There are certain invertebrates in the groups discussed that are readily available to aquarists but that are not well-suited for the reef aquarium. Here is a quick survey of some species to avoid.
There are snails that are readily available to hobbyists that are collected from temperate seas — that is, from cooler water temperatures than most of us keep in our reef tanks. These animals will not thrive at tropical water temperatures. While they may live for a while, their lives will be shortened as a result of the suboptimum conditions. Species in the genera Margarites, Norrisia and Tegula are best avoided.
While most gastropods look benign, it is important that you know about their feeding habits before plunking one into your tank. There are many species that are highly predatory (some will even eat your desirable algae-eating snails). Some hobbyists also fall for the cowries (family Cypraeidae). This is easy to do because their shells are often attractively marked, and their mantles can be adorned with ornate appendages. Only some cowries eat algae — most that enter the aquarium trade feed on sessile invertebrates, such as sponges and corals.
Even though crabs discussed in the accompanying article may provide a valuable service in the reef tank, most crustaceans are way too destructive to include in a community tank. There are many larger hermit crabs (Dardanus spp.), for example, that will not only eat other invertebrates but will hunt fish as they sleep at night. Many of the true crabs are also too predatory to introduce to the reef tank.
There are only a few algae-controlling urchins that are suitable for the home aquarium. Most sea urchins should be avoided; for example, many members of the genus Diadema get too large and will knock over sessile invertebrates that are not securely attached to the substrate. These urchins also ingest coralline algae and have long, sharp spines that will cause painful puncture wounds if the aquarist should accidently contact them. There is another group of urchins, the flower urchins (Toxopneustes) that is highly venomous. These species are best avoided because of the threat they pose to the aquarist.