The Fishes: Gasteropelecidae

CYPRINIFORMES - carps and characins: Page 5

Family: Gasteropelecidae

Family: Serrasalmidae

Gasteropelecidae
These are the unique and immediately recognizable hatchetfishes. This South American family belonging to the Characiformes is able to skim over the water surface to avoid predators. To achieve this feat the body has a deep keel and the pectoral fins are greatly developed. The family includes the genera Gasteropelecus, Carnegiella and Thoracocharax.

Gasteropelecus sternicla
This is probably the best known of the Amazonian hatchet fishes. It lives close to the water surface and tends to take food such as small flies from the surface. When alarmed it is able to rise up out of the water and fly above the surface using its long pectoral fins. The characteristic flash across the water surface accompanied by a rapid clicking, almost humming sound as the pectorals hit the water surface is frequently experienced when travelling along small channels by canoe. These fish have probably evolved this escape strategy to avoid predatory fish as they are frequently seen above the surface when large open water predators are feeding.

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A preserved specimen of Gasteropelecus sternicla, the common hatchetfish. This specimen was caught in a white water flood plain channel near Tefe

Carnegiella marthae
This is called the marbled hatchetfish because of its distinctive black and silver colouration. It is found throughout the Amazon in blackwater streams. It is a surface feeder that normally lives on small insects and possibly small pieces of flowers and even pollen. It can attain a maximum size of about 8 cm.

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Lateral view of a preserved specimen of Carnegiella marthae, the marbled hatchet fish. This specimen was caught in a small blackwater forest stream

Serrasalmidae
Some authors consider the Serrasalmidae to be a sub-family within the family Characidae with which they are clearly related. The Serrasalmidae, which includes the piranhas, is one of the most characteristic groups of S. America. Many have well-developed jaws and in the case of the true piranhas the teeth are razor sharp. The principal genera are Colossoma, Mylossoma, Myleus, Metynnis and Serrasalmus.

Colossoma macropomum
The tambaqui, as C. macropomum is called in the Amazon, is the second largest scaled fish in the region, only exceeded in size by the Arapaima. It can attain a length of more than 1 m and a weight in excess of 30 kg. Tambaqui are a fine food fish and are highly regarded throughout the Amazon. Unfortunately, fishing pressure is starting to reduce numbers and large fish are not as abundant as they once were. This species has a dentition like a horse and is also equipped with fine gill rakers. This allows it to feed on nuts and seeds, which it is uniquely able to crush, and zooplankton. This combination allows tambaqui to feed in the forest at high water and within floodplain lakes at low water. They breed in the main river channels within meanders where there are fallen trees and the young larvae drift down stream and enter the floodplain with the flood water. Initially, they enter the floating meadows where they can shelter and find food. Tambaqui is a predator when young.

Colossoma bidens
This is the smaller cousin of the tambaqui, although it is still a large fish that can attain a length of 90 cm and a weight of 20 kg. It feeds on fruits, seeds, insects and small fish within the floodplain and breeds in the main rivers. While generally viewed as a seedeater, I have caught this species by spinning, which suggests that it will occasionally actively pursue prey. Like the tambaqui, this is an important food fish for Amazonian peoples.

Colossoma bidens gut contents (P=present)
INDIVIDUAL NUMBER	1	2	3	4	5	6	7	8
LENGTH (cm)	12	10	12	9.9	13	13	14	14
MASS (g)	58	40	54	26	68	74	92	84
PISCES-FLESH				P
ARACHNIDA	1	P
CHIRONOMIDA		13	20	P
LEPIDOPTERA-LARVAE	2	1
ORTHOPTERA		P		P
DETRITUS			P
PLANT-SEEDS INDET.				P		P	P	P
CECROPIA LOTILOBA	P	P			P

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Lateral view of a recently caught tambaqui, Colossoma macropomum	Lateral view of a recently caught Colossoma bidens

Mylossoma
These beautiful deep bodied, laterally compressed fish shoal in open water in both black and white water floodplain lakes. They can grow to 20 cm in length although most individuals caught are considerably smaller. Although as adults they are essentially open water fish their shape adapts them for movement through submerged plants and the young live within floating meadow. They are plant and insect eaters.

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A preserved specimen of Mylossoma aureum	A Mylossoma species photographed soon after capture in Lago Mamiraua, near Tefe	A preserved specimen of a Mylossoma species juvenile	A painting made in the field by Peter Henderson to illustrate the damage done by piranha attack. The upper two paintings are of Mylossoma aureum and the lower two a species of Myleus

Myleus
These are abundant open water fish in river channels and floodplain lakes. They can grow to a length in excess of 20 cm. They are generalist feeders, but seem to always include plants in their diet. Their deep, laterally compressed body shape adapts them for movement between aquatic plants and the young live within floating meadow.

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A photograph of a preserved juvenile of a Myleus sp. caught in floating meadow

Metynnis
A deep bodied, laterally compressed group of fish which shoal in floodplain lakes and feed predominately on plants. They grow to 15-18 cm in length. They are adapted to move between plants and the young are found in floating meadow.

