In my recent
column discussing crabs, I wrote about a group of species
called the Malacostraca. This is the taxonomic grouping for
the crabs and a lot of other large crustaceans. In that discussion,
I used a lot of verbiage to explain that crabs were at the
top of an evolutionary pinnacle that includes a lot of other
animals, some of which don't look crab-like at all. By examining
the characteristics necessary to "make" a crab from
some sort of more primitive progenitor, it becomes apparent
the most primitive crustacean on the crab lineage really looked
nothing at all like a crab. If all of those necessary ancestral
conditions are put together, they create a "lowest common
denominator," an ancestral set of conditions called The
Caridoid Facies. No living animal has all of the characters
found in the caridoid facies and neither does any fossil,
for that matter. This animal is a "missing link"
that shall probably forever stay missing, as it is truly a
hypothetical animal, rather like an "honest politician."
Nonetheless, one rather successful group of modern animals
comes pretty close, at least in a lot of regards, to embodying
this character set and gives us a glimmer of what an ancestral
crab might have looked like. And, interestingly enough, many
of us have these animals in our aquaria. These are the mysids
or the so-called "possum" shrimp.
Figure 1. A mysid collected from my refuge tank. It
was about an eighth of an inch (3mm) long. This is a lateral
view. The animal is a male and lacks the brood pouch. Note
the shape of the appendages and contrast them to the appendages
of the true shrimp in your aquaria.
are small shrimps, common in all marine habitats. Although
most of the shallow water ones that hobbyists
encounter are small, some deep
sea forms range in size upward to about a foot (30 cm)
long. The characteristics that specifically categorize a shrimp-like
animal into a mysid shrimp, may be best seen by examining
the hierarchy of taxonomic characteristics that describe all
mysids. The names in a taxonomic hierarchy of terms are not
designed simply to be a listing of hard-to-pronounce names
to be understood only by anointed elite of the taxonomy. They
may be on such a list, but really their function is a lot
more mundane. The taxonomic hierarchy may simply be thought
of as an address of a precise group of organisms within the
larger array of all other organisms. These terms are names
of a series of sets of definable and demonstrable characters
that can be used to separate and characterize a small group
of organisms from the total chaotic array of all animal species.
These terms form a series of nested arrays
ranging from the most inclusive to the most precise. In other
words, we start with the largest group and work to ever more
precise, smaller groups. Because they have an integument modified
as an exoskeleton and move upon, or with, appendages with
obvious joints in them, these animals are placed in the PHYLUM
ARTHROPODA. Within the arthropods, they are crustaceans
(instead of spiders or insects). That means they are in the
SUBPHYLUM CRUSTACEA. These are arthropods with two
pairs of antennae. In the primitive forms, their appendages
consist of two branches, and they have a particular larva
called a "nauplius." There are a lot of crustaceans,
but mysids share characters with crabs and other shrimp-like
animals, which means they are classified in the CLASS MALACOSTRACA.
All Malacostracan crustaceans have paired
visual organs, called compound eyes. These are comprised of
several to several thousand individual subunits. Additionally,
all malacostracans have robust jaws. Furthermore, each antenna
typically has two branches, although in some species the second
branch is absent. All malacostracans are considered to have
eight pairs of thoracic appendages, although in some of the
malacostracan lineages, the front thoracic appendages have
been incorporated into the head region. They have five pairs
of abdominal appendages. Mysids are further categorized within
the SUPERORDER PERICARIDA. The name Pericarida means
"near to- or close to- shrimps," and indicates that
all the animals in this grouping we call a superorder, while
closely related to shrimps, belong to a different lineage
from the true shrimps. Pericarids have all the characteristics
of the Malacostraca, but they possess a few special ones of
their own. First, while they do have a carapace, it is normally
relatively rudimentary. Additionally, all peracarids have
at least one of the front pairs of thoracic appendages modified
as feeding appendages, and the females have a brood pouch
located at the base of some of the thoracic appendages. They
lack a free-swimming larva, so the typical crustacean larva,
the nauplius, is passed as a stage in the pre-hatching embryo.
This characteristic really does separate mysids from the so-called
true shrimp. All of the true shrimps are in the group called
the Eucarida (a name that actually means "true shrimps").
The presence of the brood pouch, in addition to separating
the mysids from shrimps, also gives these shrimp a superficial
similarity to marsupials, hence the common name, "possum
Figure 2. A diagrammatic representation of a generalized
female mysid (modified after McLaughlin,1980). The abdomen
is yellow, and the cephalothorax in red. Although the head
and thorax are not distinct, structures assigned to the head
are in blue. Compare this diagram with that of the Caridoid
Facies and the mysid in Figure 1.
