Not only are some marine inverts difficult to keep, some can be downright dangerous! Scott W. Michael lists a few that should remain in the oceans.
Sea slug
The sea slugs, the nudibranchs for example, come in a myriad of chromatic hues. Yet while you may be tempted to add one to your nano reef tank, I would discourage you.
Many are naturally short-lived, some living less than a year in the wild. Most have specialised diets that include sessile invertebrates, like sponges, hydroids, bryozoans and tunicates. Unless able to provide these food sources in the aquarium, these creatures will live an even more abbreviated life.
Also beware that there are species in at least one genus (Phyllidia - pictured above) that exude a fish-killing toxin.
Strawberry/Cauliflower coral
The soft corals in the genera Dendronephthya and Scleronephthya (family Nephtheidae) which are usually referred to in the aquarium trade as Strawberry, Cauliflower or Tree corals, are some of the most striking corals on the Indo-Pacific reefs.
They tend to occur under overhangs or in caves, always in habitats exposed to strong pulses of current.
While coveted by reef aquarists, their special dietary requirements have thwarted attempts to keep them by all but the most dedicated. Their preferred food is phytoplankton.
In reef tanks they typically remain flaccid, as they do in the wild when currents are not flowing, and rarely live for more than a month or two.
Feather star (crinoids)
The crinoids, or Feather stars, are the Dendronephthya of the echinoderm world. They would make striking display organisms but their specialised diets hamper success in the home aquarium.
Their general feeding strategy consists of capturing and transferring tiny plankton, such as phytoplankton, invertebrate larvae and detrital particulates through ciliated channels in their arms to their mouth.
What makes matters worse in the aquarium, their arms often drop off due to stress or mechanical damage incurred during collecting and shipping. When losing an arm they lose part of their feeding 'machinery' — making it even more difficult to get enough nutrients.
Venomous sea urchin
The flower urchins (Toxopneustes sp.) are lovely echinoderms that have a large, tulip-like structure on a long stalk, known as pedicellariae.
Associated with this structure is a venom gland. If a person makes contact with the pedicellariae it can clamp shut like jaws and inject venom. The result is terrible pain and possible death.
While fairly well behaved as far as invertebrate tank mates are concerned, the potential for a dangerous 'sting' is not worth the risk. They often hide among rockwork and so accidental contact when working in the tank is highly likely!
The other venomous urchins to be avoided belong to the genus Astheonosoma and called Fire urchins because of the pain that results from their sting. These animals often exhibit beautiful colours and host various small commensal shrimps.
Carpet sea anemone
Many want to set up the amazing symbiotic association that occurs between sea anemones and clownfish in our aquariums. However, not all of the host anemones do well in captivity.
The carpet sea anemones (Stichodactyla spp.) should be avoided by most hobbyists. Those that available are often colourful and usually bigger than a dinner plate. Most carpet anemones will ship poorly and tend not to acclimatise to the home aquarium.
Large hermit crab
While bulletproof, the large hermit crabs in the genus Dardanus are as welcome in the reef aquarium as a two-year-old in a china shop! They are destructive!
The Dardanus are opportunistic feeders and usually large enough to tear up a variety of tank mates. This can include soft corals, large-polyped stony (LPS) corals, echinoderms and even sleeping fish.
These crabs will also knock over unsecure pieces of rock or coral colonies. This often occurs at night when they are usually most active.
Sponges
Many sponges that are available to hobbyists do poorly in captivity. Most are filter feeders that will extract tiny plankton and bacterial aggregates from the surrounding water column. They feed most of the time and are unlikely to get enough to eat in the home aquarium, even if particulate foods of the right size are occasionally introduced.
Another potential sponge killer is exposure to the air, which can be lethal if air bubbles get trapped in the many tiny channels that permeate their bodies.
Blue-ringed octopus
You see these less frequently in the trade nowadays and they have no place in the home aquarium. The blue-ringed octopus species most often encountered by aquarists is Hapalochlaena lunulata.
Blue-rings should not be imported because they are difficult to keep, but because they are deadly! They are responsible for the deaths of several humans and it has been reported that just one has enough venom to kill 26 adults. So, why take the risk and keep one of these virulent little cephalopods in your home aquarium?
See news story Octopus hospitalises 85 people.
Sea apple
Most sea cucumbers are cryptic, hiding under the sand or live under rocks and rubble. But the sea apples (genus Pseudocolochirus spp.) are conspicuous, colourful sea cucumbers that many reef aquarists adore.
However, they produce toxic compounds (holothurin) and, if they die, may exude this into the aquarium and quickly kill fish tank mates. It may also kill some invertebrates.
Sea apples also tend to starve to death, unless you make a concerted effort to target feed them foods for suspension feeders — for example, liquid or dried phytoplankton daily.
Green brittlestar
This particular echinoderm (Ophiarachna incrassata) is easy to keep and can be a welcome resident. However, it is a proficient predator that will eat fish tank mates and in time will usually capture goby neighbours.
This animal employs a unique ambush feeding strategy. It will lift its disc above the bottom and remain motionless. When a small fish swims under, possibly seeking shelter, it will quickly turn its body, so its legs are twisted in a spiral and imprison the fish. This is then easily consumed by the crafty echinoderm.
That said, it does have a place in an aquarium with larger fish.
This item first appeared in the June 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.