The Pouched Lamprey (Geotria australis Gray, 1851) in Mediterranean streams of the Southern Hemisphere

The lampreys (Petromyzontiformes) represent an ancient lineage of agnathan (jawless) fishes that are considered one of the oldest extant groups of vertebrates (Docker et al., 2015). There are ~45 lamprey species, of which only five species are known from the Southern Hemisphere, included in two monogeneric families (Mordaciidae and Geotriidae) (Maitland et al., 2015; Potter et al., 2015; Riva-Rossi et al., 2020). Until recently, the genus Geotria was monotypic, with its sole representative being the widespread Pouched Lamprey (Geotria australis Gray, 1851). It is found in Mediterranean climatic zones of Australia, Chile and Argentina as well as in New Zealand and several interspersing islands (Neira, 1984; Nardi et al., 2020). Recently, a second Geotria species (G. macrostoma (Burmeister, 1868)) was described from Patagonia and is referred to as either the Patagonian Lamprey or the Argentinian Pouched Lamprey (Nardi et al., 2020; Riva-Rossi et al., 2020). The males of these two species, along with the Chilean Lamprey (Mordacia lapicida (Gray, 1851)), are unique in that they develop a large, sac-like gular pouch during maturation. Other parasitic lamprey species may develop a similar, though much smaller structure (Hardisty and Potter, 1971; Monette and Renaud, 2011). There has long been speculation as to what purpose the pouch plays in the life history of this (or these) anadromous, semelparous species (Potter and Welsch, 1997). Believed to have a specialised role in either courtship or during the act of spawning, or for transporting stones during nest-building (Ivanova-Berg, 1968; Potter and Welsch, 1997), it was only very recently that courtship, mating, egg-laying and gular pouch function has been described (Baker et al. 2017; Paton et al., 2020). The gular pouch was found to massage and groom eggs until they hatch, potentially ventilating developing eggs (Baker et al., 2017). In contrast to most Northern Hemisphere lamprey species, which die soon after spawning, G. australis has been shown to live for well-over a year (up to 511 days) post spawning in Mediterranean-climatic south-western Australian streams (Paton et al., 2020). This is remarkable considering that during their upstream migration phase, which in those Mediterranean-climate streams begins approximately 15 months prior to spawning, these fish don’t feed, suggesting that they can survive for up to 30 months without eating (Paton et al., 2020). To contend with south-western Australia’s hot dry Mediterranean summers, over- summering (upstream migrant) sub-adults and larvae (ammocoetes) occupy shady, often perennial, low salinity streams (Allen et al., 2017). While they have persisted for millions of years, their remaining habitats in south-western Australia are under serious threat from climate change, with a 50% reduction in stream flow observed in some parts since the 1970s. Many rivers have become secondarily salinized due to extensive land clearing in the last century, exposing non-halotolerant larval lampreys to lethal salinities (Allen et al., 2017). There is an urgent need to identify and safeguard the remaining habitats of G. australis during their freshwater phases throughout the region, and perhaps in other Mediterranean habitats within their Southern Hemisphere range.