Crustaceans

• Features

  Key features

  The segmented exoskeleton (shell) of a crustacean is generally divided into three main regions: head, chest (thorax or pereon) and abdomen (pleon). In some cases (for instance, crabs) the plates of the head and thorax are fused together to create a single shell which covers most of the animal’s back (carapace). The carapace may also have a forward projection between the eyes known as the rostrum. Crustacean shells are particularly strong due to their inclusion of calcium carbonate. Each body segment often bears a pair of appendages (limb like structures) which have different functions. On the head these include sensory feelers (antennae) and mouthpart structures (maxillae and mandibles). Bristle like hairs known as ‘setae or chaetae’ are often present in the mouthparts of crustaceans and on their feeding legs (maxillipeds). These are used to catch and trap food particles and detect changes in their environment. The thorax has a mixture of specialised walking legs (pereiopods), feeding legs (maxillipeds) These appendages also create water currents to keep oxygen flow past their gills for breathing. The abdomen often has paired swimming appendages (pleopods) and a tail like structure on the last body segment known as a telson.

  Life cycle

  The majority of crustaceans have separate sexes and reproduce sexually. Some species like barnacles are hermaphrodites (having both male and female reproductive organs).  Many crustaceans release fertilised eggs in to the water column where they develop as larvae before settling on an appropriate substrate. Others carry eggs on their pleopod legs or create a brood pouch to carry them in. As crustaceans have a hard outer skeleton, they must moult at certain stages in their development (a process known as ecdysis) in order to grow.

  

  Typical features of an Amphipod (Source: http://en.wikipedia.org/wiki/Amphipoda)

  

  Typical features of a Cumacean (Schematic created by Hans Hillewaert, Source: http://www.theanimalfiles.com/anatomy/cumacean_body_plan.html)

Descriptions of species commonly found in Waikato estuaries

• Colurostylis lemurum

  

  Common name: Cumacean shrimp or hooded shrimp

  Latin name: Colurostylis lemurum

  Scientific group or class: Crustacean

  Description: Generally small, transparent and non-descript crustaceans. They are burrowers, reworking or bioturbating the sediment surface.

  Habitat: Intertidal zone of estuaries where they burrow into the soft sediment surface. Prefer fine to muddy sand and are sensitive to pollution.

  Distribution: North Island (common in Auckland and Waikato regions) and South Island

  Indicator species: Yes

  Why is this a good indicator species? Colurostylis lemurum tolerates a sediment mud content of up to 60%, with an optimum range of 0-5%. Therefore they are usually found in sandy habitats. Colurostylis lemurum is also sensitive to lead contamination and other pollution. Where the sediment mud content increases (exceeding its optimum range) and/or becomes more polluted, the abundance of Colurostylis lemurum is likely to decline. This makes Colurostylis lemurum a good indicator species with which to assess changes in the input of sediment and pollutants into our estuaries.
• Corophiidae
  

  Corophiid amphipod (Photo courtesy of Stephen Moore, Landcare Research)

  

  Latin name: Corophiidae

  Scientific group or class: Crustacean (amphipod)

  Description: Corophiid amphipods are tiny shrimp-like crustaceans, which burrow into the sediment and emerge at high tide to scavenge or deposit feed. They have a noticeably enlarged second antennae. There are around 6000 amphipod species worldwide.

  Habitat: Normally found in muddy intertidal habitats. They can tolerate low salinity and can also be found in fresh water habitats.

  Distribution: Common in intertidal areas of North Island estuaries

  Indicator species: Yes

  Why is this a good indicator species? Corophiid amphipods tolerate a sediment mud content of 40-100%, with an optimum range of 95-100%. Therefore, they are usually found in very muddy habitats. Corophiid amphipods can also tolerate organic enrichment and pollution. Where the sediment mud content increases (exceeding 40-50%) and/or becomes polluted or organically enriched, the abundance of corophiids is likely to increase. This makes corophiid amphipods good indicators for the assessment of changes in the input of sediment and pollutants into our estuaries.
• Phoxocephalidae

  

  Latin name: Phoxocephalidae

  Scientific group or class: Crustacean (amphipod)

  Description: Phoxocephalidae are small amphipods which are surface deposit feeders and bioturbators. At least five species of phoxocephalid species are found in the Waikato estuaries we sample. They are prey for fish and birds.

  Habitat: Occur from intertidal (shallow) to deeper environments (50m depth). Burrow into the sediment. Prefer muddy sand habitats and are sensitive to pollution.

  Distribution: New Zealand wide

  Indicator species: Yes

  Why is this a good indicator species? The preferred mud content is unknown for most phoxocephalids; however, phoxocephalid amphipods are known to be intolerant to very high mud content. They are usually found in muddy sands. One species (Waitangi chelatus) is known to prefer a very low mud content of 0-5%. Phoxocephalid amphipods cannot tolerate pollution. For example, one species (Waitangi brevirostris) has been shown to be sensitive to lead contamination. If the sediment becomes muddier and/or polluted the abundance of phoxocephalids is likely to decline. For this reason phoxocephalid amphipods are good indicators for the assessment of the change of sediment input and pollution into our estuaries.