We have met quite a few extreme organisms during the course of this book. They are found among all the major groups (Kingdoms or Domains) of organisms (plants, animals, fungi, protists, bacteria and archaea). However, within these groups, often only certain species, or groupings of species (phyla or lower taxonomic groupings), have the capacity to survive in extreme environments. Among the microbes, the Archaea are the organisms which can survive the most extreme environmental stresses. Some groups of bacteria, such as the cyanobacteria, are also prominent in extreme environments. Prokaryotes (bacteria and archaea) are generally more resistant than are eukaryotes (plants, animals, fungi and protists). In the Animal Kingdom, only certain phyla and species within phyla have solved the problems of living at extremes. Let us look at the distribution of extreme organisms within the Animal Kingdom to see if any patterns emerge.
Table 8.2 is a listing of some animals found in the most extreme environments and the stresses that they survive (based largely on examples from this book). While this is an incomplete list, some patterns are apparent. Only a few groups of animals live in extreme environments. Nematodes, arthropods and vertebrates (particularly mammals and birds) feature most prominently. This may simply reflect the fact that extreme terrestrial environments are colonised by
Table 8.2 Animals surviving in extreme environments
(see Chapters 1 and3)
(see Chapter 4)
Surviving or avoiding freezing
(see Chapters 2 and 5)
Pressure (see chapter 6)
(see Chapter 6)
(see Chapter 6)
Anhydrobiotic nematodes, rotifers, tardigrades, insect larvae and crustaceans Non-anhydrobiotic invertebrates Mammals, birds, reptiles and amphibians
Nematodes and insect larvae from hot springs Annelids from hydrothermal vents Anhydrobiotic animals
Nematodes and other invertebrates
Amphibians and reptiles Mammals and birds
Crustaceans, insect larvae
Various nematodes, rotifers and tardigrades
Polypedilium vanderplanki, Artemia Ants, termites, beetles, grasshoppers and snails Camels, kangaroo rat, ostrich, dune lizards and spadefoottoad Sphicterochila boisseri (desert snail) Ocymyrmex barbiger (desert ant) Chironomid larvae
Alvinellapompejana (the Pompeii worm) See above under Desiccation
Numerous species of mites, springtails and insects Antarctic nematodes, potato-cyst nematode, earthworm cocoons, enchytraeids, intertidal molluscs and anhydrobiotic animals Notothenioids, Arctic cod Hatchling painted turtles, wood frogs Polar bears, musk oxen, lemmings, penguins and ptarmigan
Vinegar eelworm Artemia, brine fly larvae those groups of animals that are most successful at colonising terrestrial environments in general. Extreme aquatic habitats support a wider variety of animal groups, but even here only a few species have solved the problems of coping with high temperatures, low pH and high levels of salinity. The problems of coping with high pressures in the deep sea have been solved by a wide variety of invertebrates and fish, but, as I will argue below, perhaps we should not consider the deep sea to be extreme.
It seems that there are two broad groups of organisms in extreme environments. Organisms that are small and simple (bacteria, archaea, fungi, algae, protists, nematodes, rotifers and tardigrades) have limited abilities to control the conditions within them. Their cells are more likely to experience the physical conditions of the environment that surrounds them than are the cells of organisms that are larger and more complex. These simple organisms must rely on their membranes and enzymes being adapted to work in the extreme conditions, the perhaps limited regulatory abilities of their cells (capacity adaptations) and their ability to survive periods of stress by becoming dormant (resistance adaptations, such as anhydrobiosis).
The second group of organisms (birds, mammals and arthropods) are larger, more complex and appeared later during the evolution of life on Earth. Their complexity allows them to regulate their internal conditions so that they are more independent of the conditions in the external environment. Insects can retain water in a desiccating environment since they have a waxy cuticle and other ways of controlling water loss. Birds and mammals can regulate their internal temperatures, so that it is remarkably constant even in hot deserts or in cold Antarctic conditions. These abilities to regulate the internal environment in the face of extreme and changing external conditions (homeostasis) can be considered to be capacity adaptations to extreme environments. These animals also have behavioural and physiological mechanisms that enable them to avoid the most extreme conditions experienced in their habitats. Hibernation, other forms of dormancy, migration and restricting activity to the night in deserts are all ways of avoiding exposure to the most extreme conditions.
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