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Decapoda Crustaceans
by Jim McNulty

Somebody Salt Me
September 1999 newsletter - The Tropical Fish Society of R.I.

Hey Fishheads,

Welcome to the September installment of Sombody Salt Me! This month, I thought we would take a look at the order Decapoda. This is the largest order of crustaceans sporting over 8000 species. Members of this order are mostly marine and can be identified by having the head and thorax fused into a cephalothorax. From here, they are further divided into two sub-orders: the Natantia (shrimp and prawns), and the Raptantia, (lobsters and crabs). The Natantia are the true swimmers, using the pleopods, (tiny, feathery swimmerets found on their abdomen. The Reptantia are geared more for crawling than swimming. They are more adapted for using their legs. The crustaceans are among the most abundant creatures in the sea. They belong to the phylum Arthropoda (meaning joint footed). They all have an organic exoskeleton that they utilize against predation. Terrestrial Anthropods are refereed to as insects and number around 750,000 species. Crustaceans of interest to the marine aquarists are the crabs, shrimps, lobsters, barnacles and copepods. Today we will explore some of the shrimps.

Yes, while many of the crustaceans are predatory, there are large assortments that are appropriate for the aquarist to keep in their reef tanks. My personal reef is home to four species of shrimp and 2 species of crabs. They all have their own little niche and co-exist quite peacefully. Just like fish, the shrimps all share a similar anatomy with each other while looking and acting completely different. Most become accustomed to their environment and keepers and can be hand fed. While they do perform a vital function of scavenging the uneaten food in the tank, they should not be depended on for this. They require a varied diet just as you would provide for your fishes. If provided with good water quality and a proper diet they will live for many years in the home aquarium.

Because crustaceans have an exoskeleton, they can not grow the same way as fish do. They must shed this outer hard shell and grow a new one. This process is called molting. The molt is called ecdysis and is the same as the one experienced by terrestrial insects. When the time comes, usually at night, the shrimp finds a safe, secluded place on the reef and begins its transformation. A little trap door pops open on its back and the animal pulls itself out of its shell. Once free, it is vulnerable for several days until it's new shell hardens. The discarded shell looks like a dead animal and uninformed hobbyists often think their shrimp has died. Only several days later do they realize what happened when the animal emerges from it's hiding place. Most species experience 4 or 5 molts during their adolescence before they are adult size. In my opinion, a molting shrimp is a happy, well-fed shrimp.

While all the shrimp live and breed quite uniquely, one species has a real trick up its sleeve. The cleaner shrimp, Hippolysmata grabhami from the western Atlantic has the ability to change its sex! They can change from male to fully functioning female over time. As a male, it forms a pouch and fills it with sperm. Once its transformation is complete, it can yolk up some eggs and fertilize them itself. It is simply amazing the way that life refuses to be denied. There are no documented reports to my knowledge of a female becoming a male but who really knows. While the breeding of shrimp can happen, it really isn't that common so don't be discouraged if yours don't.

Compatibility is another important thing to think about. Not just with one another but with the other creatures in your tank. While one can usually keep a large number of cleaners together safely, other species don't tolerate members of their own kind well. Research should be done on any new specimen before obtaining it to be sure it is compatible. I had to laugh one time when I added a large Arrow Crab to my reef. I knew the crab was O.K. to have in a reef tank but never gave any thought to how it would interact with the other residents. Within the first 5 minutes, my two cleaners went over to it to introduce themselves and offer their services. Cleaners use their 4-inch long antenna to feel out the guest and get permission to clean if you will. The Arrow reached over with its blue claw and cut the antenna right off! I almost died right there. Not a very friendly welcoming if you ask me. The Arrow crab also decided that he liked the overhang of the big cave I have in there. Unfortunately, that is also where my Coral Banded shrimp lived. None of my shrimp were any match for the arrow crab. He continues to live in the cave and the others avoid him when at all possible. Now the CB shrimp lives in the back where you only occasionally see him. It kind of makes it a treat when he does come out.

Not all shrimps are acceptable for the reef tank. One species in particular is the Mantis Shrimp (Odontodactylus scyllaris). What a shame too as they are quite magnificent to view. They can grow to about 6 inches and are ferocious invertebrate predators. They wait in their dens for unsuspecting prey to go by. They emerge quite swiftly and grab the prey with 3 sets of legs that have holding claws on them. Their mouth is equipped with a large set of pinching jaws for tearing at their prey. As the name implies, they grab and hold their prey just like a terrestrial Praying Mantis. Fish, crabs and snails up to 5 inches are considered fair game. They also have been known to tear at some corals and calurpa during molting for camouflage. While they are truly magnificent creatures, they have no place in the reef tank. Unfortunately, they also have a nasty habit of boring right into liverock and often come in with a fresh supply. Unsuspecting aquarists buy the rock and introduce them unknowingly. If you are experiencing unexplained deaths or hear a loud clicking sound coming from your tank, you probably have one. They can usually be easily trapped if baited with food.

Shrimps and crabs are just another amazing facet of the marine aquarium hobby. It seems as though the wonders of the ocean and the parade of facenateing reef creatures will never end. My only regret thus far is that I didn't discover it sooner! Please join me next month as we continue our exploration of the invertebrate aquarium.

Jim Mcnulty

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