Saltwater Invertebrates

Marine aquarists have a wide variety of invertebrates to choose from to add to their tank.  These include various crabs, snails,  shrimp and other crustaceans similar to their freshwater counterparts.  However, there are also a wide variety of other invertebrates available in a marine aquariums such as starfish, urchins, various worms, cucumbers, scallops, clams,  sponges, anemones, cephalopods and more!

Saltwater Crabs

Saltwater Crabs

Saltwater crabs are an almost essential component of a majority of reef aquariums.  They are generally hardy and an easy way to keep the tank free of uneaten food and unwanted algae.  Dwarf Red Hermit crabs and Blue Legged Hermit crabs are a great addition to any reef tank.  Some fish such as puffers or triggers will eat most crabs but there are really large hermit crabs that even they would think twice about approaching.  Emerald crabs will eat some algae that other crabs tend to avoid and make a great addition to the tank.  The Sally Lightfoot crab is an attractive and entertaining as well as a  useful specimen.  There are large long legged arrow crabs that get up to 6″!  The Decorator crab decorates its brown body with sponges, shells, rocks, sometimes corals and other items as a method of camouflage.  There are many other interesting crabs sometimes found for sale as well.

Saltwater Snails

Turbo snails, Margarita snails, Cerith snails, Astraea snails, Nassarius snails, and Trochus snails and more are all commonly available to saltwater aquarists.  Generally they are very reef safe and compatible in most reef tanks although the same fish that will eat crabs will also eat snails.  Some crabs will also predate snails however.  They will compete with crabs for algae and food.  They are generally affordable and you can keep one every 2-3 gallons of water but most saltwater snails should be given seaweed to supplement their diet if there is insufficient algae in the tank.

Saltwater Shrimp

They tend to be a pricier investment than saltwater snails or crabs but there are many species of saltwater shrimp to add life and color to the saltwater aquarium.  They are generally considered reef safe although some have unique requirements and characteristics such as the Camelback shrimp which can nip at corals.  Peppermint shrimp are affordable and colorful additions to a reef tank while various types of Cleaner Shrimp will often pick parasites off fish in the aquarium.  Some fish such as Hawkfish that will generally stay away from crabs and snails might be more tempted by a shrimp as a snack.

Other Saltwater Crustaceans

Of course, shrimp and crabs are crustaceans but Reef lobsters are sometimes found for sale as well.  An ideal environment for a Reef lobster should have a thick gravel bed for burrowing, and rocks for hiding, as well as live rock on which to hunt. After molting, the Reef Lobster will need a secure hiding place, such as a cave, while it waits for its new exoskeleton to harden. The cave can be designed so that the lobster can be seen during the day, but it usually will spend most of the daylight hours hiding from the light. It prefers to scavenge and hunt at night. The Reef Lobster is described as peaceful, and it will ignore sleeping Wrasses or healthy fish within the aquarium. Caution must be taken when incorporating into a reef aquarium, as it may harm small fish and invertebrates. All Reef Lobsters are very territorial and aggressive towards each other, so only one specimen, or a mated pair should be kept per tank.  Most of the diet will consist of food it scavenges, but supplementing with iodine-rich foods will help ensure proper molting.


Starfish can also be a great addition to a saltwater aquarium but should be approached with some caution.  Most are fairly hardy tank cleaners and reef safe.  However, they should never be exposed to air, are generally sensitive to changes in the water parameters, and can be somewhat aggressive, with larger specimens even eating smaller unsuspecting fish.  Some like the Red Knob or Chocolate Chip Sea Star often found in stores are definitely NOT reef safe and will feed on and damage corals.


Related to starfish, urchins are certainly a fascinating and unusual to have in a home aquarium.  However, many species will actually feed on Coralline algae, the pretty purple colored covering the live rock in the tank.  Unfortunately, the species that are least suited to the home aquarium are some of the cheaper and more commonly found specimens.  Eventually, they will leave tracks all over the live rock and the tank can run out of food for the urchin and it can even be destructive to or eat corals.  Other species prefer non-coralline algae to feed on and if you supplement their diet with seaweed sheets then they can be reef compatible.  The Blue Tuxedo urchin is said to be completely reef compatible and will not damage live rock or coral although it’s diet should be supplemented with seaweed.

The Longspine Urchin can be a useful scavenger in tanks that will eat most other invertebrates because it has long mildly poisonous spines to protect it from large predatory fish.


There are several species of saltwater Fan Worms often found available for the saltwater aquarium.  Fan Worms are marine segmented worms that are sessile and attached to rocks or sand by their base. The plume of Fan Worms can measure up to 10″ in diameter on some species.  The will require microfauna in the aquarium or supplemental feeding with foods designed for filter-feeding invertebrates. 

Bristle worms are often found in the home aquarium and are often considered a nuisance pest hitchhiking their way into the tank on live rock or coral rocks.  They are ubiquitous in the earth’s oceans and there are many thousands of species.  Bristle worms are actually good tank cleaners but they can grow large enough (up to a foot long or more!) to harm fish and corals in the tank.  When they get this large they might need to be removed by hand or with a trap.  You can see them by looking at the tank with a red flashlight at night as they are nocturnal.


