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Estuarine isopods (Sphaeromatidae)

Taxonomic group

Family: Sphaeromatidae

Diagnostic features

Sphaeromatids are small, fast swimming isopods found in the estuarine reaches of streams and rivers. They are sometimes known as “pillbugs” because of their ability to roll up into a pill-sized ball when threatened. They are shorter and more compact than most other freshwater or estuarine isopods.

Typical habitats

These are more marine than freshwater species, but they can be found in streams towards the upper limit of tidal influence.


Sphaeromatids may feed on a range of estuarine foods including marine and freshwater algae, decomposing plant matter and other invertebrates.

Indicator value

  • Hard bottom: unassigned; Order Isopoda value 5
  • Soft bottom: unassigned; Order Isopoda value 4.5

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

Sphaeromatids are probably more an indication of estuarine influence than any other aspects of water quality. There are no tolerance values assigned to this group but the Order Isopoda has been assigned tolerance values of 5 (hard bottom sites) and 4.5 (soft bottom sites).