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Freshwater crabs (Amarinus)

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Taxonomic group

Hymenosomatidae: Amarinus

Diagnostic features

Amarinus (formerly Halicarcinus) is the only truly freshwater crab in New Zealand. They are small, with a round cephalothorax (the main body mass) that is flattened on top. The legs are thin and fragile. Any crabs found kilometers upstream of tidal influence are likely to be Amarinus.

[Amarinus]. Image: Stephen Moore Image
Amarinus. Image: Stephen Moore
[Amarinus]. Image: Stephen Moore Image
Amarinus. Image: Stephen Moore

Typical habitats

Amarinus is found in lakes and slow flowing streams and rivers in the north of the North Island. These crabs are often found in holes and crevices in decomposing wood.

Feeding

They appear to be omnivores, as past studies have found both plant and animal matter in their guts.

Indicator value

  • Hard bottom: unassigned
  • Soft bottom: 5.1

The tolerance values (ranging from 0 to 10) give an indication of which are the sensitive taxa (values of 8 or more) and which are the tolerant taxa (values of 3 or less). For more information see: Indicator species

Amarinus are found in streams, rivers and lakes in farmland areas where water quality is less than pristine. They have been assigned a soft bottom tolerance value of 5.1 (no hard bottom value has been assigned).