FINS IN FISHES

Introduction
• Fins are chief organs of locomotion in fishes.
• These are folds of skin or projections from body surface &
supported by fin rays .
• Fin rays may be bony ,cartilaginous ,fibrous or horny .
Types of fins
2 types of fins
Unpaired/median and Paired/lateral

Unpaired & Paired fins are supported by skeletal rods called

radials & dermal fin ray .
Unpaired fins
• Include one or two dorsal fin along mid dorsal line, a ventral
anal fin behind anus or vent (cloaca) and caudal fin around
the tip of tail.Dorsal may be in a series or reduced or absent.
Anal may be absent esp. in bottom dweller
Paired fins
• Paired fins include pectoral and pelvic corresponding to fore
& hind limbs of terrestrial vertebrates. Posterior pelvic called
thoracic when placed below pectoral & abdominal /ventral
when in front of anus
Use of fins
• Fish swim by lateral movement of tail fin .
• Paired lateral serve to maintain equilibrium .
• caudal fin serve as rudder .
• Lung fish use them as legs in walking .
• Exocoetus use pectoral fin for gliding
• Pelvic in male chondrichthyes modified as claspers .
Cont…
• In Echeneis anterior dorsal fin form sucker on head .
• In some teleosts anal fin form intromittent organ or ovipositor
Origin of fins
• Median fins are believed to have originated from continuous
fold of tissue. Fold extend from posterior region of head &
continues posteriorly around tail & extend upto anus. Fold of
skin supported by series of parallel cartilaginous rods. The
rods divided into lower basal piece in body wall & upper
radial piece lying in fin fold. From fin fold D, C & A-- fin
have evolved by restriction of radial at certain area &
degeneration of fold between them .
Theories to explain origin of paired fins
• Gill arch theory
• External gill theory
• Fin fold theory
• Fin spine theory
• Ostracoderm theory
Gill arch theory
• Proposed by Gegenbaur
• paired fins are modified gill structures , girdle represent gill
arches & Fin fold with skeleton represent gill flaps or septa .
• Posterior gill arch shift posteriorly forming pelvic fin .
• This theory is not supported by morphological ,
embryological & palaeontological evidences .
External gill theory
• By Graham Kerr
• Paired fins & skeleton derived from external gills which
occur temporarily in some larval forms .

(no supporting evidence)

Fin fold diagram
Fin fold theory
• By Balfour & Thacher & supported by Mivart.
• The continuous median & lateral folds of body wall were
present in some ancestral fish.
• The fold were separated in anterior part & fused in caudal
region forming ventral fin fold.
• It has anatomical & embryological evidences.
Cont…
• The fold became enlarged at certain portion & intermediate
portion disappeared.
• A pair of pectoral near gill arch ,a pair of pelvic near anus , a
median dorsal & caudal fin also developed.
Evidences to support this theory
1. Skeletal structure of paired & unpaired fin similar in basic
plan indicating common mode of origin .
2. Embryos of certain elasmobranch , early stage show
continuous series of muscle band which disappear except at
paired & unpaired fin .
3. In extinct Devonian acanthodian shark a row of numerous
small accessory spiny fin on either side between pectoral &
pelvic fin as remnant of fin fold .
Cont…
4. In extinct shark Cladoselache pectoral & pelvic fin broad
without basal notches & supported by parallel cartilaginous
rods.
5. In Climatius there was series of small spines & finlets
between pectoral & pelvic fin.
Fin spine theory
• Proposed by Gregory & Raven –supported by
palaeontological evidences .
• Several pairs of fins in acanthodians as defensive spines
often support membranous structures .
• All spines & webs lost except anterior & posterior pair
which later evolved into pectoral & pelvic fins for
locomotion .
Cont…
• Fins appeared from spines & membranous structures
between spines & body wall .
Ostracoderm theory
• Some ostracoderm possessed lateral fleshy lobes & dermal
spines projecting from either lateral sides & developed to
pectoral and pelvic fins .
Caudal fin
• It plays role in forward propulsion during swimming .
• Caudal fins are Diphycercal / Protocercal ,Heterocercal &
Homocercal
• In Hippocampus tail is prehensile .
• It is reduced in eel, rays & lacking in sting ray .
Diphycercal ( protocercal )
• Most primitive in which vertebral column extends upto tip of
tail & divide caudal fin into 2 equal halves dorsal
epichordal & ventral hypochordal lobe.
• Both lobes equal in size & symmetrical .
• It occurs in cyclostomes, primitive sharks, Holocephali ,
Dipnoi, Latimeria ,larval teleost .
Heterocercal
• Intermediate type, vertebral column bend upward & reach
upto tip of prominent dorsal lobe & make caudal fin
asymmetrical .
• Found in modern elasmobranch , chondrosteans, extinct
dipnoans & osteolepid .
• Characteristic of bottom feeder with ventral mouth ,without
swim bladder.
Cont…
• Stroke of dorsal lobe direct fish towards bottom.
• Opposite of it hypocercal found in Exocoetus & Ostracoderm
Homocercal
• Most common
• Found in teleosts.
• Externally symmetrical but internally asymmetrical .
• Vertebral column turned upward & greatly reduced .Tip of
which do not reach posterior limit of fin .
• In deep sea form [chimaera] vertebral column straight &
elongated forming Isocercal .
•
Cont…
• Caudal disappear completely forming –Gephyrocercal in
fieraspis .
• In abbreviated homocercal caudal fin --vertebral column a bit
elongated & upturned with or without fleshy lobe .
• It is intermediate between hetero & homocercal type &
found in Polypterus ,Amia & Lepidosteus .
• Protocercal is considered primitive ,heterocercal intermediate
& homocercal advanced .