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What Sea Slugs Are Right For Your Reef Tank?


Find out what sea slugs are beneficial to your reef tank and which ones will eat your corals.


courtesy to : / By Scott W. Michael 

Sea Slugs

So, why talk about them? The fact is that they do enter the aquarium trade. For example, on a recent visit to Los Angeles, I observed hundreds of these animals waiting to be shipped to retailers at wholesale facilities. There are also a handful that will do well in captivity and that are even employed to help reduce pest num-bers in the reef aquarium. There are also some that come in as unwanted hitchhikers with live corals we buy. And they are a fascinating group of animals! So, why not talk about them? Let’s look at some sea slug basics. 

Sea slugs are well-known to divers and underwater photographers, but are less familiar to aquarists. It is not that they don’t occasionally make their way into the ornamental fish trade — they do. Yet, many stores refuse to carry them because they tend to be difficult to keep and are naturally short-lived, even when they have an adequate captive microhabitat. 



The chromodorids, such as Chromodoris coi, are some of the most stunning sea slugs, but they have very specialized diets and can starve in the home aquarium. Photo by Scott W. Michael

Sea Slug Basics :


The sea slugs belong to the phylum Mollusca. This diverse group is thought by some to consist of more than 100,000 species, which include snails, clams, octopuses and squid. Within this phylum, the sea slugs make up the subclass Opisthobranchia, which is comprised of more than 3,000 species. These can be grouped into five different orders: the cephalaspidea (headshield slugs), sacoglossa (sapsucking slugs), anaspidea (sea hares), notaspidea (sidegill slugs) and nudibranchia (nudibranchs). Some characteristics common to all members of this subclass is that they do not have a shell, or if there is a shell, it is greatly reduced in size. They have advanced gills for respiration and are simultaneous hermaphrodites (that is, they have both functional male and female sex organs at the same time). The sea slugs have oral tentacles at the edge of the mouth that are used to help locate food; they also have rhinophores, which are tentacles on top of the head that sense chemicals in the water. 



Sea Slugs in the Trade :


In my early days of marine aquariumkeeping, there used to be more sea slugs available to hobbyists. Now because of increased education, more shops and aquarists are refusing to sell or buy them because they can be difficult to keep alive and are naturally short-lived (many live less than a year in the wild). 


The most attractive of those collected belong to the order nudibranchia. Many of these exhibit flamboyant colors that make them attractive selections for the uninformed hobbyist. But these are also the most difficult to keep because they tend to be very specialized feeders. Most feed on sponges (e.g., Chromodoris spp.) or bryozoans (e.g., Tambja spp.). Those that feed on sponges may only feed on a particular species or genus of sponges, so it is not as simple as just throwing any Atlantic sponge you find at your aquarium store in with your western Pacific sea slug. Because they are so difficult to feed, it is best that these animals are left in the natural habitat. Some of these come into our tanks unexpectedly as hitchhikers, and they can cause problems in the reef tank. Let’s look at this group first. 

The attire of this Tambja morose may advertise the presence of chemicals that mean this slug tastes bad. Photo by Scott W. Michael

Unwelcome Hitchhikers :


A number of nudibranchs are unintentionally introduced into the reef aquarium. Some of the most fascinating of these belong to the genus Phyllodesmium. These amazing animals may go unnoticed in a reef aquarium for many weeks or months. They almost always come in with their favorite food: soft corals (e.g., Sarcophyton and Xenia). Because of this, when they are discovered (because of their destructive habits), usually aquarists quickly purge them from the tank.



Soft corals produce toxins that dissuade generalized predators — but these slugs specialize in eating them. The sea slugs not only acquire nutrients by consuming the coral’s flesh, they also exploit the unicellular algae (zooxanthellae) that live within the tissues of their prey, as well as chemicals the coral produces. The Phyllodesmium transport the coral’s zooxanthellae in to their cerata (elongate structures on the back) and other parts of their bodies via digestive glands. The algae cells photosynthesize, providing the slug with needed nutrients. It’s because of this unusual mutualistic association that some people refer to these slugs as the "solar-powered nudibranchs.” The most famous of these is the largest and most spectacular member of the genus,Phyllodesmium longicirrum. This is the least likely member of the genus to be encountered by aquarists because of its large size. Not only do the Phyllodesmium recycle the soft corals’ algal cells, they also accumulate toxic compounds produced by their prey, which makes these sea slugs less attractive to predators. 

