Australia: The Land Where Time Began

A biography of the Australian continent 

Fish Origins

Long suggests that the large shape changes seen in the life cycle of certain invertebrates appears to be linked intimately with the origin of fish. There are usually large morphological differences between the juvenile stage and adult stage of fish, as well as invertebrates. Adult sea squirts have the appearance of a baglike structure attached to a rock and feed by filtering sea water for tiny particles of edible material. The fact that they can squirt seawater when they are picked up indicates that they have a coordinated muscular system. The larvae of sea squirts are quite different from the adults, being free-swimming and shaped similar to a tadpole in which there are fin rods supporting a muscular tail, and a cartilaginous rod, a notochord, stiffens the body. They also have gill slits and a dorsal nerve cord that is hollow, which make them similar to early fishes.

When the lifestyle of an organism changes, developmental changes may be precipitated in their bodies. This occurs when a sea squirt larva moves from open water to shallow, settling on the substrate.

Echinoderms such as sea stars, urchins, etc., and other groups of invertebrates also have larval phases that are mobile that also settle onto the sea bed to become sessile or slow moving forms when they become adult. A number of theories have been proposed for the origins of the first fish that involve the free-swimming larvae of the invertebrates mentioned above. These theories suggest that the fish arose by early sexual maturation of the swimming larvae, paedomorphosis, the adult continues to be free-swimming instead of settling to become sessile organisms. The author suggests the true origins are not as simple as is proposed by these theories, as not all invertebrate larvae have a notochord and are capable of developing some types of tissue known to be unique to vertebrates. Among all vertebrates a universal feature is the derivation of hard tissues from embryonic neural crest cells.

Ammocoetes are the larva of Petromyzon marinus, a marine lamprey. They live in burrows in sandy river beds for about 3 or more years before metamorphosing, after which they migrate down the rivers to the sea where they take another 3-4 years to become sexually mature. The basic anatomical structure of the ammocoete larva is not very dissimilar from the anatomical structure of Branchiostoma (cephalochordate) or urochordates (tunicate larvae). The author suggests the link to the first fish is quite clear. It has been determined by recent molecular analysis that sea squirts are closer to fish origins than either Branchiostomes or echinoderms.

According to Long all the steps in the evolution of the first fish have now been demonstrated in the fossil record.

Sources & Further reading

  1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 2011.


Last updated 26/10/2011 


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