Nudibranchs, often referred to as sea slugs, are boneless, slime-oozing creatures that belong to the Nudibranchia order and are invertebrates. More than 3,000 species exist, and many of them display brilliant colors and eye-catching patterns on their skin, making them some of the most beautiful animals in the world. They typically inhabit shallow, tropical waters. They range in size from a quarter of an inch to 12 inches long and can weigh up to 3.3 pounds. These carnivores spend their time sliding on their bellies around their habitat in search of snacks.
Anatomy and Feeding Mechanisms
Sea slugs, like most molluscs, possess a radula, a toothed ribbon used to scrape, pierce, or grip prey. The diversity of sea slug diets is reflected in the diversity of radula configuration, which is long used as a species marker by taxonomists. They have a set of curved teeth, which they use to eat coral, sponges, and fish eggs off the ocean floor. Nudibranchs use tentacles on their heads to poke around for grub.
Diet Diversity
Across the entire group, sea slugs have an extremely diverse source of food types. Conversely, each species usually has an extremely specific diet, often restricted to one or only a few, usually closely related, prey species. A list of the animal types preyed upon reveals that all the major invertebrate phyla are exploited, plus certain algae from the plant kingdom and even cyanobacteria.
Sea slugs consume a wide array of food sources, including:
- Cyanobacteria
- Diatoms
- Dinoflagellates
- Foraminiferans
- Algae
- Sponges
- Hydroids
- Soft & hard corals
- Sea pens
- Anemones
- Sea jellies
- Siphonophores
- Ctenophores
- Entoprocts
- Bryozoans
- Brittle stars
- Flatworms
- Polychaete worms
- Bivalve molluscs
- Sea slugs themselves
- Sea slug eggs
- Copepods
- Barnacles
- Small crustacea
- Even the fin webbing of fish
Specific examples of feeding behaviors include:
Read also: Diet of the Leopard Slug
- The cryptobranch dorid Hypselodoris zephyr feeding on a Dysidea species of sponge with its buccal tube extended.
- The phanerobranch dorid Nembrotha lineolata feeding on an ascidian.
- The dendronotinid Bornella anguilla stripping polyps from an Aglaophenia species of hydroid.
- The aeolid Flabellina lotos (or Coryphellina lotos) devouring polyps along the stem of a Eudendrium species of hydroid.
- The aeolid Favorinus tsuruganus eating egg capsules inside a dorid nudibranch spawn spiral.
- The sap-sucking sacoglossan Elysia pusilla (or Elysia sp.) sucking protoplasm out of algal cells of Halimeda sp., leaving a white feeding track.
Diet-Related Adaptations and Phenomena
The reduction or loss of the shell is thought to have permitted radiation into different food niches, many of them not greatly exploited by other taxa. Food availability and competition drive evolutionary change, as do ways to overcome the defenses developed to prevent predation. This is diet niche exploitation. Diet has caused changes in color and pattern, and even anatomical shape and features have revolved around diet for functional reasons as well as mimicry and camouflage.
Significant diet-related phenomena developed by sea slugs include:
- Concentrating noxious compounds in mantle glands for defensive purposes: Some nudibranchs absorb toxins from certain prey and secrete the poison from their own skin, which allows them to fend off enemies such as fish.
- Kleptoplasty: Plastids from chloroplasts are sequestered from host algae and farmed for nutritional benefits.
- Zooxanthellae symbiosis: Zooxanthellae are symbiotically hosted, also for nutritional benefit. Some aeolids will even fill their cerata with the zooxanthellae of their prey, using the photosynthetic algae in the same way as their former victim had to produce sugars for themselves under intense sunlight.
- Nematocyst sequestration: Nematocysts from cnidarian and siphonophore diets are stored for future defensive use.
Specific Feeding Behaviors and Examples
Nudibranchs exhibit diverse feeding habits, with some species displaying remarkable adaptations:
- Some nudibranchs are cannibals, feasting on other nudibranchs.
- Berghia verrucicornis is marketed as the Aiptasia-eating slug. This species munches on those pesky anemones with gusto. Yet all good things come to an end and, on gorging, clearing out the tank and breeding, these animals will then simply starve to death.
- Glaucus atlanticus, also known as the Blue sea slug or Sea swallow, spends its life belly side up at the surface, ingesting bubbles of atmospheric air to maintain buoyancy. It feeds on the deadly Portuguese Man-o’-War, absorbing the dangerous nematocysts and storing them inside its own body, on the ends of the finger-like cerata that jut out either side of its body. In doing so it often becomes more concentrated and powerful than the Man-o’-War’s sting.
Coloration and Diet
The nudibranch’s meals don’t just satisfy its hunger-the food also gives the animal its coloring. When the sea slug eats, it absorbs and displays its prey’s pigment-the substance that gives the prey its color.
Sea hares are herbivores 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.
Read also: Glaucus atlanticus: Detailed Guide
Defense Mechanisms Related to Diet
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.
Nudibranchs in Aquariums
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.
Don't be taken in by the striking colours of nudibranchs. For many reasons the hobby isn't ready for them yet, says Nathan Hill. In part this is down to a very specific dietary requirement, varying from species to species. Some are cannibals, feasting on other nudibranchs. Some will prey on fish. The vast majority have evolved over millions of years to feed on one particular variety of sponge, coral, anemone or hydroid. Some will simply sit and excrete enzymes, sucking in the resultant digested slurry of their unfortunate victim. Assuming you’re lucky enough to find out what your particular species feeds on, I wish you the best of luck in tracking down a ready source. It’s not uncommon for the food species to be far more delicate than even the most expert reefkeeper could accommodate, let alone undertake as a project.
Read also: The Hoxsey Diet