A number of aeolid sea slugs eat soft coral. This Phyllodesmium longicirrum is munching on a leather coral. Photo by Scott W. Michael

There are other nudibranchs that feed on stony corals. For example, there is at least one species of Phyllodesmium that is a known Goniopora-eater. But the most notorious of the stony coral-eating sea slugs is the Montipora-eating nudibranch. This species is considered by many stony coralkeepers as the worst reef aquarium pest. This is an aeolid species (no species name is available, and it may actually consist of more than one species) that reaches about 2 to 3 mm in length, is light in color and has multiple cerata adorning the dorsal surface of the body. It feeds mainly on the tissue of corals in the genus Montipora, but it may also dine on Porites and Anacropora species.

What makes it so dangerous is that it can rapidly multiply, and as the numbers grow, the Montipora corals in the tank suffer. As they feed on the coral tissue, they leave white patches of bare skeleton. These sea slugs don’t store the algae like the Phyllodesmium, but instead sequester the stinging cells (nematocysts) of their coral prey and use them for defense. Getting rid of these little sea slugs is tricky. There are wrasses that will eat them, including members of the genus Thalassoma (namely the saddled wrasse, Thalassoma duperrey), Coris and some Halichoeres species. Chemical baths (e.g., Lugol’s solution) have also been prescribed that may or may not work, as well as blasting the sea slugs off the corals using a powerful jet of water (e.g., direct stream of water from a powerhead).  



Beneficial Species  : 


While there are sea slugs that are considered to be pests, there are also species utilized to deal with pestilent plants and animals. The sea hares, which get their name from the pair of long tentacles on their head, are occasionally employed to feed on algae. When threatened, these animals expel an inklike substance that apparently serves to distract potential predators. If your sea hare inks, activated carbon and a water change can prevent it from polluting your aquarium. The problem with keeping sea hares is that their dietary preferences can be species-specific. So, while one species may ingest red macroalgae only, others feed entirely on blue-green cyanobacteria. 

Some sea slugs, such as this Elysia crispata, feed on microalgae and can serve a useful purpose in the reef aquarium. Photo by Scott W. Michael

There are some sacoglossan species that also feed on algae (e.g., Elysia crispata, E. ornata). Many of these suck the chlorophyll from the cells of macroalgae (e.g., Caulerpa, Halimeda), but may also eat green filamentous algae. Like the Phyllodesmium, the Elysia incorporate the cells from these algae into their own tissue and utilize carbon compounds produced by the hijacked chloroplasts. One of the most common species in the trade is an Atlantic species, Elysia ornata. This species eats Bryopsis, a common filamentous algae pest in the home aquarium. 


Another species of sea slug that is sometimes introduced to control pests is Aeolidiella stephanieae (incorrectly sold as Berghia verrucicornis — a colder water species from the Mediterranean). This sea slug is a known predator of Aiptasia (glass) sea anemones, which can overgrow the aquarium and sting more desirable cnidarians. If you put several adults in your tank, they should breed. Once the eggs hatch, the larvae will also begin feeding on these anemones, and within a short period of time, they can rid an entire tank of Aiptasia. Of course, once their food source is depleted, the slugs will begin to starve to death. The life span for one of these slugs (with an unlimited amount of food present) is about five or six months, and they can lay eggs as soon as 50 days after hatching.  There are a number of resources on the Internet on culturing these animals in captivity.

The feathery structure on the back (the gills) of this Chromodoris kunei are used for respiration, while the "horns" (known as rhinophores) on the head are used to detect odors. Photo by Scott W. Michael

There is one other slug that has been employed by aquarists to deal with invertebrate pests: Chelidonura varians, which is sometimes sold in the hobby as the velvet sea slug. As mentioned, this species specializes in feeding on acoel flatworms, which can become a plague once they get established in a reef aquarium. This lovely slug (it is black with blue lines) slides over the substrate, vacuuming up the acoels in its path. They have sensory bristles on each side of the mouth that help them find their prey, and they simply suck the flatworms into their mouths once they overtake them. Unfortunately, as with the Aiptasia-eating slugs, once the food runs out, these specialized opisthobranchs will starve to death. If food is not in short supply, or if they are not attacked by resident fishes (e.g., wrasses, sharp-nosed puffers) or sucked into filter intake tubes or into powerheads, the normal life span is short, reported around three or four months. They will spawn in captivity, but raising the larvae is difficult. When they reproduce, they produce a white, wavy egg mass, and the eggs embedded in this structure will hatch in three or four days.