Finally, this brings us to the next smaller
subdivision, called the ORDER MYSIDACEA. Mysids are
pericarids with a well-developed carapace. Although it covers
most of the thorax, it is only connected to the anterior three
thoracic segments. This carapace is the "mini-skirt"
of the crustacean world, and doesn't extend down the sides
of the animal far enough to cover the bases of the legs. The
legs have two large, evident and almost equal, branches. There
are large and evident paired statocysts (balance organs) which
are found in, of all places, the tail fan. These latter structures
are unique in the crustacea, and their presence serves to
identify any small shrimp as a mysid. The statocyst is an
organ consisting of spherical structure filled with liquid
and containing in its center a small calcareous concretion
or "statolith." This small stone is balanced in
the center of the sphere on stiff sensory cilia which look
like very tiny microscopic hairs. The cells that give rise
to the cilia are connected to the nervous system, and as the
animal moves or changes orientation, the concretion shifts
position, bending the cilia and thereby triggering the nerves
to fire. These nervous responses allow the animal to discriminate
changes in velocity and orientation. A lot of animals, including
a lot of crustaceans, have statocysts, but no other animals
have them in their tails.
Figure 3. Left. A juvenile mysid showing the
position of the statocysts at the base of the tail fan. Compare
with the short movie
below and note that the statocysts are occasionally
visible as bright dots at the back end of the moving mysid.
Right. Statocysts showing the concretions in the capsules
that comprise these balance organs.
The mysids found on reefs, in our aquaria,
or those we add to our aquaria as food, tend to be small animals,
and like most small animals, they are the food of larger animals.
The mysids have opted for the ultimate in camouflage; many
of them have become masters of transparency. Additionally,
unlike many of the shrimps we are accustomed to in the aquarium
hobby, mysids are often wholly, or at least partially, planktonic
in their habits. On reefs, and in other marine habitats, the
careful observer often sees swarms of them moving over the
substrate. They often cruise close to the surface, generally
within a foot (30 cm) or so of the surface. Such swarms are
actually relatively common in many reef areas and provide
food for many reef animals. Unfortunately, mysids from coral
reef habitats have seldom been studied, so we really do not
know much about their normal biological interactions.
Figure 4. A mysid swarm photographed about 3 feet (1
m) above a Caribbean
coral reef. The animals are visible only
because their eyes and part of the gut are
from the camera's strobes. The mysids were about 0.25 inches
(6 mm) long.
The body shape and appendages reflect the
mobile lifestyle of these demersal or "bottom dwelling"
plankton. This is particularly evident in their appendages.
Their thoracic appendages are not really well adapted for
walking, instead they are designed rather for touch-and-go
swimming. Although many species will "touch down"
on the substrate, they don't really walk around much, and
they seldom pick things off the bottom with their legs. This
is in decided contrast to most of the other shrimpy animals
we keep in our aquaria. These shrimps are supremely well-adapted
to their bottom dwelling mode of existence. Not only are they
very adept at walking around, such animals are constantly
picking at things with their appendages tipped with pincher
type claws. Mysids, in contrast, are almost always in motion
as the following movie
Mysids are often animals that are at intermediate
levels in food chains. They tend to be omnivorous, eating
smaller planktonic organisms; both phyto- and zooplankton.
In turn, they are eaten by larger zooplankton, often small
fishes, or larger shrimps. Because of their tasty nature and
small size, natural selection has undoubtedly worked on them
to produce animals that are about as transparent as animals
can be. I have included an illustration taken of a single
mysid resting on a sediment substrate, and about the only
things that are visible are the eyes, and the thin nerve cord
that runs down the center part of the bottom of the body.
The rest of the animal is "visible" only as "space"
where nothing else is seen. All of the guts, muscles, blood,
and even food are transparent. When seen swimming, often the
only things that are visible are the eyes and the statocysts.
The latter are so small that the light and the animal's orientation
has to be "just right" to see them. Even if the
mysids in our aquaria are visible, as some of them are, they
get mistaken for other things. Because of their small size,
shrimp-like demeanor and often slightly reddish color they
are often assumed to be the larvae of cleaner or peppermint
shrimps. In reality, the
fragile larvae of these other shrimps probably don't survive
for more than a few hours in most aquaria.
Figure 5. A solitary mysid photographed on sediment.
Left: The unaltered image. Right: the mysid's
The photograph is actually in crisp focus, any ambiguity of
structure is due to the poor
resolution imposed by online
imagery and the transparency of the animal. The animal was
about 0.5 inch (1.2 cm) long.
Small mysids thrive in reef aquaria, particularly
those with refuge aquaria. Mysids are good food for a lot
of small predators such as small fishes, and some of the zooplankton-catching
suspension feeders such as many of the so-called Large Polyped
Scleractinians. All that is necessary for their cultivation
and maintenance is the presence of some safety zone in the
aquarium, and some food. Although their natural food is unknown,
I would suspect it is microzooplankton of the nature produced
by the deep sand bed fauna, particularly the larvae of the
numerous bristle-type worms found in such
For a short mysid video, click
here. The video and pop-up window may take a few minutes
to load depending on your connection speed. The QuickTime
plug-in is necessary to view this video. To download the plug-in
click on the image below:
Frozen mysids may be found as food in pet
stores, and some supply houses stock live ones, although these
tend to be a bit expensive. Nonetheless, they are excellent
foods for many marine animals, and are really a more natural
food source than are Artemia. Some aquarists do culture them,
and the culture methods are about the same as for Artemia.
Mysids are much more muscular animals than are brine shrimp,
and should be a more nutritious food source; however, like
Artemia, they would themselves need to be fed for optimal
nutritional quality. Like any live or frozen food as well,
they will not be a totally balanced food source. They are,
however, an excellent supplementary food.