Crustaceans or Crustacea are a very large group of Arthropods (invertebrates with exoskeleton and segmented bodies). They are generally considered as as a Subphylum. Crustaceans include familiar Animals as Crabs, Lobsters, Crayfish, Shrimp, Krill and Barnacles. Most Crustaceans live in water, but some live on land like the Woodlice. Crustaceans are the most numerous Animals in the oceans, but they are not limited to oceans as they are found in fresh water as well. Some of them are parasitic like Fish Lice, Tongue Worms etc. and some are Sessile (attached to a surface) like Barnacles). There are about 52,000 described Species of Crustacea and the number of undescribed Species may be 10-100 times higher. Their origin can be traced back to Cambrian Period (544-500 million years ago) and includes living fossils such as 'Triops cancriformis', which has existed apparently unchanged since the Triassic Period. Crustaceans form an important source of food for the fishing industry. More than 10 million tons of Crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimps and prawns. Krill and Copepods form a vital part of the food chain in the ecosystem with large number of Species like Whales dependant on them for their existence.
The classification of Crustaceans has been quite variable, the system used by Martin and Davis is the most authoritative, and largely supersedes earlier works. Mystacocarida and Branchiura, here treated as part of Maxillopoda, are sometimes treated as their own Classes.
Classes and Orders of Crustaceans
1. Branchiopoda - It includes Brine Shrimp, Fairy Shrimp, Water Fleas etc. There are about 800 Species.
2. Remipedia - It includes primitive Crustaceans. There are about 130 Species.
3. Cephalocarida - It includes Horseshoe Shrimp etc.
4. Maxillopoda - It includes Barnacles, Copepods etc.
5. Ostracoda - It includes Seed Shrimp. There are about 65,000 Species.
6. Malacostraca - It includes Crabs, Lobsters, Shrimp, Krill etc. There are about 22,000 Species.
Physical Features / Characteristics of Crustaceans
Segmented Body - The body of a Crustaceans is composed of body Segments.
These Segments are grouped into three regions:
i) The Cephalon or Head - The distinctive head usually bears five pairs of appendages (limb-like attachments). Two pairs of these appendages are Antennae that are used to detect food as well as to sense changes in humidity and temperature. Another pair of appendages are Mandibles (jaws) that are used for grasping and tearing food. The final two pairs of appendages 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). A Brain exists in the form of ganglia close to the Antennae. The head also consists of one median eye and two lateral eyes.
ii) 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 surface 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).
iii) 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.
Exoskeleton - Crustacean 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 means that the right and left halves of the body are similar to each other and they typically have a definite front and back end.
Open Circulatory System - Respiration in 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 Animal body . Most Crustaceans breathe dissolved oxygen from their surrounding water either through gills or through most of the body surface; exceptions to the rule are the Woodlice and the extreme parasitic forms.
Presence of Haemocyanin - The blood is colourless and contains slightly bluish coloured Haemocyanin in place of the Haemoglobin. The respiratory function of Hemocyanin is similar to that of Haemoglobin.
Ectothermic - The body temperature of Crustaceans depends on the temperature of the water or their surroundings. 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, an Animal rhythmically waves many fine oar-like structures known as Setae, back and forth. This motion creates a mini 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).
Weight - The 'Largest Crustacean' weighs about 44 lb (20 kg) and the 'Smallest Crustacean' weighs about few milligrams.
Size - The 'Smallest Crustacean' measures about 14 ft (4.3 m) including leg span and the 'Smallest Crustacean' is about 0.1 mm (0.004 in) in length.
Geographical Range and Habitat
Crustaceans are distributed far and wide. They frequent the seas at all depths, occur in fresh-water lakes and rivers, and a few dwell in the bark of trees, but a few groups have adapted to terrestrial life, such as Terrestrial Crabs, Terrestrial Hermit Crabs and Woodlice.
Diet - The principal food of Crustaceans includes small Animals like Snails, Insect Larvae, Eggs and Tadpoles, Plankton and decaying organic matter. They even prey upon each other. Some are parasites like Water Fleas which attach themselves on the skin and gills of Fish. A few of them are wholly plant-eaters.
Predators - Predators of Crustaceans include various Species of Fishes, Octopuses, Squids, various Species of Birds like Heron and Egrets, Mammals like Otter and other Crustaceans.
History and Evolution
Crustaceans are considered to have evolved during Cambrian Period (544-500 million years ago) and includes living fossils such as 'Triops cancriformis', which has existed apparently unchanged since the Triassic Period. The 'Decapod Palaeopalaemon', a Shrimp-like form, occurs in the Devonian Period (416 million to 359.2 million years ago), Crayfish occurs in the Late Permian Period (260.4 million to 251 million years ago), and allies of the Hermit Crabs (Anomura) are found in the Jurassic Period (199.6 million to 145.5 million years ago). The True Crabs (infraorder Brachyura), which represent one of the pinnacles of Crustacean evolution, do not occur until the beginning of the Cretaceous (145.5 million to 65.5 million years ago).
The majority of Crustaceans have separate sexes, and reproduce sexually. The sexes being nearly always distinguished by appendages on the abdomen called Swimmerets or, more technically, Pleopods. The first (and sometimes the second) pair of Pleopods are specialized in the Male for sperm transfer. A small number are Hermaphrodites (having both sexes), including Barnacles, Remipedes and Cephalocarida. Some may even change sex during the course of their life. Parthenogenesis is also widespread among Crustaceans, where viable eggs are produced by a Female without needing fertilisation by a Male. This occurs in many Brachiopods, some Ostracods, some Isopods, and certain higher Crustaceans, such as the Marmorkrebs Crayfish. Fertilization usually occurs sexually between two individuals. The fertilized eggs then mature either in a specialized brood chamber in some part of the Female's body or attached directly to some external appendage such as a claw. Most Decapods (having ten legs) carry the eggs attached to the Pleopods, while those belonging to Order Peracarids, Notostracans, Anostracans, and many Isopods form a brood pouch from the carapace and thoracic limbs. Female Branchiura do not carry eggs in external ovisacs but attach them in rows to rocks and other objects. Most Leptostracans and Krill carry the eggs between their thoracic limbs; Some Copepods carry their eggs in special thin-walled sacs, while others have them attached together in long, tangled strings. Most aquatic Species hatch into a free-swimming Larvae that progress through a series of body moults (where they shed their skin) until finally arriving at the adult size. Larvae development is generally in several stages, but most Species have a Nauplius Larva (development stage characterized by the presence of three pairs of head appendages).