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biodiversity explorer

the web of life in southern Africa

Class: Crustacea (crabs, lobsters, shrimps, prawns, barnacles)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Ecdysozoa > Panarthropoda > Tritocerebra > Arthopoda > Mandibulata > Crustaceomorpha

Introduction

The Crustacea are a subphylum within the Arthropoda that are primarily marine with a few fresh water forms and even fewer are terrestrial. This large and very diverse group includes marine lobsters, crabs, shrimps, prawns and barnacles. The name crustacean is derived from their hard, crusty exoskeleton made from chitin. Although quite diverse, crustaceans are characterised by having three distinct body parts; the head, thorax and abdomen. The head region usually bears a pair of compound eyes and five pairs of appendages (ie., two pairs of sensory antennae and three pairs of mouthparts for feeding). Biramous (Double branched) appendages on the thorax and abdomen are arranged segmentally. One branch is usually the gill branch while the other is the leg branch.

There are over 40,000 species of crustaceans known globally which are classified further based on complex body, carapace and appendage morphology. The Ostracoda or seed shrimps have a carapace shaped like that of a bivalve mollusc, which covers the entire body. The Copepoda or krill are small, often planktonic, lack a carapace, have 4-6 thoracic limbs and no abdominal appendages. The Cirripedia or barnacles have a body enclosed by a shell of calcareous plates. The Malacostraca or shrimp-like or crablike forms have eight pairs of thoracic limbs and usually possess swimming appendages on the abdomen. At present ~2333 crustacean species are known from South Africa.    

Ecology

The Crustacea occur in almost every habitat of the marine environment, and are therefore a highly diverse group of organisms. Small crustaceans like copepods and krill feed on green algae. Barnacles, which are attached to various substrata live very inactive lifestyles by filtering the surrounding waters for food. Decopods, which include lobsters, crabs, hermit crabs and shrimp possess a thorax that bears walking legs and either scavenge for food or in some instances capture prey using claws.

Crustaceans are able to form unusual interactions and associations with other organisms. Zooplanktonic crustaceans like copepods and euphausids provide a vital link between the phytoplankton they feed on and larger bodied organisms like pelagic fish, basking sharks, whale sharks and other whales, all of which in turn feed on them. Some barnacles occur exclusively on whales like the goose barnacle, Conchoderma aurita. Hermit crabs are well known for their behaviour of colonizing empty gastropod shells to protect their soft skinned abdomens. When they outgrow their shells they seek out new, larger, ones and on occasion will fight each other for the best shell. Some crabs may be covered in a coat of long, stiff, brown hairs, thereby mimicking a sponge. In fact, they sometimes carry a cloak of sponge, ascidian or seaweed on their backs to ensure that the crabs are unpalatable and well camouflaged from potential predators.

The west coast rock lobster, Jasus lalandii, occurs in shallow water (0-50 m) off the south-western coast from just north of Walvis Bay (Namibia) to about East London on the east coast of South Africa. They are fascinating animals, because they make use of large-scale ocean current systems to transport young away from parent populations, only for the young to return several months later to resettle on “home territory”. This large range is accomplished by the drifting existence of their phyllosoma (”leaf-organism”) larvae. These larvae moult through at least 11 stages, each time adapting to the prevailing environmental conditions, before they metamorphose into a swimming stage known as a puerulus (”little boy”). These transparent miniatures of adult lobsters are capable of swimming long distances thereby returning to the area in which the parent stock lives.

Crustacea and humans

The South African rock lobster fishery is based on two species, one a limited fishery on the south coast taking Palinurus gilchristi, and a second on the west coast targeting the shallow water species Jasus lalandii. The latter is caught inshore by traps and hoopnets deployed from small vessels, and is also harvested by recreational divers, and the former is a deep-water species caught by means of traps set by larger freezer vessels.

The commercial fishery for West Coast rock lobster is controlled by company-allocated quotas within a Total Allowable Catch (TAC) subdivided by geographical area. A reduction in the minimum size that can be legally harvested, from 89 to 75 mm carapace length, was introduced in April 1992. The TAC was set at 2 200 t for the 1992/93 season, but, with growth rates staying low, the TAC has since been reduced progressively.

The South Coast rock lobster fishery has been in existence since 1974. No minimum size limit is enforced and animals are caught from a size of about 60 mm carapace length upwards. As this means that little protection is afforded to breeding females, a conservative TAC of 450 t tail mass was set each year from 1984 in order to retain enough surviving adults in the stock to ensure adequate egg production and recruitment. It was later increased to 475 t, but it is now declining. For the 2003–2005 seasons (1 October to 30 September each year) the TAC remained constant at 382 t tail mass.

The vagaries inherent in the harvesting of wild lobster resources and the possible negative implications that reduced supply may have on the export markets stimulated interest in the possible aquaculture of these crustaceans. However, artificially replicating the environmental conditions necessary for the phyllasoma larvae to metamorphose has been difficult and so immediate efforts have been focused on the capture of wild pueruli for on growing in aquaculture farms.
 

Classification

Ostracoda

 

  

Copepoda

 

  

Cirripedia

 

  

Malacostraca

 

  
 

Hoplocarida (mantis shrimps)

 

  
 

Peracarida

 

  
   

Mysidacea

 

  
   

Cumacea

 

  
   

Amphipoda

 

  
   

Isopoda

 

  
 

Eucarida

 

  
   

Euphausiacea (krill)

 

  
   

Decapoda (shrimps, crabs, rock lobsters)

 

  
     

Natantia

 

  
     

Reptantia

 

  
       

Macrura (rock lobsters and sand prawns)

 

  
       

Anomura (hermit crabs and porcelain crabs)

 
       

Brachyura (true crabs)