved effective and sometimes novel defenses to avoid being eaten.

For example, some species use camouflage to make them hard to spot in their environment. Other species exhibit vivid, eye-catching colors to warn predators that they contain noxious chemicals. Some sea slugs produce toxins themselves while others extract them from the foods they eat. 

There are sea slugs that use sting cells obtained from other organisms they consume. They have evolved unique ways to ingest and handle these cells without their minute poison-tipped harpoons exploding. The cells are pushed into sacs on their own skin to use in defense against predators. There is even a species that can squirt hazardous sulfuric acid from its skin when attacked.

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