On the lookout for introduced marine isopods
Science Centres: Aquatic Biodiversity and Biosecurity
The new species of Cirolana found at Island Bay.
How much do we really know about what lives on our shores? Well, not as much as we might like to think. Recently a common grey isopod crustacean was found while turning over rocks at low tide during a routine search at Island Bay in Wellington. A check through the literature suggested it might be Cirolana australiense, a common Australian species – but it was not. This discovery highlights two important points: how little we know about what species exist on even our most readily accessible shores, and the difficulty in identifying these and separating them from introduced alien species.
One might think there are few opportunities for an alien isopod species to find a vacant niche to exploit and become established, but at least 25 species of isopod are recognised as being introduced at various locations throughout the world. These include two of the most abundant marine isopod families – the Cirolanidae and the Sphaeromatidae.
Only one marine isopod species, Cymodoce tuberculata, has been identified as being introduced into New Zealand, but this has since been discounted as a misidentification. New Zealand has ‘exported’ the cirolanid Eurylana arcuata to Australia and the Pacific coast of the USA. Another New Zealand species, Pseudosphaeroma campbellensis, has recently been found in harbours and marinas in California and may also have been introduced to southern Australia. The North Pacific cirolanid Cirolana harfordi is established in most southern Australian ports, but it has not been reported in New Zealand even though conditions seem to be suitable.
There are numerous criteria that can be used to identify an invader, including belonging to groups of animals not known from New Zealand but characteristic of another geographic region; having habits suitable for translocation (e.g., the ability to live on the fouled hulls of ships); and being found only in harbours or marinas. However, all these criteria rely on first being able to identify the species.
Recognising an introduced species is no simple task. It requires high-resolution taxonomy and a working knowledge of the likely evolutionary relationships of the animal. As scientific techniques and knowledge advance, so does our ability to identify species that are nearly identical in outward appearance. In recent decades, revisions of isopod species long believed to have a global distribution have resulted in ‘species swarms’ of between 20 and 30 species or more. One such swarm is the so-called ‘Cirolana parva group’ of species, once considered to be a single, globally distributed species. This group now has 25 named species and a similar number of undescribed species, all of which are nearly identical in general appearance. The nominate species is now restricted to the Caribbean and Pacific Panama, while all the other species are restricted to island groups or extended coastal regions.
The only record of this group of species in New Zealand was of Cirolana australiense (Hale, 1925), which was reported from the Chatham Islands in the early 1960s. There was no suggestion at the time that this species might have been introduced, but this did seem likely. To most identifiers this animal would appear to be identical to Australian specimens. The alternative was that this distribution was natural, albeit inconsistent with the distributional patterns shown by other species in the family.
Bearing this in mind, collections were made at Island Bay and other nearby locations for a detailed taxonomic study. Examination of collections held at the Auckland Institute and Museum in Auckland and at Te Papa in Wellington revealed a second subtidal species, also very similar to C. australiense. Neither of these two species, both which are found in the Auckland region to at least Kaikoura in the south and east to the Chatham Islands, are C. australiense. The critical differences lay in the details of the spines and ‘hairs’ (setae) on their anterior legs, and the shape of some of their body segments. The descriptions of these two common species, which are new to science and endemic to New Zealand, have been submitted for publication.
A conservative estimate suggests that the potential number of marine isopods around New Zealand may exceed 1000. Clearly ‘new’ isopods can be readily discovered, but how do we know which of these may be introduced? The important message here is that without solid background knowledge of both New Zealand and world fauna based on accurate taxonomy, it is often not possible to identify indigenous ‘new’ species from those that may have been introduced.