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Nudibranchs of Florida

Discover the nudibranchs of Florida. These colorful sea slugs lack shells and carry their gills on the outside of their bodies. Find out how they are useful in scientific research.

Molluscs include familiar invertebrates such as snails, clams, scallops, oysters, slugs, squids, and octopuses. Used as food, decoration, tools, medicine, and in the aquarium trade, they have benefited people for millennia. A lesser-known molluscan group is the nudibranchs, commonly called "sea slugs," which display a fascinating variety of colors and body forms.

Radula of a Nudibranch

Nudibranch (pronounced nu-da-brank) means "naked gill," an apt name because their gills are external. Some nudibranchs have gills towards the rear. Others have rows of respiratory projections called cerata situated along the body. On the front end of the nudibranch is a pair of rhinophores (rhino = nose, phore = carrier). These structures are sensory organs that detect chemicals in the water, similar to a sense of smell. Both the gills and the rhinophores can be retracted into specialized pockets for protection. A nudibranch feature that is unique to molluscs is the radula, which they use to eat sponges, corals, anemones, hydroids, bryozoans, tunicates, algae, and sometimes other nudibranchs. The radula acts like a cheese grater, moving back and forth to grasp and shred the food. Nudibranchs that have cerata are capable of storing the tiny stinging cells called nematocysts that are in the corals, hydroids, or anemones that they eat. Then they use those stinging cells for their own defense.

Diagram of a nudibranch

Many nudibranchs have bright color patterns, which warn predators to keep away. Several nudibranchs have glands along the bottom of the mantle that store the poisonous chemicals derived from their food, and by associating bright color with bad tastes, predators may leave them alone. Such warning coloring is called aposematic coloration. However, not all predators are deterred. Several animals, such as sea spiders, polychaetes, sea stars, and some crabs, target nudibranchs as food.

Most nudibranchs live up to a year, although four-year-old nudibranchs have been found. They come in various shapes and sizes, ranging from 1/8th inch (4 millimeters) to 2 feet (60 centimeters). Nudibranchs have an interesting life cycle. They are hermaphroditic, which means they possess both female and male sex organs, and reproduce by cross-fertilization. When nudibranchs reach maturity, they find a partner. Shortly after mating, they lay their eggs in a ruffled ribbon-like strand or in a cluster. The young may develop indirectly, hatching as free-swimming planktonic or pelagic larvae called veligers (the only time they have a shell); or they may develop directly into small, crawling, adult-like juveniles.

Scientists study nudibranchs for several reasons. They are an "indicator species" showing the health of their environment. They help us understand evolutionary processes, such as having a shell or no shell and using nematocysts for defense. 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 who study learning and memory use nudibranchs' large, simple neurons in their research.

The Fish and Wildlife Research Institute (FWRI) collects data directly from people who harvest aquatic species, including nudibranchs, in order to monitor harvest rates and assess the health of fish and invertebrate populations. The Fisheries-Dependent Monitoring (FDM) section at FWRI maintains marine-life landings data dating back to 1994. To view landings data, view the article Commercial Fisheries Landings in Florida.

For more information about Florida's marine-life fishery for the aquarium trade and the data collected, view the article Marine Life and Tropical Ornamentals.

A few other nudibranchs that are also found in Florida waters:

Hypselodoris edenticula (© Tracy Jobsen), Thordisa diuda, Hypselodoris bayeri

Unless otherwise noted, all images are credited to the Florida Fish and Wildlife Conservation Commission (FWC).