FINS

Structure and types

The fins constitute the major locomotory organs in fishes. These
are either folds of skin or projections from the body surface. The
fins are supported by fin rays. These supporting rays may be bony,
cartilaginous, fibrous or horny. There are mainly two types of fins
in fishes: (1) unpaired or median fins, and (2) paired fins
The unpaired fins include dorsal fin on the back, an anal
fin on the ventral side behind the vent and a caudal fin at the
end of the tail. In some species, an adipose fin is present between
the dorsal and caudal fins. The paired fins are the pectorals and
pelvics or ventrals corresponding to the fore and hind limb,
respectively of the terrestrial vertebrates. In some species, tiny fin
like structures, known as finlets, are present between the dorsal
and the caudal fins or between the anal fin and the caudal fin

Finlets are generally without fin rays and
non-retractable. The shape, size, color and position of the fins
are greatly varied among fishes. The diversity in the fin system in
fishes is due to their adaptive responsiveness.

Origin of fins
It has been believed that fins of fishes are evolved from the ancestors
that had no fins at all. Presumably, ancestral forms responded to
movement in water and developed a median ridge or keel on the
body, from near the head around one end of the tail to the anus.
These ridge, perhaps initially without supporting rays, gave rise to
median or unpaired fins. Similar ridges appearing on the sides and
then coalescing at the proper sites have been presumed further to
be the ancestral antecedents of the paired fins.

Origin of unpaired fins
The unpaired fins in fishes are held to be originated from a
continuous fold of tissues. This fold extends from the posterior
region of the head and continuous posteriorly around the tail and
forward up to the anus. The fold is supported by series of parallel
rods. In course of development, each supporting rods divides into
a lower piece, i.e., basal, embedded in the body wall and an upper
piece laying in the fin-fold, i.e., radial. From such continuous
fin-fold, the dorsal, caudal and anal fins have been evolved by
restriction of the radials at certain areas and the progressive
degeneration of the fold between them.

Origin of paired fins
Most of the Ichthyologists are convinced about the derivation of
the unpaired fins from the continuous fin-fold, but the origin of
paired fins has been much debated, and many theories have been
put forward in explanation. Two major conventional theories are:
(1) Gill-arch theory, and (2) Fin-fold theory.

Gill-arch theory
This theory was put forward by Carl Gegenbaur (1898), a German
Anatomist. According to this theory, the paired fins with their
girdles are derived from the gill arches. The two pairs of gill arches
transformed into the pectoral and pelvic girdles and their gill rays
gave rise to skeleton of the fins. Actually, this theory was proposed
on assumption that the primitive gnathostomes possessed gill
arches extending further behind than they are at present.
Soon on the basis of this theory the direction of fins has evidently been
dorso-ventral and both the pectoral and pelvic fins were very close
together behind the head. However, the present position of pelvic
fins is explained on the assumption that some of the posterior gillarches
have been shifted posteriorly. This hypothesis has found no
support from the embryological, morphological, placontological
and molecular studies. It has been confirmed that the endoskeletal
girdles of paired fins never exhibit characteristics of gill arches.

Fin-fold theory
According to this theory, the paired fins are derived from
longitudinal lateral folds of epidermis extending backward along
the body from just behind the gills to the anus (Figure 4.14). By
accentuation of the anterior and the posterior, and suppression
and reduction of the intermediate portions of the folds the
pectoral and the pelvic fins were formed. Into these folds, muscle
buds migrated from the ventral border of the adjoining myotomes,
giving rise to radial muscles. The muscle buds disclosed a
metameric arrangement and derived their nerve supply from
ventral roots of the spinal nerves. There are many placontological
and embryological evidences in favor of the origin of paired fins
from the lateral fin-folds.

Fin rays
The fins of fishes are supported by dermal rays or spines. The
dermal fin rays of elasmobranchs and bony fishes are known
collectively as dermatotrichia or lepidotrichia. These supporting
rays may be bony, cartilaginous, fibrous or horny. They may be
segmented or unsegmented, branched or unbranched, biserial
or uniserial. The movements of the fins are due to the action of
the muscles, these movements beings possible because of the
articulations and often flexibility of these rays. In teleosts, the fin
rays are of two types

1. Soft rays: These are thin, flexible, most often branched, Y-shaped,
segmented and always biserial (two lateral components paired
on the midline). These are called lepidotrichia.
2. Hard rays (spines): A number of soft rays united solidly to
compose hard rays which are stout, rigid, unsegmented,
uniserial and sometimes sharply pointed. These are called
actinotrichia.