Two Similar Slugs :


One word of warning: Make sure you don’t mistake C. varians with the very similar Philinopsis gardineri in coloration and general form. In the latter species, the "tail” is rounded and not as long and filamentous, and it lacks the hammerlike head found in C. varians. The diet of the two species is very different — P. gardineri feed only on an unusual opisthobranch known as the bubble shells (family Aplustridae), not on flatworms.  

That ends our brief look at the opisthobranchs that are wanted and unwanted in the home aquarium. While most species are not well-suited to life in captivity due to their specialized diets, there are some sea slug species that by chance or by choice make interesting aquarium inhabitants! Happy slug-watching! 

Sea Slugs list of available species :


courtesy to : 



Sea Slugs, sometimes called Nudibranchs, display vivid colors and an unusual body shape. Typically found in tropical reefs, their most notable characteristic is the pair of stalked rhinophores or horns at the head of the Slug. 


1-Blue Velvet Nudibranch  

Chelidonura varians 


Minimum Tank Size: 10 gallons

Care Level: Expert Only

Temperament: Peaceful

Reef Compatible: Yes

Water Conditions: 72-78° F, dKH 8-12, pH 8.1-8.4, sg 1.023-1.025

Max. Size: 2¾"

Color Form: Black, Blue

Diet: Carnivore

Origin: Western Pacific

Family: Aglajidae

The Blue Velvet Nudibranch, also known as the Head Shield Sea Slug, is black in color with blue lines around the outside of its body and down the middle of its head. It has a hammer shaped head and two appendages at the rear, mimicking a flatworm.


This specialized eater will consume flatworms in the aquarium. It sucks up the flatworms using a tube-like proboscis and makes a great natural alternative for flatworm control. However, it must be provided with a steady diet of flatworms in order to thrive. In addition, it is sensitive to rapid changes in water conditions and does not have a long life expectancy. Because of this, only expert aquarists should attempt to maintain this species.


Spawning occurs regularly when more than one of this species is maintained together. Unfortunately, there has not been a good success rate reported in raising the larvae.


The Blue Velvet Nudibranch should be acclimated using the Drip Method. It can be harmed by pump intakes and drains, so care should be taken to screen these off. It is sensitive to high levels of nitrate and will not tolerate copper-based medications.


Approximate Shipping Size: 1-1/2" to 2"

2-Lettuce Sea Slug, Green  

Elysia spp. 


Care Level: Moderate

Temperament: Peaceful

Reef Compatible: Yes

Water Conditions: 72-78° F, dKH 8-12, pH 8.1-8.4, sg 1.023-1.025

Max. Size: 3"

Color Form: Green, Tan, Yellow

Diet: Herbivore

Compatibility: View Chart

Family: Elysiidae

The Lettuce Sea Slug is an unusual and entertaining addition to the reef aquarium. It has highly folded parapodia (side appendages), which give it a ruffled appearance similar to lettuce. The Lettuce Sea Slug is captivating not only for its interesting shape but also for its purposeful and comical manner in which it creeps along your reef aquarium. The coloration of the Lettuce Sea Slug generally ranges from green, yellow, or brown.


The Lettuce Sea Slug may also be referred to as the Lettuce Nudibranch, or Green Lettuce Nudibranch, although it is not a true nudibranch. The Lettuce Sea Slug will do well in established aquarium systems, if provided ample room to forage. It prefers an aquarium with live rock so that it can graze on algae.


The Lettuce Sea Slug incorporates the chloroplasts (the portions of the cell responsible for photosynthesis) from the algae into its tissues, and thus relies on photosynthesis for part of its energy. Pump intakes and drains can harm the Lettuce Sea Slug, so care should be taken to screen these off. It is sensitive to high levels of nitrate and will not tolerate copper-based medications. The Lettuce Sea Slug has no distinguishing characteristics to help differentiate it from its mate.


Approximate Purchase Size: 1/2" to 1-1/2"



3-Sea Hare  

Aplysia sp. 


Care Level: Expert Only

Temperament: Peaceful

Water Conditions: 72-78° F, dKH 8-12, pH 8.1-8.4, sg 1.023-1.025

Max. Size: 1'

Color Form: Black, Tan

Diet: Herbivore

Origin: Indo-Pacific

Family: Aplysidae

The body of the Dwarf Sea Hare is a combination of speckles and patterns. The location of its rhinophores (organ used to smell) and its oral tentacles make it look a bit like a rabbit, hence its common name.


