at the Lake Biwa Museum, Japan
Robin James Smith
AppendagesOstracods typically have seven pairs of segmented appendages, the caudal rami (furcae or uropods of some authors), and sexual organs. The ostracod body has been reduced so that the carapace can entirely enclose the non-calcified parts of the body. Because of this, the division between the head, thorax and abdomen are obscure.
The head supports four pairs of appendages, the antennules, antennae, mandibles and maxillula (or first maxilla). There is debate as to whether the fifth pair of appendages belong to the head, or are the first pair of thoracic appendages because of a lack of a clear boundary between the head and thorax.
Crustacean limbs, with the exception of the antennule and caudal ramus, are derived from a biramous limb (a limb consisting of two main branches). The base consists of a coxa and basis, which together form the protopod (also called a protopodite). Endites are protrusions on the inner edge of the protopod, and epipods (= epipodites) from the outer edge. The two branches, typically segmented, are called the exopod (= exopodite) and endopod (=endopodite), the exopod being the outer most one.
The reduced nature of ostracod limbs means that identifying the main parts is open to some debate.
AntennuleThe antennule (also called the antennula or first antenna) is the first appendage of an ostracod, located near the anterior end of the hinge.
It is uniramous (consisting of a single branch) in all groups.
The antennule consists of between five and eight segments.
The antennules of some groups, such as some Cypridoidea, are long and slender, and are flexible enough to be used for swimming in conjunction with the antennae. When the ostracod is at rest, the long setae are fanned out to sense movements in the water.
In other groups, the antennules are much more robust, and used for moving through sediments.
AntennaThe antennae, (singular antenna, sometimes called the second antenna) is the second pair of limbs.
It originates from the head, either side of the forehead and and is biramous, meaning that it has two branches (the endopod and exopod) originating from its basis.
The exopod is usually quite small, consisting of a single segment with three setae protruding from it, but in the Cytheroidea, it forms the spinneret seta. The spinneret seta is a robust tube connected to the spinneret gland, and is used to extrude a sticky thread-like substance.
MandibleThe mandibles are located each side of the mouth region. They are used for moving food to the mouth, and to masticate the food in the oral cavity.
The first segment of the palp is the basis. The rest of the palp is the endopodite. The small branchial plate protruding from the basis is probably the exopod.
MaxillulaThe fourth limb is called the maxillula. This has two main parts to it. The anterior parts point towards the mouth cavity and are used for feeding (endopod and endites). The posterior part consists of a large branchial plate, which is either an exopod or epipod (the origin is unclear). The movement of the branchial plates create a current of water through the cavity of the carapace, and hence help with respiration.
Fifth LimbThe fifth limb is located just behind the mouth. This limb shows the most variation in morphology.
In other groups the waking leg is reduced, and the protopodite is enlarged towards the mouth, forming an endite. This is used for feeding.
In some groups, the endopod is reduced into a non-segmented palp in the females, and a functions as a clasper in the males, used during copulation.
Typically, the fifth limb has a branchial plate (probably an exopod), similar but much smaller than the branchial plate of the fourth limb, the maxillula.
Sixth LimbThe sixth limb is situated in the posterior section of the carapace.
It is a walking limb in most groups, but can be a clasping appendage (male Platycopina), reduced (female Platycopina, some Myodocopida) or absent (some Myodocopida).
The first segment of the sixth limb is the protopod, while the other segments are the endopod.
Seventh LimbThe seventh limb is located in the posterior region of the carapace. It is either a walking leg, similar in shape to the sixth limb, or a cleaning limb bent upwards into the carapace.
Caudal ramusThe caudal ramus (plural caudal rami) is the last appendage, found at the posterior of the body. Other names used for this appendage include furca and uropodal ramus.
The caudal ramus is used for locomotion, while in some groups it is also used to manoeuvre the male sexual organs during copulation.
In some groups, the caudal ramus can be reduced or absent altogether in the adults, but it is present in the early juvenile stages of these groups.
Sexual organsThe male sexual organs (hemipenes) are very useful for ostracod taxonomy because the morphology is usually different for each species.
They consist of two penes, which are joined at the base, and are located just behind the seventh pair of appendages.