Corophium mucronatum, a small amphipod native to the Ponto-Caspian region, was discovered in September 1997 in Lake St. Clair. A single individual was collected using a bottom sled dredge in littoral waters adjacent to Seaway Island, Ontario, Canada. No other Corophium specimens were detected despite extensive sampling on 60 sites in the lower Great Lakes during 1997 and 1998. Corophium mucronatum was apparently alive at time of its collection because its integument was normal in appearance. Because no other Corophium individuals were found during repeated sampling over two years, it is highly unlikely that this species has established in the Great Lakes.
The genus Corophium consists of at least 50 species, most of which are native to marine and brackish water environments (Dediu 1980). Only six Corophium species, namely C. curvispinum [synonym = C. sowinskyi], C.robustum, C. maeoticum, C. chelicorne, C.nobile, and C.mucronatum are confined exclusively to fresh and brackish water habitats (Mordukhai-Boltovskoi 1960, 1964). These species are considered native to the Caspian Sea, to freshened estuarine regions of the Black and Azov Seas, and to the lower courses of rivers draining into these seas. The original distribution of these species was apparently determined primarily by the salinity and ionic composition of the water (Mordukhai-Boltovskoi 1960, Birshtein and Romanova 1968, Dediu 1980). In the Caspian Sea, the ratio of total ion to chloride ion is ~2.4 and Corophium occur in habitats ranging in salinity from 0.3 to 13‰ (Mordukhai-Boltovskoi 1960). In the Black and Azov Seas, where this ratio is 1.8, Ponto-Caspian Corophium spp. do not occur at salinity values >5‰.
Corophium spp. have been widely introduced in Eurasian freshwaters owing to human activities including canal and reservoir construction, changes to hydrological regimes of rivers, transfer s by commercial ships, and by intentional transplantations. C. curvispinum is the most widespread species. It has spread to upper reaches of many Ponto-Caspian rivers and has penetrated or been introduced to Lake Balkhash, to the Baltic and Northern Sea basins, and to the Rivers Ob, Ural and Danube (Mordukhai-Boltovskoi 1964, Dediu 1980). Other Ponto-Caspian corophiid amphipods (e.g. C. robustum, C. chelicorne, C. nobile, C. maeoticum) have extended their ranges in the lower and middle courses of Ponto-Caspian rivers, and to adjacent canals and reservoirs. Some of these range extensions apparently resulted from natural dispersal, though intentional transplantations were also important. C.robustum also has been discovered beyond the Ponto-Caspian basin in a lake in the Marmara Sea basin (Mordukhai-Boltovskoi et al. 1969).
Corophium mucronatum current distribution is restricted to Black and Caspian sea basins. Species extended its range in the lower and middle courses of Ponto-Caspian rivers and to adjacent canals and reservoirs. Corophium mucronatum was likely transported to the Great Lakes in ballast waters from its native region.
Corophium mucronatum belongs to the family Corophiidae, which exhibit marked morphological differences from amphipod families in the Laurentian Great Lakes. No other corophiid amphipods were reported from the Great Lakes today. Corophium mucronatum is one of the smallest species in the genus, with an adult body length that rarely reaches 5–6 mm. For example, Corophiidae amphipods can be easily distinguished from members of the Gammaridae (e.g. Gammarus,Echinogammarus), Talitridae (e.g. Hyalella) and Haustoriidae (e.g.Diporeia) by their possession of a dorso-ventrally compressed body, and enlarged antennae II that are modified for grasping. The families Gammaridae, Talitridae, and Haustoriidae are characterized by a laterally compressed body and antennae I and II that are of almost equal widths. Corophium also differs from these families in that it possesses a mandible palp with two segments, while the other families have either a three segmented palp (Gammaridae, Haustoriidae) or lack a palp (Talitridae). Corophium can be further discriminated from Gammaridae and Haustoriidae by the absence of an accessory flagellum on antennae I and by possession of an entire telson. Corophium can be discriminated from Talitridae in that its coxal plates that are small and scale-like, while they are large in the latter group.
Features of C.mucronatum include a urosome with separate segments, a well-developed and sharply pointed rostrum , and the presence on segment 4 of antenna II of a large curved process which extends beyond the side tooth on segment 5 and which extends at least half the length of segment 5 (Fig). At the base of this process there exists a single tooth which is less than half as long as the process. Also, segment 5 of antenna II lacks teeth (tooth) on its distal edge. Antenna II is more strongly developed in males than in females.
C.mucronatum is multivoltine, producing either two or three generations per year depending on water temperature (Dediu 1980). In the Dniester River estuary, C.mucronatum occurs at a variety of depths, though it is most commonly encountered at 0.5 – 3 m. In the Caspian Sea,C. mucronatum descends to 50m, though it achieves greatest abundance at 3 – 4m where fresh and saline waters meet (Birshtein and Romanova 1968, Dediu 1980). In the Dniester tidal estuary, C. mucronatum occurs frequently in areas where salinity is low (0.3 – 3‰) and infrequently in brackish water (3 – 5‰). C. mucronatum also was reported in freshwater regions of the Dnieper and Danube river basins (Dediu 1980, Zimbalevskaya 1989, Shevtsova 1991, Moroz 1993, Romanenko 1993), though the species may be established only in the Dniester tidal estuary and Caspian Sea (Dediu 1980).
C.mucronatum occurs in a wide array of substrates including clay, grey mud, silted sand, sand, stones, smashed mollusc shells, on macrophytes and wooden structures, and in amongst colonies of zebra mussels (Dreissena polymorpha). Population density may be as high as 1,000 ind. m-2 (Dediu 1980), though densities of between 1-140 ind. m-2 is typical (Dediu 1967). Thus if the species were to establish in the Great Lakes, it would almost certainly occur at very low density with little ecological impact. By contrast, C. curvispinum may achieve population densities as high as 200,000 to 750,000 ind. m-2 on stone substrates after it invaded the lower River Rhine, and adversely affected D. polymorpha via competition for food and space (van den Brink et al. 1993).
Recent mass invasions of the Great Lakes by Ponto-Caspian species suggest that ballast water management may be insufficient to prevent invasive species from reaching North American freshwater habitats. Appearance of the sexually reproducing amphipodCorophiummucronatum, lacking both pelagic stage and parthenogenetic reproduction, appeared to be related to its high salinity tolerance. Tolerance of brackish or saline conditions permits many of these species to survive in estuarine habitats where ballast water is frequently loaded, again increasing the likelihood of transport to North America. Prevention of further invasions of Ponto-Caspian species into the Great Lakes will require examination of the efficacy of the current ballast water exchange program.