Animal Diversity Web

Geographic Range

Oscars, Astronotus ocellatus, are freshwater fish found in areas with warm water temperatures. Oscars are native to South America, throughout the Amazon and Orinoco Rivers and tributaries. These river basins are a complex of waterways that include Argentina, Brazil, Colombia, Paraguay, Peru, Uruguay, and Venezuela. Oscars are also found in parts of Surinam and French Guiana.

There are many varieties of this species that have been bred and introduced to southern Asia, North and Central America, and Africa. Oscars have been used in Florida as game fish and are common aquarium fish. (Crumly, 1998; Kullander, 2003; Page and Burr, 1992)

• Biogeographic Regions
• nearctic
  • introduced
• palearctic
  • introduced
• oriental
  • introduced
• neotropical
  • native

Habitat

Oscar habitats are found within shallow, calm, freshwater rivers or basins. The water they inhabit has a fairly neutral pH (6-8), with moderate hardness, and tropical temperatures of 22-25 degrees Celsius. Because they are mostly bottom-dwellers and feeders, they live in areas that have muddy or sandy substrates. (Barlow, 2000; Crumly, 1998; Kullander, 2003; Mills, 1993; Page and Burr, 1992)

• Habitat Regions
• tropical
• freshwater

• Aquatic Biomes
• pelagic
• benthic
• rivers and streams

Physical Description

The average length of an oscar is between 30-40cm (35cm average), and average mass is 1.58kg by adulthood. Although there are many varieties of this species, the wild-types are a dark green/brown color with varied markings of orange, red, and yellow on the sides. They have characteristic ocelli at the base of the dorsal fin, which are black with an orange and red outline along with a lighter-colored vertical mark at the base of the anal fin. The fan-like fins are dark with light edging. The eyes are dark with a thin red outline and the lips are thick and fleshy.

Characteristic of cichlids, oscars have a single nostril on each side of the snout, and have pharyngeal jaws. Color pattern changes occur based on different behaviors and developmental stages, as reported by Beeching (1992), using dummy fish. The author found that the fish displayed different color patterns based on how the oscars responded.

Variations of Astronotus ocellatus include longer fins, black coloration, red coloration in the markings, and a white body with orange markings throughout. (Barlow, 2000; Beeching, 1992; Crumly, 1998; Mills, 1993; Page and Burr, 1992; Toffoli and Farias, 2012)

• Other Physical Features
• ectothermic
• bilateral symmetry

• Sexual Dimorphism
• sexes alike

• Average mass

  1.58 kg

  3.48 lb

• Range length

  30 to 40 cm

  11.81 to 15.75 in

Development

Although little is known about the development of Astronotus ocellatus, there have been two studies focusing on embryonic and larvae development for this species. A study published by do Carmo et al. (2015) described the early development histology of the oscar by induced reproduction. A selection of oscars in secondary maturation, with rounded abdomens and visible urogenital organs, were induced with two different hormones for artificial reproduction: extract of carp pituitary and synthetic human chorionic gonadotropin. The eggs from the initial collection were in the gastrula stage, the forming of a top (epiblast) and a bottom (hypoblast) layer due to migration movement. Fifteen hours after the initial collection, the forebrain, cerebellum, notochord, and eyes of the embryo were formed, along with 2 pairs of adhesive glands in the upper sections of the head. Hatching of the eggs occurred 46-58 hours after initial collection, with larve measuring about 3.25mm in length and about 1.55mm in height. These newly-hatched oscars had non-pigmented eyes, a closed mouth and anus, and a large liver.

Within just 6 hours after hatching, skeletal striated muscular tissue was observed along with the digestive tract containing visceral or smooth muscle tissue. After 12 hours it was noted that the skeleton was only supported by hyaline cartilage. The digestive tract tube was opened after 24 hours, while the mouth remained closed. When the mouth finally opened, the digestive tract began separating into the oesophagus, stomach, and intestines. The full development of the first sense organ, the eyes, quickly happened 42 hours post-hatching (hPH). This was also when a pigmented layer with melanin granules and apical extensions covered the retina, along with a photoreceptor cell layer and outer and inner nuclear layers. Seventy-seven hours after hatching, there was separation between the oral cavity and pharynx, and the oral valve and the supporting muscle of the lower jaw were located at the back of the maxilla and mandible. Gills were set up in four brachial arches and pseudo-gills on the anterior of the first arch, posterior of the eyes, on both sides of the pharynx, and the inside of the ventricle operculum. The brain was not yet fully developed 89 hPH. However, the olfactory lobe in the frontal section, dorsal part, central pineal organ, cerebellum, and hypothalamus were all seen. It was noted that the swim bladder was not yet inflated, but pronephros were observed above it, and in the body cavity 113 hPH. In the pericardial cavity, the heart was completely formed and was the first functional organ of the oscar larve. Goblet cells were found near the pharynx and oesophagus 125 hPH, also noting the start of exogenous feeding. The layers of the eye and a functioning inflated swim bladder located below the notochord and above the digestive tract were found 137 hPH. As the larve grew, there was an increase in cell number seen 161 hPH, and the cytoplasm appeared large and clear. Simultaneously, the walls of the stomach were thin and folded and the edging of the intestines were thicker. Also during this phase was an observation of enterocytes and secretory cells, and the mandibular and oral maxillary valves were formed for the use of mechanical respiration.

