External anatomy

Crustacean bodies are segmented, and a pair of appendages can be found on each segment. Generally, the body of a Caridina species can be divided into two major parts called tagmata, the cephalothorax and the abdomen. The cephalothorax is the fused cephalon (Latin cephalicus, ‘head’) and thorax (Ancient Greek thōraks, ‘breastplate’), consisting of 5 and 8 segments respectively, forming one large tagma of 13 segments. The cephalothorax is covered by a helmet-like plate of exoskeleton called the carapace, which is smooth and hydrodynamic, reducing drag in the water and so allowing for better swimming. The carapace protrudes forward to form a beak-like structure called the rostrum (Latin rōstrum, ‘beak[-like]’). Behind, at either side of the rostrum, is a pair of stalked, compound eyes. These are of course used for photoreception, giving a wide field of vision for detecting movement. The appendages of the 13 cephalothoracic segments are as follows:

Cephalon

1. First antenna (antennule)
2. Second antenna (antenna)
3. Mandible
4. First maxilla
5. Second maxilla

The first antennae (Latin antenna, ‘pole’) are biramous of two whip-like flagella and are concerned with chemoreception and spatial orientation. The second antennae have a single flagellum each and are concerned with chemoreception and tactile reception. The mandibles (Latin mandibula, ‘jaw’) are muscular and are used to cut and/or grind food. The first and second maxillae (Latin maxilla, ‘jaw’) are used for food handling.

Thorax

1. First maxilliped
2. Second maxilliped
3. Third maxilliped
4. First pereiopod (first cheliped)
5. Second pereiopod (second cheliped)
6. Third pereiopod (first walking leg)
7. Fourth pereiopod (second walking leg)
8. Fifth pereiopod (third walking leg)

The three pairs of maxillipeds (Latin maxilla, ‘jaw’; Latin –ped, ‘foot’) are used for food handling, although the third pair is often modified for other functions. The first and second pereiopods are chelated (have claws) and are concerned with food gathering, grooming and signalling, whereas the posterior third, fourth and fifth pereiopods are involved with locomotion.

The abdomen consists of 6 segments, each covered in a band-like plate of exoskeleton, allowing the abdomen to flex. This is important for shrimp survival as it allows for what is known as the Caridoid escape reaction, where the shrimp flexes its abdomen powerfully and quickly to propel itself at great speed through the water to escape danger. The regions on either side of the abdomen where the band-like plates end are termed the pleura. In Caridean shrimp the second pleura overlap the adjacent first and third pleura, whereas in prawns the pleura overlap sequentially. The abdomen terminates with the telson of the tailfan which bears the anus.

Abdomen

1. First pleopod
2. Second pleopod
3. Third pleopod
4. Fourth pleopod
5. Fifth pleopod
6. Uropod

The pleopods (Latin ple-, ‘swim’; Ancient Greek –pod, ‘foot’) are used to power swimming and for the incubation of embryos. The uropods (Ancient Greek uro-, ‘tail[-like]’; Ancient Greek –pod, ‘foot’) are splayed, flanking the telson (together making up the tail fan), and are involved in steering during swimming and Caridoid escape reaction.

Sources:

Bauer R. T., (2004). Remarkable Shrimps: Adaptations and Natural History of the Carideans. Chapter 2, pg 15 – 35. University of Oklahoma Press.

Ruppert E. E., Fox R. S., Barnes R. D., (2004). Invertebrate Zoology. Chapter 19, pg 625 – 629. Brookes/Cole, Thomson Learning.

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