Crabs

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jhnrb

Reef enthusiast
Criteria for Suitability

So-Called "Safe" versus "Unsafe"

The description of any reef animal as "safe" or "unsafe" must be accepted as an arbitrary assignment. Such valuations can still have merit and are offered to guide aquarists to make successful choices. However, inevitably all reef animals must eat something else on a reef and in the strictest definition are not "safe" species here in realtive scope with their likelihood to eat other popular and commonly kept fishes and invertebrates. For Species that can be identified accurately, we generally have good to very excellent information about their behavior and husbandry. Rest assured, however, that reliable deductions of such information are also possible with unidentified species by careful observation of physiological characteristics. We can tell a lot about a crab's behaviour by studying the form of key features and appendages. Form almost certainly follows function, as they say. As such, a specimen with large, sturdy pincers like a Calappa species, the Box or Shame-faced crabs, is likely to use them for similar purpose; chipping and crushing pieces of thick mollusk shells to reach living snails, hermit crabs and bivalves (clams and the like). The large and powerful claws indicate a formidable predator not to be trusted in a tank with mixed invertebrates. In suit, an unknown species of crab with feathery appendages (maxillipeds) like the porcelain crab species (Neopetrolisthes and Petrolisthes) is certainly some kind of filter feeder and likely to be quite safe with most other fishes and invertebrates. Diminished claws and filter feeding appendages indicate a very compatible crab in mixed species displays. In between these two extremes of "safe" and "unsafe", most crabs fall. It is no surprise then as most are very opportunistic feeders, that this large group of crabs in the middle has mediated appendages that serve a true omnivore well. A sleuthing aquarist will evaluate a new crab carefully and can be assure that larger claws are for crushing (prey capture), and smaller claws are for cutting (algae and invertebrate grazing). Thus armed with this infromation you can make an informed decision with reasonably good confidence as to the suitablility of a crab for your marine aquarium.

Excert from Reef Invertebrates by Anthony Calfo and Robert Fenner.
 
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Hairy Versus Smooth Carapace Legend

In popular aquarium lore, a legend exists about crabs with hairy carapaces. Although it often proves to be true, there are more reliable methods of evaluating suitability. It is said that an unidentified crab species can be assessed as "reef-safe" by the texture of its carapace (back/body). Hairy crabs are believed by some to be categorically dangerous while smooth shelled species are thought "safe" with mixed invertebrates. The truth of the matter is that such distinctions are neither practical nor reliable. There are some notorious hairy bandits that have made their way into reef aquaria and wrought havoc like the Dardanus species, however, quite a few crabs without hairy carapaces or appendages can be equally destructive and predatory like the box or shame-faced crabs. These Calappa species (bald as a pool cue ball) seem like they will eat anything including volkswagens given the chance to (and usuing their powerful claws like a can opener!). As a rule, the shape (form) and function of a crab's claws are a better indication of likely behavior as described above (crushing versus cutting pincers). Generally, your best bets for safe selections are species that get no larger than a coin as adults (less than 3 cm) and have diminished princers. Smaller species pose less risk to fishes and a modest risk to invertebrates. Beware however, of hearsay opinion regarding "average" adult sizes of popular crabs. Commonly imported at 1-2" (2.5-5cm) in diameter, Mithraculus (Mithrax) spinosissimus, The red channel crab, grow to nearly 6" (15cm) in diameter and are quite aggressive to fishes.

Excert from Reef Invertebrates by Anthony Calfo and Robert Fenner.
 
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