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A painting made in the field by Peter Henderson to illustrate the damage done by piranha attack. The illustration shows two different species of Metynnis, both caught in a white water lake near Tefe	Adult pacu, Metynnis species	Adult pacu, Metynnis species, showing a close-up of the head. This picture should be compared with a similar view of Serrasalmus nattereri	View towards the head of Serrasalmus spilopleura. This is a group hunting species that will take bites from almost any vulnerable animal within the water	Adult pacu, Metynnis species

Serrasalmus
This genus holds the famous piranhas, which are one of the most important fish groups of the Amazon basin. Because of their high abundance piranhas are an important food for people, dolphins and birds, and their predatory behaviour affects the lives of many other aquatic organisms, particularly fish. The power of their bite can hardly be exaggerated and their teeth are as sharp as razors. On a number of occasions when fishing with a rod and line I have caught a piranha which has escaped by biting through the shaft of my hook. While they can certainly inflict terrible bites on man, their reputation for turning animals to skeletons the moment they enter the water is exaggerated. They can be rather shy fish and like most freshwater biologists working in the Amazon I have frequently swum with them. The trick is to make sure no one has been in the habit of throwing food to the piranha in the area where you propose to dive or they may take you for another generous offering. Piranha are not obligate carnivores and will take a wide variety of foods. One of the most common and carnivorous of species is the red-bellied piranha, Serrasalmus (Pygocentrus) nattereri which is a general fish eater and scavenger (see table below). This species has to be regarded as essentially a floodplain species even though its young are also found in river margins. The piranha method of feeding has its special advantages in two circumstances: firstly, during the overcrowded conditions of a v�rzea low-water lake when the species attacks large prey in groups (as well as certainly continuing its normal and individualistic fin and tail biting habit); secondly, at high water within the forest where its dentition allows it to scavenge flood-stricken victims like a dog. Plausibly ancestors of Serrasalmidae at some point specialised from the plastic repertoire of a tambaqui to become unique neotenous chunk-biting predators, thus initiating the radiation of form and behaviour which we now see. The observation that most neo-tropical fish have an African analogue does not hold for the piranhas, and the idea of the arrival of a fertile pair of piranhas in an African river leads to interesting evolutionary speculations.

Some taxonomists have moved the true piranha, such as Serrasalmus nattereri to the genus Pygocentrus. Here, for simplicity, they are all discussed as members of the genus Serrasalmus. When a species has been allocated by some workers to the genus Pygocentrus this is shown in brackets.

Serrasalmus (Pygocentrus) nattereri gut contents (P=present)
INDIVIDUAL NUMBER	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
LENGTH (cm)	15.4	18.5	13.8	9.4	8.8	7.8	8.2	8.4	8.2	16.5	19	17	21	13	12.5
MASS (g)		214	98	30	18	14	16	20	12	162	256	194	334	88	82
MAMMALIA										P
PISCES-FLESH			P	P		P						P
PISCES-BONES	2								P		P
PISCES-SCALES				1				P					P
PISCES-FINS								P						P
ARTHROPODA-INDET.						P								P
ARACHNIDA														P
CHIRONOMIDA					1
ISOPODA			1
ORTHOPTERA							P							P	P
DETRITUS		P		P	P
PLANTS-ROOTS														P

Another common floodplain species, Serrasalmus elongatus, is known as a fin and scale eater, although larger specimens will also take fish. During the low water season, when fish are concentrated with the drying floodplain lakes piranhas become highly piscivorous and it can be almost impossible to catch a fish which does not have one or more of its fins damaged. See the pictures below for illustrations of the type of damage other fish experience.

Piranha release their eggs amongst vegetation and within the flood plain the young spend their early life within floating meadow. When they are large enough to survive, they enter the open waters where they search widely for food entering the forest when flooded.

Piranha are remarkably adapt at surviving low oxygen periods. Over a number of years of observation of mass fish kills I have never seen a dead piranha. What makes this observation so surprising is that they are not known to possess any particular mechanisms to collect atmospheric oxygen like most of the other species that are not killed. The locals say that as the oxygen falls the piranha quickly move off to main channels or parts of lakes where oxygen is available. In this respect, as in many others, the piranhas show themselves to be exquisitely adapted to their habitat. They are the true masters of Amazonian lakes.

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This large catfish shows on its anal fin a typical example of a piranha bite, probably made by Serrasalmus (Pygocentrus) nattereri	A living adult Serrasalmus spilopleura	Lateral view of a recently caught adult Serrasalmus (Pygocentrus) nattereri	View towards the head of Serrasalmus spilopleura, the red bellied piranha. This is a group hunting species that will take bites from almost any vulnerable animal within the water
A lateral view of a recently caught Serrasalmus elongatus	A preserved specimen of Serrasalmus spilopleura caught in a white water floodplain lake	A painting made in the field by Peter Henderson to illustrate the damage done by piranha attack. The upper two paintings are of Mylossoma aureum and the lower two a species of Myleus	A painting made in the field by Peter Henderson to illustrate the damage done by piranha attack. The illustration shows two different species of Metynnis, both caught in a white water lake near Tefe