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Odd Pods, Tanaids
in Reef Aquaria
Most aquarists are familiar with
some of the crustaceans found in their aquaria. Other than
the shrimp or crabs that are added to aquaria by the reef
hobbyist, a number of these bugs seem to show up more-or-less
by themselves. Although they actually migrate in on live sand
or live rock or, on decorative animals such as corals, their
appearance is largely unpredictable.
Often the first of these to be seen in
reef aquaria are the harpacticoid copepods. These are minute
crustaceans often visible only as tiny white dots moving on
the aquarium walls. Sometimes they also form swarms in the
water, but as these animals are more-or-less natural fish
food, such aggregations are very rare in tanks with fish in
them.
There are three groups of larger crustaceans
that commonly have representatives found in our aquaria. These
are the Mysidacea; also known as "the possum shrimps" or "mysids,"
the Amphipoda; also known as "scuds," "side-swimmers," or
"amphipods" and the Isopoda; also known as "pill bugs," "rolly-pollies,"
or "isopods."
These latter three groups have a lot in
common and are actually probably closely related. Taxonomists
put them all in the group called the "Superorder" Peracarida,
in the great assemblage of animals referred to as the Crustacean
Class Malacostraca. Taxonomic "Classes" are groupings that
are subdivisions of larger units, each called a phylum. A
phylum is easiest conceptualized as a basic and unique way
of making an animal, or put another way, "a body plan." Classes
are subdivisions of that body plan that are coherent unto
themselves. As an example, all birds can be grouped into the
Class Aves of the Phylum Chordata. Other groups of Chordate
species at the Class level include Mammalia, the mammals,
and Reptilia, the reptiles. Likewise there are many other
Crustacean classes, such as the Class Cirripedia or the barnacles,
and the Class Copepoda or the copepods. However the largest
crustacean class, and really the most diverse crustacean group
in forms of body structure, is the Class Malacostraca.
About this time in any discussion of crustaceans,
as somebody who tries in my feeble way to convey information,
I feel the beginnings of dread. Trying to discuss crustaceans,
"marine bugs," is hard. It is hard not because they have too
few characters for easy identification but because, in a very
real sense, they have too many characters, and additionally
they are often small and unfamiliar.
Crustaceans have a hard exoskeleton, and
it has a lot of architecture. And EVERY little tiny
bit of it has a name... And NONE of those names are
familiar to anybody other than a specialist in the given group.
So, if I am going to discuss some small bug found in our tank,
I have to have to try to make sense of a lot of nomenclature
for you. This is not an easy task. I can't simply ignore all
these names and terms. While there is a LOT of what appears
to be "useless" terminology, some of it actually conveys useful
information. So bear with me here, things will get worse as
I introduce a lot of terms, but then they should get better
as I pass into the discussion of the particular small bugs
in question, the tanaids.
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Figure 1.
A male tanaid crustacean crawling on the sediment surface.
This is an individual of the NE Pacific species, Leptochelia
savignyi, photographed in nature in Barkeley
Sound, British Columbia, Canada. The animal is about
5 mm (1/5 inch) long. The large claws characteristic
of male tanaids are evident. The whole animal is visible
the pair of light dots at the lower left image edge
are the terminal ganglia of the animal's nervous system.
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Malacostracans, Shrimps and Similar Animals
The variation within this large Crustacean
group, the Class Malacostraca, notwithstanding, it does form
a real cohesive animal group, readily defined and easily characterized.
A bit of background is necessary first, though. Arthropods
such as Crustaceans are one of four major animal groups in
which the animals are composed of serially replicated body
units called "segments." This replication of segments is probably
best seen in something like a millipede, but all arthropods
are composed of segments. Each segment has a pair of appendages,
which may or may not be like those appendages on the segment
in front or in back of it. The sequences and groupings of
the segments and the modifications seen in them and the appendages
are the major characters used to distinguish one small buglet
from another. And each and every one of these little bloody
whiskers on the side of each bug has a name. Not only that
each section of each leg has a name. And often each part of
each section has a name or term that describes it. So... you
have to look past the fog of names to see the patterns that
define the animals.
