Web Page Produced for:
New Zealand Secondary Schools
Extracts from UE, Bursaries and Scholarship Biology Prescription
1. Investigate and develop an understanding of diversity, structure, function, and interrelationships of living organisms, and their interactions with the abiotic environment.
Investigate and identify aspects of animal behaviour and plant
responses in relation to biotic and abiotic environmental
Investigate an aspect of the ecological niche of an animal and a plant.
Investigate and describe gene expression
Investigate and explain speciation and identify patterns of evolution, with emphasis on New Zealand examples.
Adaptive radiation within one group of organisms should be studied.
As the pieces of Gondwana continued to move away from each other, the Antarctic continent shifted to its now polar position, Drakes passage opened up and the circulation pattern of the Southern Ocean developed. The study of the origins of the circulation pattern of the Southern Ocean has been the subject of a recent deep sea drilling program off the Chatham Rise. This led to Antarctica as the place we know today, a frozen continent surrounded by ice-covered seas. What happened to the temperate fish species that originally inhabited Antarctic coastal waters? We now know that virtually all of these species were wiped out! One species survived probably because of the evolution of antifreeze. Antifreeze evolved from a precursor of one of the stomach enzymes (trypsinogen. This ancestral species founded a sub-order of fishes known as the Notothenioidei. Because of the known time scale of the radiation of this group, and the very cold conditions of the Antarctic seas, these fishes form a model system for the study of issues of evolution, and adaptation to environment.
Family Tree of Antarctic Fishes
In McMurdo Sound, the most common notothenioids are the group Nototheniidae, these are the so-called Antarctic cod. The top 3 pictures shown below are fish from this group. They include (left to right) Trematomus hansoni, Pagothenia borchgrevinki, and the giant Antarctic cod Dissostichus mawsoni. On the bottom row are members of other families, the dragonfish Gymnodraco acuticeps, the plunderfish with the barbel on its chin, and 1 of the icefish (Channichthyidae).
The Channichthyidae have no haemoglobin in their blood, so the blood is colour-less. Haemoglobin is an oxygen carrying pigment. Most adult vertebrates cannot survive without its assistance. It is only the very cold waters of Antarctica which make this possible. At some point, right at the start of the evolution of this group the ability to make haemoglobin was lost. This loss was probably detrimental, but not lethal. We can tell this because subsequent evolution has increased the blood volume, and the size of the heart to make good the loss. This is one of the very few instances where we can document the progress of adaptive evolution going backwards (a little), and then recovering.
Antifreeze is one of the most interesting adaptations to environment. Because the blood of fishes is less salty than that of seawater it would freeze at the normal freezing point of seawater (why is that?). This means that to stay unfrozen the fish need antifreeze in the blood just like antifreeze is added to your car radiator to prevent freezing. In fish the antifreeze is a glycoprotein.
Sensory Adaptations to the Antarctic Winter
The water temperature in Antarctica is right on the point of freezing all year round. It gets no colder in the water over winter, but it is dark. In order to survive the Antarctic winter the fish need to be able to feed in the dark. they have a specialised sensory system called the mechanosensory lateral line, which enables them to feel the motion of other animals in the water around them. All fish have this system, but it is particularly well developed in deep sea fishes and in the Antarctic fish. In the picture of Trematomus hansoni, you can see the lateral line running down the side of the body. It consist of patches of hair cells covered by a dome. Water pushing on the dome deflects the hair cells and this message is sent by nerves to the brain. The system exists as superficial patches on the surface of the skin (left diagram) or patches sunk within canals (right diagram).
The last article on this page is an abstract of a paper in the 1998 SCAR Symposium (Antarctic Ecosystems: Models for Wider Ecological Understanding Eds W. Davison, C. Howard-Williams and P. Broady (2000), Caxton Press. Christchurch, 2000, ISBN 0-473-06877-X).
ADAPTIVE EVOLUTION OF ANTARCTIC FISHES
John Montgomery, School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand
The morphological and physiological attributes of animals change through time. Adaptive evolution is the component of this process where selection pressures have shaped change in particular directions. However, objectively recognizing "adaptive evolution" as opposed to other mechanisms of change is a fraught business. Fortunately, the Antarctic fish fauna provides an excellent system in which these issues can be explored. Notothenioid fishes form a monophyletic radiation that dominates the Antarctic shelf and upper slope ichthyofauna. This phylogenetic grouping, coupled with reasonable historical re-constructions of Antarctic climate and the generally extreme nature of the environment all contribute to the possibility of identifying important selection forces and resulting adaptive responses. This review discusses the general nature of the adaptations, and the confidence with which we can label particular attributes "adaptive". It will also attempt to put aspects of the adaptive evolution of Antarctic fishes into a comparative context. Discussions of adaptive evolution in Antarctic fishes have centered around three areas: adaptation to low temperature; re-invasion of pelagic habitat; and adaptation to low light. This review will pose the questions: How do low temperature adaptations of Antarctic fish compare with low temperature adaptations found elsewhere in Nature? What are the suite of characters that describe pelagic fish in other oceans, and to what extent are these matched by notothenioid pelagic species? How do the sensory systems of Antarctic fish compare with those of deep-sea fishes? This comparative perspective provides a backdrop of "what is possible elsewhere" against which to view the nature and the extent of the adaptations of Antarctic fishes.