Sea cucumbers are an interesting option for saltwater aquarists.  However, if they get sucked into a pump intake by accident there is a possibility they might poison the aquarium.  The cucumbers available that do not have branching feeding arms will most likely just disappear under the substrate and live there.  They will need a deep substrate and will clean it but you will not likely see them often.  The cucumbers with branching arms will likely spend a lot more time out in the open and need microfauna in the aquarium.   If there is not enough microfauna in the system, their diet should include liquid or dried phyto and zoo plankton. They will also benefit from the substrate being stirred regularly releasing bacteria and detritus into the water. When malnourished, they will shrink in size, and may lose feeding arms. If these signs are noticed, increase the number of feedings, and target the cucumber with the appropriate food.


The scallop usually found for home aquariums is usually called the Flame or Red Flame Scallop and has a bright red mantel with red or white tentacles.  The Red Flame Scallop builds a small nest of small rocks and pieces of coral in which it embeds itself. In an aquarium, this may not offer much protection. When threatened, it may escape by clapping its valves together, propelling it through the water, with assistance from its tentacles. It tolerates all animals that do not try to eat it, and can be kept in groups. It is a filter feeder, and requires floating micro-plankton to be kept alive in the aquarium, and will do best in a mature reef aquarium system. It also needs proper calcium levels and alkalinity.

The flame scallop is non-photosynthetic and requires supplementation with phytoplankton provided by using a pipette to dispense the food upstream of its location.  Microfauna in the aquarium will also help keep them healthy.  They can be a challenge to keep healthy in the home aquarium over the long term.


The Tridacna genus has many different species of clams that have become very popular among marine aquarists. The most distinguishing feature of clams is their large two-part shell and interior mantle. Most Tridacna clams have intricately colored patterns in the mantle, making them attractive additions to the marine aquarium.

Tridacna clams can get quite large in nature and have a broad range in the wild. They occupy outer reef habitats on both sandy and hard packed substrates, and can be found either solitary or in small aggregations.  Although they can be found in a variety of colorations and patterns in the wild and for sale in the aquarium trade.

They prefer to be placed in the sand or in a recess of a roc. In the home aquarium, they require moderate to intense lighting to thrive as they contain the symbiotic algae called zooxanthellae, and receive the majority of their nutrition from the light through photosynthesis.  Care should be taken to adapt them to the lighting. It’s best to set them on a small rock or on the substrate at the bottom of the aquarium. Over time, the clam can then be slowly moved up higher in the aquarium. When in a good environment, smaller clams can double or triple their size within a year.  Clams are also filter feeders and constantly filter the water for small particulates.  They can be fed micro nutrients such as phytoplankton if maintained in a microfauna poor reef aquarium.  So good lighting, good water flow, microfauna or supplemental feedings, good calcium levels, and maintaining other correct tank parameters will be essential for keeping Tridacna clams


Sponges are animals of the phylum Porifera (meaning “pore bearer”). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them. Sometimes a sponge will be offered for sale to the home aquarist.  Sponges are found in a wide variety of colors such as red, yellow, and orange and a few different forms.  They require a strong current and good water conditions.  They can never be exposed to the air. If it is, air becomes trapped in the matrix of channels that line the inside of its body. With air blocking the path for planktonic food to reach its cells, it will basically starve.  Other than that sponges are pretty hardy and difficult to kill.  If they are not in a tank with active microfauna then sponges will require supplemental feeding with liquid plankton and other dissolved organic foods.


Many Anemones will host a clownfish just like in that famous movie!  However, the commonly found Haitian Anemone and Condy Anemone will not.  You also have to get a species of clownfish that is likely to accept an anemone as it’s home.  These include Amphiprion clarkii, A. perideraion, A. akindynos, A. rubrocinctus, A.barberi, A. bicinctus, A. ocellaris, and A. percula.  Anemones can live singly or in groups in the ocean but typically you will only want one in the tank.  This is because they can and will move around and their tentacles can harm corals or other anemones.  However, once they find a place they like they will usually stay there.

Anemones require a tank with strong lighting and strong water movement. The aquarium should have a variety of sandy and rocky locations as this animal oftentimes can move about and seek refuge in a place it prefers. They can attach to a rock or bury its foot into the sandy bottom at the base of a rock, and it will oftentimes attach itself to the bottom of the aquarium glass.

Some Anemones can eat fish in the aquarium and it is best that they be regularly fed small pieces of cut up shrimp, scallops, or other seafood.  This and hosting a clownfish will reduce the possibility of a fish becoming eaten.  When attached, it is very difficult to convince it to let go without damaging the animal. Some anemones can cause a severe reaction in humans, especially when it comes in contact with areas of the skin that are more sensitive like the underside of the arms, or back of the hand.  When in a healthy environment they can even split into two!

Cephalopods and other invertebrates

Sometimes other invertebrates are offered for sale to marine aquarists such as Octopus or Sea Slugs.  An Octopus requires a very specific aquarium set up in a single species tank and so is only recommended for the absolute most dedicated and advanced aquarists.  Sea Slugs are easily sucked into intakes and need algae supplements to survive.  It’s best to leave anything too exotic to the real experts.