By the final observed time, 383 hPH, the teeth were formed. There was no evidence of bone tissue throughout the larve or gastric glands in the stomach. However, folds were noticed in the mucosa of the bowel which makes the passage of food slower, resulting in a longer digestion and increases the absorption of nutrients.

Paes et al. (2012) used stereomicroscopy and scanning electron microscopy to explain the early development of oscars. The authors followed results of natural reproduction and found a few differences in the development process. The formation of the embryo began about 12 hours after the initial collection of the eggs. Tail extension was observed 15 hours after initial collection. Increase in body pigmentation, appearance of pectoral fin, detachment of head from yolk, pumping heart, and clearly pigmented eyes were all seen 15-30 hPH. Fifty-three hPH, the mouth opened with a complete lower jaw and the pectoral fins were able to freely move. The anus opened 101 hPH. Exogenous feeding was initiated along with vertical swimming and observed taste buds on the lip surfaces 125 hPH. The yolk was completely absorbed 257 hPH and at the final time of 383 hPH, the eyes and digestive tract had completely formed, the swim bladder was inflated, and there was beginning formation of the rays of the dorsal and anal fins.

Not much information is available following early development. Gonadal maturation occurs between 10-12 months of age and although considered a sexually-monomorphic species, males have been known to grow quicker. Because of visual predators and feeding habits, most of the sense organs develop relatively quick in order to survive so young. (Paes, et al., 2012; do Carmo, et al., 2015)

Reproduction

Oscars are a monogamous species that are sexually mature around 14 months of age. Being sexually monomorphic, it is easy to distinguish between male and female when the breeding tubes appear before spawning occurs. The tubes of females are short and wide with a flat tip as opposed to the long, thin, curved, and pointed tip of a male tube. Spawning will take place within 48 hours of the tube appearance from both partners. Males will lock jaws when in a dispute over female selection or territory.

When both partners are ready to spawn, their colors intensify and they begin movements of side-to-side vibration/shaking along with flared gills and spreading out of fins. Before the eggs have been laid, both male and female work to clean the site, usually the flat surface of a rock, by rubbing against it and scrubbing off any debris. The process of cleaning can take days or even weeks.

The female lays her first eggs by passing over the rock multiple times followed by the passing by the male over the eggs. This process lasts about 3 hours, resulting in organized rows of eggs in about a 15.4cm section. (Goldstein, 2015)

• Mating System
• monogamous

When oscars reach sexual/gonadal maturity, they are about 14 months old and 15.24-25.4cm long. Within a year, there are 3-4 reproductive cycles that produce about 300-3,000 eggs per cycle. The amount of eggs is dependent on the size of the female. Smaller females lay 300-500 eggs, while larger female oscars lay about 2,500-3,000 eggs. Species that are partitioned spawners, including Astronotus ocellatus, produce small amounts of semen and have a smaller number of mature males in relation to mature females within the same time period. An individual oscar has active reproduction for about 9-10 years.

The site where the eggs will be laid is cleaned by both the male and female. Spawning is dependent on temperature and occurs in warmer months when the temperature is above 25 degrees Celsius. The eggs are fragile, light colored, oval, demersal, and adhesive. They have a large yolk and have a small perivitelline space. The characteristics of oscar eggs are found in most non-migratory species. Oscars care for their young from the time of spawning until the young are almost a year old and some mating partners will continue to reproduce with each other. (Goldstein, 2015; Paes, et al., 2012; Yilmaz and Arsian, 2013; do Carmo, et al., 2015)

• Key Reproductive Features
• iteroparous
• seasonal breeding
• year-round breeding
• sexual
• fertilization
  • external
• oviparous

• Breeding interval

  Oscars reproduce in 3-4 cycles per year.

