fluctuating biological interactions, with shifts in osmolarity, food sources, predators, etc. Since many adult Alosa species die after spawning, only the young generally migrate to the sea from the spawning grounds. Duration of migration varies among fish, but can greatly affect survival. Reproduction varies by species. Studies done on Alosa in Iranian waters have shown that spawning varies in time, place, and temperature of the waters they inhabit. Fecundity may also vary. Species are known to spawn as early as April or as late as August. Temperatures range from about 11 to 27 °C. Fecundity can range from 20,000 to 312,000 eggs. Eggs are pelagic. Geography and temperature are important environmental factors in egg and young-of-year development. The lifespan of Alosa species can be up to 10 years, but this is generally uncommon, as many die after spawning. Systematics The systematics and distribution of Alosa shads are complex. The genus inhabits a wide range of habitats, and many taxa are migratory. A few forms are landlocked, including one from Killarney in Ireland, two from lakes in northern Italy, and two in Greece. Several species are native to the Black and Caspian Seas. Alosa species of the Caspian are systemically characterized by the number of rakers on the first gill arch. They are classified as being "multirakered", "medium-rakered", or "oligorakered". The multirakered are primarily plankton feeders, the oligorakered have large rakers and are predators, and the medium-rakered generally consume a mixed diet. Most current species of the genus Alosa in North America can be found in Florida, whereas the distribution of most of them is broader. Morphology is notoriously liable to adapt to changing food availability in these fish. Several taxa seem to have evolved quite recently, making molecular analyses difficult. In addition, hybridization may be a factor in shad phylogeny. Nonetheless, some trends are emerging. The North American species except the American shad A. sapidissima can probably be separated in a subgenus Pomolobus. Conversely, the proposed genus (or subgenus) Caspialosa for the Caspian Sea forms is rejected due to paraphyly. Species by geographical origin North America Alosa aestivalis (Mitchill, 1814) (blueback herring) Alosa alabamae D. S. Jordan and Evermann, 1896 (Alabama shad) Alosa chrysochloris (Rafinesque, 1820) (skipjack shad) Alosa mediocris (Mitchill, 1814) (hickory shad) Alosa pseudoharengus (A. Wilson, 1811) (alewife) Alosa sapidissima (A. Wilson, 1811) (American shad) Western Europe and the Mediterranean Alosa agone (Scopoli, 1786) (agone) Alosa algeriensis Regan,