Q&A: Reef Aquaria

by J. Charles Delbeek, M.Sc.

Well here it is reefkeepers, a column devoted to our passion; reef
aquariums! In this monthly column I shall be answering questions
dealing with the inhabitants and the systems which, when combined
together, we commonly call miniature reef aquariums. I also hope to
provide you with the latest information dealing with the animals we
keep, the systems we keep them in and the many different ways we can
keep it all working. But first I have been asked to tell you a little
bit about myself.

I first began keeping marine fish when I was ten years old. My father
introduced me to the aquarium hobby much earlier but it was not until
I was ten that we started to keep marine organisms. I began to
snorkel at the age of five and by the time I was twelve, with mason
jar in hand, I was collecting baby angels, damsels and butterflyfish
around the docks and rock jetties of the Florida Keys during my
summer holidays. I would keep them in a five gallon tank with an
air-driven external filter with some filter floss and activated
carbon. The tank was bare bottomed but I would add small live rocks
and clumps of Halimeda algae. I could keep the fish like this
for weeks with only small freshwater top-offs. At the end of my
vacation I would then either let them go or bring them back home with
me. When I turned fourteen I enrolled in the only SCUBA course in
Toronto that would take me at that age, and a whole new world opened
up. I began taking underwater photographs at sixteen and progressed
slowly from those totally blue shots with little specs in the
distance that I insisted were fish during slide shows to my parents.
Sitting there patiently, my mother would smile and nod, and always
say "Oohhh!" and "Ahhhh!" at the appropriate times; my father would
just chuckle at all the inappropriate times!

I went on to study zoology at the University of Toronto obtaining my
undergraduate, graduate degrees, and a bachelors in education. After
leaving university I taught high-school science for eight years. Last
year I decided on a bit of a change and applied, and was hired, for a
position at the Waikiki Aquarium in Honolulu, Hawaii where I am
currently working as an aquarium biologist.

During the late 80's I began to experiment with reef aquariums using
live rock and corals as the basis of an ecosystem approach to
aquarium keeping. After meeting George Smit in 1986, he encouraged me
to begin writing articles about what I was doing and what I had
learned. This has lead to over 40 articles in various aquarium
magazines over the last 10 years, including an eight part series in
AFM beginning in early 1989. In 1991, my friend and fellow reef
keeper Julian Sprung and I, began to work on our first book together.
With the skills of our friend and partner, Daniel Ramiriez, we
published volume one of The Reef Aquarium, in 1994; we are
currently completing work on volume 2.

"Eeek! What's That Crab Doing in My Coral???"

Many times I have been asked just this question, perhaps not with the
same level of alarm, but definitely with some concern. When branching
stony corals are imported it is not uncommon for them to contain
numerous little hitchhikers such as shrimp, worms, sponges, coralline
algae, tunicates, and crabs. Some of these passengers can be
potentially harmful, others are not, and some we just don't know what
they do so most people err on the side of caution and remove them
anyway. This is the case with some of  the crabs we find in branching
corals such as Acropora,  Pocillopora and
Stylophora. Crabs belonging to the genus Trapezia in
Pocillopora, and Tetralia and Domecia in
Acropora, were at one time thought to be ectoparasites, living
off of the tissues of the coral. Later it was discovered that these
little crabs actually protected the coral in which they lived from
predators such as the crown-of-thorns seastar (Acanthaster
planci). When the seastar begins to climb over a branching coral,
these little crabs move to intercept it, much like the interceptors
in the Babylon 5 TV show move out to intercept an attacking space
craft! They attack the seastar by nipping at it's delicate tube feet,
driving it rapidly from the coral. The benefits to the crabs were
less easily ascertained and it appeared to be simply that the coral
offered shelter to the crabs in exchange for defense from predators.
Others speculated that the crabs were feeding on the carbon and lipid
(fats and oils) rich mucus produced in excess by the corals. Several
studies have also shown that corals contain large amounts of lipids
in their tissues, in fact 30 to 40% of a coral's dry tissue mass is
lipid. The majority of these lipids are passed from the symbiotic
algae in the coral to the coral's tissues, with the rest being
manufactured by the coral from metabolites of the algae. What these
lipids were being used for was a mystery. Most of it was used for the
formation of reproductive cells, but there was still significant
excess available. One interesting observation was that coral polyps
on branches lower in the coral head had a greater vitality when the
crabs were present than if  they were absent.

In studies of the ubiquitous Hawaiian branching stony coral
Pocillopora damicornis, Dr. John Stimson, of the University of
Hawaii, found that the presence of Trapezia sp. crabs,
actually stimulated the coral to produce fat bodies, which were
clearly visible as small, whitish globs in the tentacles of the
polyps. These were then fed upon by the crabs. How this occurs is not
clear. They could be eating the tentacles directly, they could
stimulate the polyp in some way to cause it to release the fat
bodies, or the fat may be exuded in coral mucus. When the crabs are
removed, the fat bodies get larger for about a week, but then grow
smaller. Dr. Stimson has recently discovered that if the crabs are
fed, they readily release ammonia, but if they are not fed, they
don't. This raises the possibility that when corals feed their crabs
lipids, the crabs become living fertilizers, moving amongst the
branches, releasing nitrogen-rich ammonia to the zooxanthellae. This
may explain why polyps in the lower branches retain high vitality
rates despite being exposed to less light. When crabs are removed,
the lower branch polyps often die-off, and the lower branches become
covered with algae, sponges and tunicates. It has also been
speculated that the movements of the crabs lower down in the colony
increase water circulation and prevents detritus from accumulating.
They may also stimulate an increase in mucus production by the coral,
further contributing to keeping the branches clean.

This example of mutualism, is not unlike that exhibited by the acacia
tree of Africa and a species of ant that lives amongst it's branches.
The thorns of the acacia are hollow and the ants use these as nesting
sites. The tree produces small swellings along its branches, which
are sites of sap release; rich in nutrients that the ants feed upon.
In turn, the ants protect the tree from predators by dropping down on
some unsuspecting antelope who is eating the leaves of the acacia,
and biting it. Now one ant is not much of a problem you would say,
but what about 1000's of little pinchers running all over your body?
That has got to be uncomfortable!

The findings of the activities of these crabs and their benefit to
the coral, calls into the question the relationships that similar
crabs have with soft corals. Colonies of Xenia are often
imported with commensal crabs, perfectly colored to match the coral.
Could these crabs also be feeding on coral mucus or lipids? Could
they also be beneficial fertilizers for the coral? Most observations
of these crabs indicate that they do indeed consume the tentacles but
it could be that Xenia tentacles might be retracting for other
reasons, such as reduced light levels. There are many aquarists who
have maintained these corals successfully with their crab symbionts
present but here are also those who have reported losses of their
colonies due, they believe, to these crabs.

In any event, it seems that Trapezia and other commensal crabs
found in the branches of stony corals offer definite benefits to the
coral. The next time you are lucky enough to acquire a coral colony
with one or more of these beneficial crabs present, please do not
recoil in horror, but rather leave them in place and your coral will
thank you.

Well that about covers it for now. Next month I'll address some of
the numerous questions that have already appeared in my mailbox. If
you have any reef aquarium related questions please don't hesitate to
write me care of Aquarium Fish Magazine!