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Gastropods are the most diverse group of molluscs, displaying a fascinating variety of colors and body forms. Sea slug is a common name for marine gastropods that that do not have an external shell, including sea hares and nudibranchs. Sea hares have an internal shell made of protein and nudibranchs shed their shell after their larval stage. Nudibranch means "naked gill," which is an apt name because these animals’ gills are external. Some nudibranchs have gills toward the rear, while others have rows of respiratory projections called cerata situated along the body. Both sea hares and nudibranchs have on their heads two long rhinophores, which are sensory organs used to detect chemicals in the water, similar to a sense of smell. These rhinophores are used to find food and detect predators.

Sea hares are herbivores (plant eaters) and are generally found in shallow-water seagrass beds. Their color depends on their diet. For example, sea hares that eat mostly red algae tend to have a reddish color while those that eat mainly green grasses are more green in color. Nudibranchs are carnivores and are found in all depths and marine habitats. They eat sponges, corals, anemones, hydroids, bryozoans, tunicates, algae and sometimes other nudibranchs. To eat, sea hares and nudibranchs use a radula, which acts like a cheese grater, moving back and forth to grasp and shred food.

Since they are shell-less, sea hares and nudibranchs have developed unique defense techniques. Some sea hares’ outer coloring and texture  allows them to blend in with their surroundings and avoid visual detection. They can also burrow into sediments, leaving only their rhinophores exposed. Some sea hares are even capable of releasing a toxic ink, similar to that of an octopus, that acts as visual and scent deterrent, allowing them to escape. Sea hares also have a similar toxin in their skin that makes them inedible to some predators. Some nudibranchs have developed bright color patterns that predators associate with a bad taste, and several species have glands that store poisonous chemicals derived from their food. Not all predators are deterred by the bright colors, and several animals – such as sea spiders, polychaetes, sea stars and some crabs – target nudibranchs as food.

Sea hares and nudibranchs are hermaphroditic, which means they possess female and male reproductive organs. Shortly after mating, they lay their eggs in a ruffled, ribbonlike strand or in a cluster. The fertilized eggs of most marine gastropods develop into either trochophores or veligers, which are free-swimming planktonic stages. A few species bypass this free-swimming stage and hatch directly into small, crawling, adult-like juveniles, but that is more characteristic of land snails. Most nudibranchs and sea hares typically live up to one year, although 4-year-old nudibranchs have been found. Nudibranchs come in various shapes and sizes, ranging from one-eighth inch (4 millimeters) to 2 feet (600 millimeters). Sea hares, on the other hand, are generally larger and more bulky in appearance. The largest sea hare species (Aplysia vaccaria) can reach a length of 30 inches (750 millimeters) and weigh up to 4 pounds (2 kilograms); it is one of the largest gastropods in the world.

Scientists study sea slugs for several reasons. They are an indicator species, meaning they offer clues about the health of their environment. For example, the abundance of nudibranchs may be linked to climate changes. They also help illustrate evolutionary processes, such as shell and organ development and defense mechanisms. Nudibranchs are medically important because the toxic compounds in the creatures they eat are powerful chemical agents that can deter the growth of cancer cells. Scientists studying neurobiology (learning and memory) use sea hares and nudibranchs in their research because their simple nervous systems have thousands of large, easily-identifiable neurons.

Sea hares and nudibranchs are popular in the aquarium industry because of their bright colors and aesthetic appearance. Sea hares are also used to clean algae and bacteria in reef aquariums. Scientists with the FWC’s Fish and Wildlife Research Institute (FWRI) collect data directly from people who harvest aquatic species, including sea slugs (current data include nudibranchs, sea hares and lettuce slugs), to monitor harvest rates and assess the health of their populations. To access landings data, view the article Commercial Fisheries Landings in Florida.