In the last article we concentrated on two of the Classes
within this group that are referred to as “starfish” the Asteroidea and the Ophiuroidea (true starfish and brittle starfish).
Of the remaining
the aquarist is likely to experience only two of them, and these will
be focused upon here.
Class Echinoidea - Sea Urchins
One of the most unmistakeable groups of marine invertebrates the echinoids,
or sea urchins, are a very successful group numbering some 900 described
species which have colonised all of the worlds oceans from shallow
waters to several miles depth. The typical body plan is that of a hard
exterior, usually spherical or ovoid, often covered in spines, which
protects the soft internal organs of the animal. Despite the fact that
most of the individuals of this echinoderm Class experienced by the
aquarist will follow this loose body plan it is worth mentioning some
departures from this.
In U.K. coastal areas dominated by sandy beaches we can locate the
aptly named “sea potato” (Echinocardium caudatum). This
yellowish urchin lives life completely buried in sand, sometimes to
a depth of several feet. That this species is present is sometimes
only obvious to the casual observer, as beaches will often have their
exoskeletons or tests washed up on them. Many visitors to Florida will
be aware of the sand dollar test, which is often seen for sale in souvenir
shops. This is another sand dwelling Echinoid, which is specialised
for an existence on top of sandy substrates. Due to their flattened
shape sand dollars and their relatives are referred to as irregular
The spines of regular sea urchins are calcareous extensions of the
test which are able to move with a simple ball and socket joint at
their base, thus they are able to act as the main source of “rapid” locomotion
in many urchin species or wave around menacingly in the direction of
a potential predator (or aquarist trying to remove one from an aquarium).
However, due to specialised tendons in the socket joint, the spine
can be made totally rigid which is an essential defence adaptation.
Projecting from the test are also specialised structures called pedicellariae
and the tube feet both powered by the water vascular system unique
to echinoderms. Pedicellaria are organs that resemble tube feet with
pincer-like endings. They are used primarily to clean the surface of
the animal although they also have a secondary role in protecting the
urchin from small predators to whom the defences offered by the spines
are no problem to breach.
Given the relative abundance of sea urchins in natural reefs it is
unsurprising that specimens make their way into the hobby by accident.
However, live rock is unlikely to be the origin of these animals
in our aquaria as most will be too sensitive to survive the shipping
and curing processes. Live rock from Vanuatu (Pacific storm rock/
South Pacific Rock) is particularly rich in the dead tests of urchins.
Some species do make it as accidental arrivals on the base rock associated
with soft corals and polyps. The commonest of these imports seems
to be Echinometra spp., most of which that we are likely to encounter
are characterised by a white disc surrounding the base of each spine.
These species are perhaps the only ones we may experience as an accidental
introduction on what could be called a “regular” basis.
Superficially they resemble the long-spined sea urchins Diadema spp
but the latter have much longer spines and a prominent anal bladder
(this is the large structure on top of the urchin which some aquarists
believe to be its “eye”). In fact this structure surrounds
the anus and is an extension of the intestines. It is certainly true
however, that the majority of regular sea urchins have light sensitive
organs on their dorsal surface.
Echinometra presents no more problems to the aquarist than those
demonstrated by the vast majority of ornamental urchin species that
are readily imported for the aquarium hobby. In nature, shallow water
regular urchin species are grazers of algae. The mouth, which is
located underneath the animal has five separate “teeth” (one
for each sector of the body- remember, all echinoderms exhibit pentamerous
symmetry) which are articulated and supported by small bone-like
structures and specialised muscles called collectively “Aristotle’s
lantern”. These teeth are extremely robust and in many species
can articulate in a number of ways. Their ability to graze algae
from rock is incredible and often the aquarist can determine the
route an urchin has taken by the clean rock left in its wake. This
is great news for the person inundated with nuisance algae, or so
you would think if it wasn’t for the fact that urchins are
a little more cosmopolitan in their tastes than nuisance algae. A
grazing urchin may remove large amounts of calcareous algae that
in most aquaria will not be able to recover quickly enough to withstand
the grazing pressure. The desire of an urchin for calcium carbonate
is such that you can supplement their uptake with pieces of cuttlebone,
which will be grazed to nothing in no time at all.
Many Echinometra species demonstrate their ability to consume calcium
carbonate substrates in their rock-boring activities. Some will excavate
a depression into the rock that they occupy during the day or in times
of low tide when they might be vulnerable to predation. Thus the less
well-protected oral surface of the urchin is firmly in contact with
the substrate and cannot be levered or wafted free by triggerfish or
other predators and the only exposed surface is extremely prickly.
In reef terms the grazing of sea urchins is extremely important in
the production of sand - estimates of up to 90% of all the erosion
of coral reefs caused by animals, plants and bacteria in certain
parts of the world may be due to urchins.
So, are the little Echinometra urchins suitable for marine aquaria?
Well, it depends on your feelings about calcareous algae. If you like
the latter forget about obtaining an urchin-they are just too destructive
to this beautiful alga. In their place, however, they make interesting
and hardy introductions to marine aquaria. Urchins are clumsy creatures
too - they can knock over unsecured sessile invertebrates and even “pick
up” polyps and rubble that is an adaptation to concealment in
some of the urchins.
If you have an urchin you might want to have a look and check for
the presence of commensal or symbiotic creatures on the body and in
between spines. There are several small bivalve species that can be
found around the underside of the urchin. Recently there have been
some small, elongated shrimp available in the trade that live on the
spines themselves. It is always possible to find the seemingly ubiquitous
bristleworm family on urchins too, but they are unlikely to be harmful
to their host in most cases.