Malacostracan crustaceans have a
body divided into a head region, a middle area called a thorax
and rear region called the abdomen. A carapace or
shell generally covers the head region and thorax (this is
a bit of an "iffy" character; as we will see, many malacostracans
don't have this). The thorax consists of eight segments, with
male genital opening on the eighth segment, and the female
opening on the sixth segment. Additionally, thoracic limbs,
often called "walking legs" are very different in shape from
the paddle-like appendages of the abdominal region. The abdomen
generally consists of six appendage bearing segments, and
ends in a tail fan. Finally, most of the animals called Malacostracans
have compound eyes, often on stalks. When all of these above
characters are assembled in their most generalized aspects,
the resulting "animal" is a shrimp-like animal. So malacostracans,
are the shrimps and their kin.
The Peracarids, The "Unshrimplike" Shrimps
However, several groups of these "shrimps"
are decidedly unshrimp-like. These groups, taken together,
are placed in yet another taxonomic group, referred to as
the "SUPERORDER" PERACARIDA, (pera = "near or about",
carida = "shrimps"). These are the animals similar
to, but not quite like, shrimps. This very large group combines
several forms of very different appearances, for example,
the relatively shrimp-like pelagic mysids and the decidedly
not-shrimp-like, flattened, benthonic isopods. That the Peracarida
is a successful group is evident in its species richness.
There are many thousands of species in the group, and interestingly
enough, aside from the rather ubiquitous copepods, crabs and
shrimp, virtually all the crustaceans found in marine aquaria
are peracarids; and most of them get there uninvited.
Peracarids together share some rather interesting
characteristics. Generally, they are small animals; if you
wish to examine a peracarid, you will need a good magnifying
glass or hand lens. A stereo-microscope, sometimes called
a dissection microscope, will work even better. Being good
Malalcostracans, the peracarid body is divided into three
regions, a head, thorax, and abdomen. However, the degree
to which these regions may be differentiated varies significantly
between the groups. When viewed from above, the head, bearing
compound eyes, is generally noticeable at the front end of
the animal and is followed by the thorax. Differentiation
between the thorax and the abdomen is often difficult, when
viewed from above, as these regions often seem to grade into
one another. However, when viewed from the underside, the
different shapes of the appendages on the thorax and abdomen
makes the determination easy.
Many Peracarida have a carapace, but it
is small and may be hard to see, particularly on a smaller
animal. A carapace is a fusion and extension of the exoskeleton
of the head area and the first one or two thoracic segments
to form a "shield like" plate that surrounds and covers the
top and sides of the body. In the crabs and true shrimp, the
carapace covers the entire head and thorax region, and the
segments of those areas are visible only from below. That
is not the case with the Peracarids; at best, the carapace
involves only the head and first couple of thoracic segments
and the remaining thoracic segments are visible from above
and the sides. In amphipods and isopods, the carapace is lacking
altogether.
No free-living peracarid has a swimming
larva. In fact, they have no larval development at all. The
larva characteristic of the crustaceans, the nauplius,
exists only as a stage in the prehatching embryology of these
animals. Instead of a larva, they undergo a complex embryology
to develop directly into small juveniles. The females possess
a brood-pouch-like structure, termed the "marsupium."
This is found on the underside of the thorax, and the fertilized
eggs are deposited into it. Additionally and not surprisingly,
the females often have specialized behaviors involved with
care of the developing embryos. Generally, they have a small
number of eggs in each brood, 10 to 30, depending on the species.
The Peracarida may be thought of as being
composed of several distinct, probably closely related, groups
of similar species. The taxonomic hierarchy progressively
includes relatively similar species together. Several similar
species may be grouped into a genus. Several similar genera
(the plural of genus) may be included into a larger group
called a family. Several similar families may be placed into
a still larger group called an order. Sometimes several very
similar and obviously closely related orders are consider
to form a "Superorder." The Peracarida is
such a superorder, it encompasses all of the species within
seven orders. The three largest of these groups, the ORDERS
MYSIDACEA, ISOPODA, and AMPHIPODA have many species, and at
least a few species from each one of these orders are commonly
found in reef aquaria. Two more groupings, the ORDERS TANAIDACEA
and CUMACEA, each have fewer members, and species from them
are found in aquaria less commonly than the "big three." Finally,
for bookkeeping, there are two groups from specialized habitats
each containing very few species, the ORDER THERMOSBAENACEA
(found in hot, brackish waters) and ORDER SPELAEOGRIPHACEA
(found in one cave in South Africa), which will never be seen
in reef aquaria.
The Tanaidaceans or Tanaids
Tanaids (pronounced "ta-nay-ids") are small
animals; while the largest get to about 20 mm (3/4 inch) long,
most are about one fourth that size. Their bodies are what
morphologists would call "sub-cylindrical," that is, their
basic cylindrical body is flattened a bit from top to bottom.
