Temperature responses and distribution of Australian species of Cladophora (Cladophorales: Chlorophyta)

Abstract

Temperature ranges for survival, growth and sporulation of isolates of 11 Cladophora species from Australia, as well as one isolate from Japan, were tested in constant temperature conditions from 0 to 35°C at 5°C intervals over 3 months. These ranges were compared with those previously determined for Cladophora isolates from the North Atlantic Ocean. The Australian endemic species, Cladophora feredayi Harvey, Cladophora valonioides Sonder and Cladophora rhizoclonioidea van den Hoek et Womersley, had stenothermal responses. They are unable to survive below 5–10°C or above 20–25°C and have a very narrow growth range (∼ 15–20°C) which confines them to the southern coasts of Australia. Another group of species occurs in both hemispheres, or has closely related “sister” species in each hemisphere, but is absent from the intervening tropics. Cladophora att. ad pellucida, Cladophora hutchinsioides van den Hoek et Womersley and Cladophora flexuosa (Mueller) Kuetzing from Australia, were less cold tolerant and/or more heat tolerant than their North Atlantic counterparts ( Cladophora pellucida (Hudson) Kuetzing, Cladophora hutchinsiae (Dillwyn) Kuetzing, C. flexuosa and the related species, Cladophora sericea (Hudson) Kuetzing). A third group of species occurs in both hemispheres and is widely distributed through the tropics. Cladophora vagabunda (L.) van den Hoek, Cladophora laetevirens (Dillwyn) Kuetzing and Cladophora lehmanniana (Lindenberg) Kuetzing isolates from Australia, and Cladophora albida (Hudson) Kuetzing isolates from Australia and Japan had similar responses to isolates from the North Atlantic. In contrast, Cladophora montagneana Kuetzing, from Australia and Curaçao, had very different responses. Only two of the 11 species studied ( C. feredayi and C. vagabunda) have sufficient collections in Australia to show that they occur over the full potential geographic range allowed by their physiological tolerance. All other species could be expected to extend beyond their present known range on the basis of their temperature responses and the seasonal sea temperatures around the Australian coast.