Types of caudal fin
Of the unpaired fins, the caudal fin plays the most important role
in forward propulsion during swimming. The caudal fin differs
from the dorsal and anal fins in the nature of its supporting
skeleton. Four major types of caudal fins have been reported in
different fishes.
1. Protocercal or diphycercal
This type of caudal fin is regarded to be the most primitive type. The
posterior end of the vertebral column exrends up to the tip of the
tail and divides the caudal fin into two equal halves (Figure 4.16-
A). The dorsal half is called epichordal lobe and the ventral one
is known as hypochordal lobe. The epichordal and hypochordal
parts of the caudal fin are equal in size and symmetrical. It is
found in Amphioxus and Cyclostomata.
2. Heterocercal or epicercal
In this case, the posterior end of the vertebral column bends
upwards. As a consequence the caudal fin is divided into two
unequal halves. The vertebral column is bent upwards and
continues almost up to the tip of the fin (Figure 4.16-B). The
epichordal part is greatly reduced while the hypochordal lobe
is specially enlarged to make the caudal fin asymmetrical both
internally as well as externally. This type of caudal fin is found
in elasmobranches, extinct crossopterygian and primitive
actinopterygians.
3. Hypocercal
This type of caudel fin is seen in the fossil Agnatha. The tail is
asymmetrical, as the vertebral column is bent downwards so that
the epichordal lobe is larger than the lower hypochordal lobe
(Figure 4.16-C). It is also known as ‘reversed heterocercal’. This
type of caudal fin is not present in any modern fishes.
4. Homocercal
This type of caudal fin is the characteristic of the higher bony fishes.
The fin is symmetrical externally but internally it is asymmetrical.

The posterior end of the vertebral column is turned upwards and
becomes greatly reduced (Figure 4.16-D). The tip of the vertebral
column does not reach the posterior limit of the fins. There is no
apparent dorsal lobe but the ventral lobe is greatly enlarged and
divided into two equal superficial lobes.
In many teleosts belonging to Anguilliformes, Clupeiformes,
Osteoglossiformes and some others, the tail is tapering and
symmetrical. This is not a truly protocercal and is called isocercal or
leptocercal. During development, the hypochordal lobe is greatly
reduced, while the dorsal and the anal fins become elongated so as
to form a contineous fin.
The protocercal or diphycercal type of caudal fin is considered as
the most primitive type, the heterocercal as the intermediate stage
and the homocercal condition represents the advanced stage. In
most teleosts, the caudal fins start as diphyceral then becomes
heterocercal and finally becomes the homocercal condition. The
transition of three types of caudal fin in the development history
of fishes is significant from the phylogenetic point of view.

Functions
1. The principal function of the fins is regarded for locomotion in
the form of progression, steering or balancing.
2. The basic function of a rudder is fulfilled by the pectoral and
pelvic fins. These fins permit the fish to steer, stabilize, and stop.
3. The dorsal and anal fins do not perform the function of forward
movement of the fish. But they assist in upward and downward
turning. They also serve to stabilize the fish while it is swimming
and stop the fish from rolling over.
4. The caudal fin is used to propel the fish forwards. Fish that have
forked caudal fins are fast-swimmers. Fish that have rounded
caudal fins are fish capable of quick action like predators.
5. In some fishes (e.g., Colisa spp.), the pelvic fins prolonged into
trailing filaments serving as tactile organs.
6. In some fishes (e.g. Gambusia affinis), the anal fin of male is
modified into gonopodium to briefly hook into the vent of a
female fish to deposit sperm.
7. In some hill-stream fishes, pectoral and pelvic fins are modified
to form an adhesive surface for attaching them to the stones
and rocks of the river.
8. In some fishes, pectoral fins are modified as long, pointed and
serrated spines, containing poison glands, which act as defense
organs (e.g., Heteropneustes fossilis).
9. The climbing perch (Anabas spp.) can walk on land or even climb
up the trees with the help of its pectoral fins and operculum.
10. In the anglerfish (e.g., Bufoceratias wedli), the dorsal fin has
been modified into a fishing rod and lure.
11. The dorsal fins have been modified in the suckerfish (e.g.,
Remora spp.) into a sucking disc that allows them to cling to
sharks or other large fishes.
12. In some fishes, the spines present in the fin acts as a sound
producing organ by stridulating.
13. In some species, paired fins are used for nest building and to
hold the eggs during spawning.