If it becomes startled, it may release a purple dye to repel attacking fish. In the home aquarium, the Sea Hare will need a good chemical filter system to quickly remove this toxic dye before it causes problems.


In the wild, it is usually found in shallow areas of seagrasses and coral rubble, preferring the shaded areas. Cover from the lights in the aquarium should be provided. It requires a large area in which to move. It prefers an aquarium with live rock and open sandy areas so that it can graze on algae, eating any Caulerpa in the tank. It is sensitive to high levels of copper-based medications and will not tolerate bad water conditions (high nitrates).


If regular feedings of Caulerpa are not possible, it will need a supplemented diet of parboiled lettuce and dried kelp.


Approximate Purchase Size: 2" to 4"

4-Glaucus Atlanticus — The Blue Dragon


Now this might seem like a fancy brooch to you that you might want to pin to your clothes or evening gowns, but actually it’s not!
Yes it’s a tiny creature that spends most of its life time floating upside down in the deep blue water of the Atlantic, Indian and Pacific Ocean. What keeps it floating on the surface of the water is the air bubble that it fills in its belly, whichever way the wind blows it takes this little creature along with it, so all it does is relax while floating in the water. Can you imagine a more carefree life than Glaucus Atlanticus?


But don’t get confused by its carefree attitude, as this creature is not that harmless. This 3 cm long invertebrate is known for its appetite for hydrozoans (same group to which jelly fish belongs) and its most favorite ones are the most poisonous, Portugese Man-O’-War. The Portugese Man-O’-War are known to be fatal for human and its sting is extremely painful, but it doesn’t take much time for our little friend Glaucus to swallow it without getting hurt. How can it do that? Well, inside its skin are hard disks that acts like a barrier and it also releases mucus that forms a protective layer against stings.

Another astonishing trick this creature can perform is that not just it protects itself from the poison but it stores it for future use. The poison is sucked in by cerata (finger like structures sticking out from its body) that are 84 in number and stored in sacs called canidosacs, and later this poison is used to defend itself from other predators. (What a strategy!) Other poisonous hydrozoans that it likes to feed on are blue button and by the wind sailor. And yes, it has been known to cannibalize as well in situations where there not enough prey available to feed on. Scientists say that Glaucus can have more deadly sting than the Man-O’-War depending on the amount of venom it has collected in its body.


Another trick that this not so innocent animal can play is that it fools its predators by camouflaging; the trick is also called countershading. As mentioned before, this creature floats upside down meaning ventral side up and dorsal side down wards. Now, how does it help? The ventral side is bright blue which helps it blend into the ocean’s surface while the grayish dorsal side merges with the inner surface of the water. This trick is used for both swimming and flying predators.


This slender creature is known by the names of blue dragon, blue sea slug, blue ocean slug and blue glaucus. The scientific name is Glaucus atlanticus and it belongs to the group of molluscs. But what differentiates it from typical molluscs is that it doesn’t possess any shell and therefore known as blue sea slug. This animal has been named after Greek god of the sea, Glaucus, this sea god was forced to live in the sea for eternity. The creature was first studied by Forster in the 17th Century. Blue dragon was once thought to be an insect living in marine and was later classified as marine gastropod in 19th Century.


Glaucus Atlanticus is also believed to be a close relative of Glaucus Marginata, also known as “the blue fleet” comprising of its potential prey Physalia, Velella and Porpita.


These sea slugs are also known to be Hermaphrodites; now what happens is that when the two sea slugs mate both produces egg strings. They lay their eggs on driftwood and sometimes on the skeletons of their prey (that’s one brutal creature). This enables the young glaucus to float on the sea surface while its air sac under development.


When out of water these creatures tend to roll themselves turning into a spiny ball, as soon as they get back into the water they open up again. Now Glaucus are very particular about which side should be facing upward and in any case if their grey side is facing the surface these creatures instantly flip sides.

You can find these creatures anywhere in the world, from temperate to tropical waters. Now that you know about this deadly creature it is highly recommended that you keep your eyes open when the next time you go to the beach for surfing.

Glaucus atlanticus
Forster, 1777




Circumglobal in temperate and tropical waters.


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