Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Guide to New Zealand's freshwater invertebrates

Estuarine isopods ( Sphaeromatidae : 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

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).