The head and first two thoracic segment are covered with a
carapace. In the species found in aquaria, small compound
eyes are present on small bumps on either side of the front
of the head. All crustaceans have two pair of antenna or "feelers"
arising from the front of the animal. Often, however, one
pair is much more evident than the other one. That is the
case here, and often only the large first antennae are evident.
Arising from under the carapace, are a
pair of pincher-bearing appendages, and in some individuals
of most populations these pinchers are positively enormous,
compared to the size of the animal, often being more than
half the animals total body length. These appendages serve
to immediately distinguish tanaids from all other aquarium-dwelling
Peracarida. Mysids and cumaceans do not have pinching or grabbing
appendages at all. Isopods generally do not have grabbing
appendages, but if they do, they are appendages that fold
back onto themselves in a manner similar to a jackknife. Amphipods
have the first two pairs of appendages formed into grasping
or cutting appendages, but they also close like a jackknife.
Only in the tanaids are true pinchers or forceps-like appendages
found.
Six pairs of smaller, and more normal appearing,
walking legs are found on the bottom of the remaining thoracic
segments, and smaller flap-like appendages are found on the
bottom of the abdomen. These latter appendages serve to ventilate
the animal's tube, and in aquaria they move a relatively large
volume of water into the sediment, particularly if the population
of tanaids is relatively dense.
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Figure 2.
Diagrammatic representation of tanaids, the male is
at the top, the female at the bottom. Note the disparity
in the size of the claws (Modified from McLaughlin,
1980).
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The natural history and ecology of tanaids
is exceptionally poorly known. The links at the end of the
article can lead you to a bibliography of every article published
on tanaids published in the last 20 years, and there are only
a few that are not simply species descriptions. Additionally,
ALL of the published works detailing anything at all about
the ecological interactions discuss temperate, polar, or deep
water forms. There is NOTHING at all about
the tropical species. Sigh... we lose again!!
It is evident from some the works on temperate
species, however, that they may be very important ecologically.
Some species have a very bizarre reproductive biology. Leptochelia
savignyi from the North Eastern Pacific (Highsmith,
1984), has been shown to start out life as a sexless neuter,
called a "juvenile". Juveniles have no sexual characteristics
and are neuter. As a juvenile grows, it becomes a female.
At any given time, about 95 percent of the population is composed
of individuals that are either neuter or female. As they molt
and grow, the females finally reach a full sized condition,
and may reproduce several times. If food is abundant they
eat a lot, and undergo a final molt (being crustaceans means
they must molt their exoskeleton to grow), and change into
a male. At this time, a couple of important changes occur:
first, their front pincher-claws become very enlarged, and
second, the gut degenerates and becomes non-functional. These
males don't feed. They have only one function, and that is
to reproduce. They become "Roving Romeos" searching out and
fighting with other males in their quest to mate with as many
females as they can while they live off the stored energy
reserves accumulated during the last female stage. Finally,
they run out of fuel and die, and the next set of females
molts to take their place.
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Figure
3. A group of tanaids
of the NE Pacific species, Leptochelia savignyi,
photographed in nature in Barkeley Sound, British Columbia,
Canada. The largest animal is a bit less than 5 mm (1/5
inch) long. Walking legs are visible on the male emerging
from his tube. These animals are micro-predators and
scavengers.
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Ray Highsmith,
who did a lot of pioneering work on coral growth as a graduate
student, also studied tanaids (Highsmith, 1982, 1984). He
worked on these North Eastern Pacific species and found a
number of interesting things. No peracarid has dispersal larvae,
yet they are often geographically widespread, and one can
ask the question. "How do they get to where they are going?"
Highsmith found that these animals can disperse great distances
as tubes attached to bits of flotsam and jetsam. Additionally,
he found that they may form large aggregations or beds, wherein
they become very abundant, up to more than 50,000 animals
being found in a square meter of substrate. Here they are
predators on drifting larvae, small zooplankton, and other
small animals. They are very important animals in these assemblages.
Not only do they eat a lot of small animals, but they also
are able to exclude many larger ones, such as sand dollars,
from their beds, presumably by "gluing" the sediment together
with their tubes.
They are very ecologically important in
many areas, apparently, but also poorly known. They do make
an interesting and odd addition to the fauna of a deep sand
bed. By their action of moving in and out of their tubes,
they undoubtedly help move water through the sand beds. Additionally,
they are probably very good micro-scavengers, eating detritus
and food particles.
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