Taxonomy of Finfish (1+2)

6.1.1 Introduction

From about 20,000 fish species estimated to occur, the basic karyotype characteristics, i.e., diploid chromosome number (2n) and the number of chromosome arms (NF), are known for not more than 3,000 fish species, which represents only about 15% of the total number.

The first investigations of chromosomes were made by means of histological sections in the late nineteenth century. Then, squash techniques were performed which permitted a more accurate count of chromosome number. Later, the air-drying technique combined with colchicines and hypotonic treatment (to accumulate cells at metaphase and to ease chromosome spreading) greatly increased our knowledge not only of chromosome number, but also of chromosome morphology. Karyotyping, i.e. paring of the chromosomes according to their size and morphology, then became feasible. Recent development of new differential staining techniques allowed the demonstration of characteristic banding of chromosomes. These techniques have led to a better understanding of the fine characteristization of individual chromosomes and to a better standardization of karyotypes.

Aquatic invertebrates have been investigated mostly with squash or air-drying techniques from tissues, while in fish, since 1960 there has been a boom in cytogenetic studies mainly due to the application of human cell culture technique. Because of this, there is more information on banding techniques in fish than in invertebrate species.

There has been a biased recording of chromosome numbers in aquatic organisms depending on the group studied. Looking at the aquatic invertebrates, some phyla are poorly investigated, e.g. the sponges or the cnidarians. With regard to minor pseudocoelomate or eucoelomate animals, published data are even more rare.