Small to medium sized, slender and elongated, with a scaleless, eel-like body. Adults are commonly 300–440 mm long in freshwater habitats (maximum size ~500 mm). Two low dorsal fins are situated well back on the body near the rounded caudal fin, and paired pectoral and pelvic fins are absent. Lampreys lack jaws; instead, adults have a well-developed suctorial oral disc with sharp, radially arranged tooth plates. There are two large tricuspid teeth on the oral disc above the mouth and no fimbriae (fringing filaments) surrounding the oral disc. In mature adults, the radial toothplates disappear. The eyes of adults are dorsolateral, and both adults and ammocetes have seven small oval gill apertures on each side of the body behind the head.
Adults are bluish-grey dorsally just before and after their marine phase; during their spawning migration they are a duller grey. The larval juvenile life phase (ammocete) is usually less than 140 mm long, worm-like, lacks eyes and tooth plates, and the dorsal fins are very low, not extending far above the body surface. A large, hood-like upper lip overhangs the small mouth. Brownish in colour, ammocetes are darker on the dorsal surface and reddish around the gills because of the underlying blood vessels. They can be distinguished from ammocetes of the Pouched lamprey by the position of the vent, which is below a point about halfway along the length of the second dorsal fin.
Most of the adult life is spent at sea or in estuaries. Young adults migrate upstream from the sea from late winter to early summer to breed in rivers. In the Basin, adults are sporadically sampled migrating upstream at fishways on the Murray Barrages between the Coorong and Lower Lakes. Riverine olfactory cues, including pheromones from juvenile lamprey, are believed important in stimulating upstream migrations in northern hemisphere lamprey species. While migratory cues are unstudied for short-headed lamprey, the species is only encountered at the Murray Barrages during periods of freshwater discharge, with moderate peak flows associated with increased capture rates, suggesting freshwater-related migratory cues are likely important for this species.
Little is known of many aspects of the species’ ecology in the Basin, so much of the information below is drawn from studies elsewhere. Adults may inhabit freshwater habitats for about a year before they spawn the following spring. During upstream migration, individuals are usually nocturnal, and burrow into the substrate during the day. Females have 3,800–13,400 small eggs, (0.3–0.5 mm diameter), which are deposited in a shallow nest (small depression) in the substrate, often in shallow, relatively fast-flowing water. Adults die shortly after spawning. The juveniles, known as ammocetes, are sedentary and live in slow flowing streams, burrowing in silt or mud, for about three years before metamorphosing (at around 100–140 mm length) and migrating down to the sea, usually in spring. The ammocetes are toothless, feeding on algae, detritus and micro-organisms filtered from the water. After metamorphosis to adulthood and emigration to marine habitats, they become parasitic on other fish, rasping a hole in the side and feeding on blood and/ or muscle. Adults cease feeding prior to their upstream spawning migration. Short-headed lamprey pass through the Murray Barrages from July to Nov, with peak movement in Sept-Oct. Several individuals have been PIT tagged at the Murray Barrages in recent years and two individuals have subsequently been recorded on the tag readers at upstream fishways. This included an individual tagged at the barrages in August 2019 that ascended 8 fishways and passed 10 weirs and was last detected in November 2019 825 km upstream of the Murray Mouth.
Generally restricted to the lower to mid Murray River in the Basin (occasionally recorded as far upstream as Yarrawonga and even Narrandera on the Murrumbidgee), but otherwise found in coastal rivers in Vic, NSW, SA and Tas. In recent decades, this species has been rarely recorded in the Basin because of its cryptic habits, limited connectivity and absence of freshwater discharge during the upstream migration period. In recent times adults are rarely seen, but formerly they could be seen in large numbers in the lower Murray on their spawning run at migration barriers such as weirs. They were occasionally recorded in large numbers moving through the fishway at Euston Weir. A significant influx of lampreys (recorded as ‘freshwater eels’ but considered to be short-headed lamprey) was reported in the Murrumbidgee commercial fishery records in high flow years in 1972–74, but this result was not repeated before the fishery closed in 2003. The installation of fishways on the Barrages and the completion of the Sea to Hume fish passage program means the species is likely to be recorded in more upstream locations in coming years, with the 2019/20 catch of this species in the Coorong-Lower Lakes monitoring program being the highest since the program’s first year in 2006/07. No individuals were captured in the Sustainable Rivers Audit (2004-2013) or the MDB Fish Survey (2014/15-2021/22). Ammocetes can be reasonably common in suitable silty habitats. It is listed as Endangered in the Action Plan for South Australian Fishes, Vulnerable by the Australian Society for Fish Biology, and considered Rare by the SA Department for Environment and Water.
Barriers to fish movement can interfere with spawning migrations, although the species can climb wet vertical surfaces.
Lack of freshwater outflows from the barrages and Murray Mouth during the winter–spring upstream migration season probably limits cues for movement from marine to freshwater habitats.
Allen et al. 2002; Bice et al. 2020a,b, 2021; Cadwallader & Backhouse 1983; Gilligan 2005a,b; Hammer et al. 2009; Hughes & Potter 1968; Koehn & O’Connor 1990; Miller et al. 2021; Potter 1970, 1996a; Wedderburn et al. 2017; Zampatti et al. 2010.
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