Ecology Laboratory - ENCM 21062
Practical No. 15 - Freshwater Vertebrates
Adaptations seen in a small number of freshwater vertebrates are given in this hand-out. The students
should also refer to all freshwater vertebrates studied under Animal Diversity Laboratory.
Learning outcomes: At the end of the practical session, students should be able to
(1) recognize the animal (common name is adequate) and describe morphological features of each
animal that are adaptations to the aquatic mode of life
(2) discuss the role of each animal in the functioning of freshwater ecosystems
A. Fish
1. Stone Sucker (Gara ceylonensis)
(Order: Cypriniformes, Family: Cyprinidae)
An endemic species, distributed throughout the country; inhabits benthic substrates. Body is sub–
cylindrical with a flattened ventral surface. Pectoral and pelvic fins with broad bases (act as supports
when settling on the substrate). The lower lip has an adhesive sucker (which facilitates the
attachment to the substrate). Feed mainly on benthic microalgae growing on rock surfaces. Ventral
mouth has a rough brush on upper lip (to scrape algae). This animal has an ability to camouflage.
2. Climbing Perch (Anabas testudineus)
(Order: Percifornes, Family: Anabantidae)
Generally lives in still water habitats such as marshes, reservoirs, ditches, ponds and paddy fields. A
carnivorous fish, mainly feeds on aquatic insects, fish eggs and larvae. Mouth bears prominent teeth.
Gill rakers are poorly developed. An accessory air breathing organ (i.e. Labyrinthine organ) lies
within the opercular cavity (enables the tolerance of poor oxygen environments and breathing
atmospheric air). It can live out of water for short periods and cross land from one water body to
another, usually, at night. Opercular edges of this fish are armed with sharp spines (facilitate
terrestrial movements).
3. Level – finned Eel (Anguilla bicolor bicolor)
(Order: Anguilliformes, Family: Anguillidae)
A nocturnal fish; generally inhabits aquatic vegetation, rock crevices, boulders in the rivers and
reservoirs. Very long and flexible body is adapted well for swimming through rock crevices and
aquatic vegetation. Dorsal and anal fins are very long and combined with the caudal fin. Undulating
movements of long fins facilitate backward movements. Body is covered with very tiny scales and a
thick layer of mucus (cutaneous respiration is facilitated through the skin and therefore can tolerate
poor oxygen levels). They migrate to the sea for breeding and larval forms enter rivers and grow to
maturity in the freshwaters. It has a wide gape and minute teeth in mouth (a carnivore).
4. Walking Catfish (Clarias brachysoma)
(Order: Siluriformes, Family: Clariidae)
An endemic species. Found in reservoirs, marshes and flowing waters. It lives in the bottom and
shaded areas with aquatic vegetation and rocks. It has four pairs of long barbels (for olfaction and
tactile sensation). An accessory air breathing organ lies in the opercular cavity (facilitates short
distance movements on land while breathing atmospheric oxygen).
5. Bar-eyed Goby (Glossogobius giuris)
(Order: Perciformes, Family: Gobidae)
This is widely distributed in streams, rivers and reservoirs in the lowlands. A bottom-dwelling
species with a sub-cylindrical body. The ventral surface is flattened and pectoral fins are broad (act
as supports when settling on the bottom). The pelvic fins united to form an adhesive sucker (use to
attach to the substrate). It has a superior mouth, broad gape and well-developed teeth (carnivore).
Body colours can be quickly changed according to the surroundings (camouflage).
6. Dwarf Panchax (Aplocheilus parvus)
(Order: Cyprinodontiformes, Family: Aplocheilidae)
This is a small fish that inhabit shallow waters such as marshes and paddy fields. It is normally found
close to the water surface. The dorsal surface of the body is flattened and the dorsal fin is set far back
on the body. Mouth is superior and mainly feeds on surface living small animals, small terrestrial
animals and mosquito larvae.
B. Amphibians
1. Six –toed Green Frog (Euphlyctis hexadactylus)
(Order: Anura, Family: Ranidae)
This animal is widely distributed in low country reservoirs, rivers and marshes. Hind legs with welldeveloped muscles and webbed feet facilitate swimming in the waters. It is able to camouflage in a
shaded green surrounding (colour of aquatic vegetation). They are omnivorous and usually feed on
insects and plant matter. It has a wide gape and a large protrusible tongue. The tadpole has external
gills for aquatic respiration, long muscular tail and a caudal fin for swimming, a ventral sucker to
attach to the substrate and horny jaws to scrape plant matter.
C. Reptiles
1. Swamp Crocodile (Crocodylus palustris kimbula)
(Order: Crocodilia, Family: Crocolilidae)
The largest reptile found in freshwaters. It is generally found in swamps, tanks and rivers. The
external nasal openings are in upper position; when swimming they keep this above the water level.
When diving, the nasal openings are closed with a valve. The bilaterally flattened tail produces the
propulsive force when swimming and diving. The streamlined body and webbed feet also facilitate
swimming and diving. The increased lung capacity helps them to maintain buoyancy. Eyes are
covered with transparent lids and ears are covered with flaps when they swim and dive. Young feed
on insects and small animals but as they grow older they feed mainly on fish. They have elongated
jaws with sharp pointed interlocking teeth and a wide gape. The vision and olfaction well developed.
2. Water Monitor (Varanus salvator)
(Order: Squamata, Family: Varanidae)
Normally found in marshes, paddy fields, streams and reservoirs. They are excellent scavengers and
feed on both dead and live freshwater crabs, insects, snakes, rats and domestic animals. The
bilaterally flattened tail produces the propulsive force when swimming and diving. The stream-lined
body and webbed feet also facilitate these activities. The increased lung capacity helps them to
maintain buoyancy. The external nasal openings are in an upper position; when swimming they keep
this above the water level. When diving, the nasal openings are closed with a valve.
3. Chequered Keelback (Natrix piscator)
(Order: Squamata, Family: Colubridae)
This snake is common in all types of freshwater habitats especially where the surrounding vegetation
is damp. It is non-poisonous and harmless. It mainly feeds on small fish and amphibians and has a
wide gape. It maintains the buoyancy while in water due to the increased lung capacity.
D. Birds
Oriental Darter (Anhinga melanogaster)
(Order: Pelicaniformes, Family: Anhingidae)
Legs are short and webbed (useful in swimming and diving). Commonly found in the reservoirs and
rivers of low country dry zone. Rarely found in wet zone or in hills. It has a very long thin kinked
neck and very thin pointed bill for hunting fish, the major food source of the bird. The body is boatshaped and legs placed at far posterior position, (act as paddles) facilitates swimming and diving.
E. Mammals
The Ceylon Otter (Lutra lutra ceylonica)
(Order: Carnivora)
An endemic species; generally found in and near rivers, lakes and paddy fields. Its body is
streamlined and ears are very small; more or less water proof due to the short fur on the body. Limbs
are short and the toes are webbed for swimming. The tail is thick at the base and flattened. It is an
excellent swimmer, capable of diving and travelling long distances under the water. It is nocturnal
and generally feeds on aquatic animals such as insects, crabs, frogs, waterfowl and small rodents.
Questions:
Describe how each animal given under (A) – (E) contribute to trophic dynamics and energy flow in
the freshwater ecosystems.