I have never found a crab yet that has
hitchhiked into my tank that I would consider reef safe. The only crabs
that I consider to be truly reef safe are the commensal types such as
the porcelain crab and coral crabs. The only reason such
crabs can be considered reef safe is that they never leave their
hosting spot. The hermit crabs that are intentionally introduced to our
tanks are not to be trusted or considered reef safe either, all crabs
are omnivores and will eat what ever they can. While most corals are
safe from hermit types, other inverts and fleshy polyp corals are not.
The Xanthidae Crabs
(link - extensive photo gallery) None of which can be
considered reef safe and can be the most destructive of the crab types.
While very small, they seem to pick at algae on the rocks, but just as
with other crabs species, once they gain a bit of size, their ability
to pick off and eat any number of invertebrates and corals also
increases. They also have a bad habit of enlarging their hiding places
within live rock and will, in due time, weaken the rock considerably.
Below, a commonly found Xanthid species living
within live rock.
Below, yet another rock living Xanthid species.
Below is a Xanthid species found on Acropora corals and
assume they prey upon the corals polyps or tissue.
Various other Xanthid species.
The Hermit Crabs
Paguritta Hermit Crab
( Cling Crabs) None of which I would consider to
safe, members of this family include the popular "emerald" crabs that
are used for algae control, which may be fine for awhile, but with all
crabs being omnivores, it is usually only a matter of time before these
crabs discover that there are meatier items to be had within your
Crabs Photo Link #2 Photo Link #3 Can
be considered Reef Aquarium Safe
The Anomuran crabs, part of the
Porcellanidae family, relatives of Petrolisthes and Porcellanella.
Galatheid Crabs -
known as squat lobsters for their appearing very lobster like. All of
the species that I have found on live rock have been very small and
seem to be somewhat harmless, although they may feed upon small sessile
Allogalatheid Crabs -
also known as squat lobsters but only lives on crinoids. Color
patterns usually consists of alternating dark & light colored bands
with white leg tips & claws; the colors always match the host
crinoid. They are klepto-parasites that don't directly harm the
host but steal its food.
in carrying sponges and even zoanthids as a means of camouflage, it
does so by means of its last set of legs which are modified to grasp
onto its captive audience. I have had this specimen drop its sponge and
snip out a square inch section of zoanthids off of their rock and form
the zoanthids into the same shape as the sponge pictured below. I have
seen others on the reef slopes carrying about both sponges and
zoanthids in this manner.
( Swimming Crabs) Note the paddle
shaped last pair of legs.
Below: A Lissocarcinus laevis
, a symbiont of sea anemone.
Below: Various other species
The Majoidea SuperFamily
( spider crabs / decorator crabs)
Below: Xenocarcinus tuberculatus A member of the Pisidae family
Below: Achaeus japonicus
Below: Various other unknown species
The Parthenopidae Family
commonly called the elbow crabs. Below is a possible Daldorfia horrida
Below, possibly a
juvenile Daldorfia glasselli.
The Calappa Crabs
( box crabs )
Ready to hatch eggs
Two day old
The Leucosiidae Crabs
( Pebble Crabs)
Below, a member of the
The Cryptochirid Crabs
( Gall Crabs )
( Pea Crabs) - Pea crabs are commensals
that associate with various echinoderms, molluscs, polychaetes,
brachiopods, etc. Some are external while many of them are
internal living inside their hosts' gill chambers or body cavities.
(photos by Beth LeBlanc)
arrow crabs ) Normally not considered reef aquarium safe
Presented by Charles & Linda Raabe
Mactan Island, The Philippines
© 2009 All Rights Reserved
" Stony corals are the foundation of coral reef ecosystems and form associations
with other reef species. Many of these associations may be ecologically
important and play a role in maintaining the health and diversity of reef
systems, rendering it critical to understand the influence of symbiotic
organisms in mediating responses to perturbation. This study demonstrates the
importance of an association with trapeziid crabs in reducing adverse effects of
sediments deposited on corals. In a field experiment, mortality rates of two
species of branching corals were significantly lowered by the presence of crabs.
All outplanted corals with crabs survived whereas 45-80% of corals without crabs
died within a month. For surviving corals that lacked crabs, growth was slower
and tissue bleaching and sediment load were higher. Laboratory experiments
revealed that corals with crabs shed substantially more of the sediments
deposited on coral surfaces, but also that crabs were most effective at removing
grain sizes that were most damaging to coral tissues. The mechanism underlying
this symbiotic relationship has not been recognized previously, and its role in
maintaining coral health is likely to become even more critical as reefs
worldwide experience increasing sedimentation. "
It is our hope that by documenting the coral and crab species
found as commensals we can help to determine which crab species should
be targeted for breeding efforts. If we are to ever transplant any of
the branching corals out onto the reefs as a restoration project, we
will have to include a variety of coral crab species as well.
Click Links to view photographic details
Jonesius triunguiculatus Tetralia cinctipes Trapezia septata
: Very common and is considered a coral symbiont, with the caveat that such
symbiosis are a trade off between the coral and the crab, the coral gains
protection and house cleaning services while giving up a few polyps, mucus and
captured/settled food particles in exhange for those services. A healthy coral
should have no problem repairing or replacing lost polyps, but again, in an
aquarium environment, an eye should be kept on the coral for excessive damage
being done due to the coral being unable to recover as fast or faster than the
damage being done by the crab. In short, its a judgement call that you will
have to make.
Four New Species of Coral Crabs Discovered
Trapeziid Crabs of New Caledonia, Eastern Australia and the Coral Sea
Identifications made by:
Peter Castro, Ph.D.
Curator of Fish and Invertebrates Biological
Aquarium of the Pacific in Long Beach Ca. California
State Polytechnic University
Download a .pdf version of the publication shown below
Every Crab in the World
(click above link)
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since 24 Jan.08