Creatures of the Coral Reef – Crustaceans

Crustaceans or Crustacea are a large group of Arthropods (invertebrates with exoskeleton and segmented bodies).  Crabs, Lobsters, Crayfish, Shrimp, Krill and Barnacles are among the best-known crustaceans, but the group also includes an enormous variety of other forms without popular names. Crustaceans are generally aquatic and differ from other arthropods in having two pairs of appendages (antennules and antennae) in front of the mouth and paired appendages near the mouth that function as jaws.  Some of the Crustaceans are parasitic like Fish Lice, Tongue Worms, etc. and some are Sessile (attached to a surface) like Barnacles.

These animals are known for their hard outer shell. This shell must be removed and discarded as the animal grows. Once this takes place, the new shell takes time to harden. During the time the new shell is hardening, the animal is without its primary means of protection and is vulnerable to attack from predators. However, many crustaceans have a number of defensive mechanisms or capbilities at their disposal. Some crustaceans have claws that are capable of exerting hundreds of pounds of pressure while others have the ability to produce a deafening sound waves with which they stun their prey. For instance, the mantis shrimp can break the glass of an aquarium or split a man’s thumb to the bone with one strike.  But in spite of their impressive capabilities, crustaceans do routinely meet their match. The teeth of the triggerfish and the beak of the octopus can crack through the toughest shells of crabs and lobsters.  The life of the Crustacean is challenging and requires these creatures to be constantly on guard.

Kingdom – Animalia

Phylum – Arthropoda

Subphylum – Crustacea

Body

The body of a Crustacean is segmented and has three distinct parts or sections:

The Cephalon or head – The distinctive head usually has five pairs of appendages. Two pairs are Antennae that are used to detect food as well as to sense changes in humidity and temperature. Another pair are Mandibles (jaws) that are used for grasping and tearing food. The final two parirs are Maxillae, arm-like projections used for feeding purposes. Crustacean appendages are typically Biramous (divided or separated into two branches), meaning they are divided into two parts; this includes the second pair of Antennae, but not the first, which is Uniramous (a single series of segments attached end-to-end). The Brain exists in the form of ganglia close to the Antennae. The head also consists of one median eye and two lateral eyes.

The Thorax – The Thorax bears numerous Appendages which are usually Biramous (forked) and also the gills, but the smaller Species may breathe through their body survace by diffusion. The Head and Thorax may be fused together to form a Cephalothorax, which may be covered by a single large Carapace (hard outer covering or shell).

The Pleon or Abdomen – The Abdomen bears Pleopods (appendages used for carrying eggs and swimming) and ends in a Telson, which bears the Anus, and is often flanked by Uropods (posterior abdominal appendage) to form a Tail Fan. Each segment bears a pair of appendages. A collection of major ganglia is found below the gut.

Physical Characteristics

Exoskeleton (or shell) – The Crustaceans body is protected by the hard Exoskeleton, which is moulted as the Animals grow. The Exoskeleton is generally harder than it is in other Arthropods because it contains Limestone in addition to Chitin (tough semitransparent horny substance).

Bilaterally Symmetrical – All Crustaceans have Bilaterally Symmetrical bodies which mean that the right and the left halves of their bodies are similar to each other and that they typically have a definite front and back end.

Open Circulatory System – Respiration in the Crustaceans is performed chiefly by a series of gills contained in a special chamber on either side of the Thorax. The Circulatory System consists of a heart and several principal arteries and a number of spaces called Sinuses into which the blood passes from the arteries. The blood is circulated to the Haemocoel (body cavity) which is present in the dorsal part of the body. Most Crustaceans breathe dissolved oxygen from their surrounding water either through gills or through the surface of their bodies; exceptions to the rule are the Woodlice and the extreme parasitic forms of Crustaceans.

Presence of Hemocyanin – The blood is colorless and contains slightly bluish coloured Hemocyanin in place of the Hemoglobin. The respiratory function of the Hemocyanin is similar to that of Hemoglobin. However, there are a significant number of differences in its molecular structure and mechanism. Whereas hemoglobin carries its iron atoms in porphyrin rings (heme groups), the copper atoms of hemocyanin are bound as prosthetic groups coordinated by histidine residues. Species using hemocyanin for oxygen transportation are commonly crustaceans living in cold environments with low oxygen pressure. Under these circumstances hemoglobin oxygen transportation is less efficient than hemocyanin oxygen transportation.

Ectothermic – The body temperature of Crustaceans depends on the termperature of the water of their surroundings. Typically an Ectothermic animal’s internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature. Such organisms rely on environmental heat sources, which permit them to operate at very economical metabolic rates. When the temperature around them drops their activities become sluggish.

Feeding Techniques – Crustaceans exhibit a wide range of feeding techniques. The simplest of these techniques are used by Species such as the tiny Shrimps and Copepods that practice “filter feeding”. In Filter feeding, many oar-like structures known as Saetae are rhythmically moved back and forth to create waves, which in turn direct a water current towards the mouth. Plankton and other suspended materials are carried into special filters and then transferred to the mouth.

Excretory System – The Excretory organs of Crustaceans are a pair of glands placed laterally on either side of the Gullet (passage between the pharynx and the stomach). Two different excretory organs are found among crustaceans: the antennal gland and the maxillary gland. Both have the same basic structure: an end sac and a convoluted duct that may expand into a bladder before opening to the outside. In most adult crustaceans only one or the other gland functions. The functional gland may change during the life cycle.
 The antennal and maxillary glands primarily regulate ionic balance. The total balance of salts and water is also controlled in part by the gut, which can absorb both.

References

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