• Breeding season

  Spawning occurs in warmer months with temperatures >25 degrees Celsius.

• Range number of offspring

  300 to 3,000

• Average number of offspring

  1,650

• Range time to hatching

  2 to 3 days

• Range time to independence

  8 to 12 months

• Range age at sexual or reproductive maturity (female)

  10 to 14 months

• Range age at sexual or reproductive maturity (male)

  10 to 14 months

Parental care is evident for oscars following the spawning. Both male and female prepare and clean the site where the eggs are laid and guard the nest before and after hatching. The females fan the eggs to keep them from collecting debris and mouth them to coat them with an antibacterial mucus. Eggs that are unfertilized are eaten by the parents. Within 2-3 days the eggs hatch but are stuck to the surface of the rock by secretions. The parents will rearrange the embryos and move them into a sandy nest after a day or two. Within the next 5-7 days, they are moved a second or third time. The young cling to the parents even after they are able to freely swim. (Goldstein, 2015; Paes, et al., 2012; Yilmaz and Arsian, 2013; do Carmo, et al., 2015)

• Parental Investment
• male parental care
• female parental care
• pre-fertilization
  • provisioning
  • protecting
    • male
    • female
• pre-hatching/birth
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• pre-independence

Lifespan/Longevity

Although there is little information on the actual lifespan of oscars, they are known to live to at least 10-14 months, which is the point of sexual maturation. Less scientific aquarium web pages (e.g., http://fishnet.org) estimate lifespan in captivity to be 8-10 years, although no scientific publications can support this. (Paes, et al., 2012; do Carmo, et al., 2015)

Behavior

Although they are common subjects for behavioral studies in fish, there is little known about the behavior of oscars in their normal setting. When studying oscars, they are often stimulated to react to a certain variable in order to get a behavioral response. Beeching (1997) examined the functional groups shown in the social behavior of oscars. Dummies were presented to 25 isolated adult oscars to stimulate a response. The five functional groups in the social behavior that were shown to have the greatest observed responses were investigation, attack, nesting, boldness, and distress. The individuals were videorecorded for 10 minutes when in the presence of the dummy. The main behavior shown in the investigation functional group was lateral display. Lateral display is considered a low intensity aggression that is the oscars way of showing their size to the intruder and showing their residency status in the territory. This activity was sometimes seen with head or tail biting. This interest in the intruder (the dummy) was the most frequent activity seen throughout the study and is common in both agnostic and mating behavior. The second functional group associated with all cichlids, attack, was accommodated with frontal display, charging, and head biting. For this activity, it was shown that oscars who were quick to attack were also found to attack more frequently than the oscars who were slow to attack. Not much activity was seen for the nesting functional group because it only included the oscars visits to the nest, or fighting at the nest. It was stated that if nesting activities are reflecting courtship, it is not related to attack activities. The fourth group, boldness, had a main variable of body size. The larger oscars were quicker and more persistent in the dummy attacks, common in cichlid social behavior. The last group, distress, showed a relation with body size and certain behaviors. Smaller oscars displayed both tail beat and tail flutter more frequently than the larger oscars. Tail beat is an activity in order to demonstrate the size of the individual. Tail flutter is an activity specifically common to Astronotus ocellatus. The authors concluded no relationship between sex and behavior.

Goncalves-De-Freitas and Mariguela (2006) examined behaviors in 10 juvenile oscars. Mirrors were introduced at different intervals and recorded. Their findings supported the theory that Astronotus ocellatus may establish a dominance hierarchy and defend its territory. During the trial, 4 types of aggressive activities were directed toward the mirror: mouth fighting, butting, tail beating, and frontal display. Although the frequency of the fighting didn't change, there was an increase in mouth fighting, assumed to be the most aggressive behavior in oscars. (Beeching, 1997; Goncalves-de-Freitas and Mariguela, 2006)

• Key Behaviors
• natatorial
• motile
• sedentary
• territorial
• social
• dominance hierarchies

Home Range

Home ranges have not been reported for Astronotus ocellatus.

Communication and Perception

Most cichlids produce sounds of communication at very low frequencies, using pulses or grunts for recognizing the species, sex, or assessing the other fish. Visual and vocal displays can be used almost equally and interchangeably when reacting to or communicating with individual fish. Other cichlids can change color or pattern with behavior. Eye changes are also characteristic based on behavior. The eyes of oscars become black when they have lost a fight with another fish. (Barlow, 2000)

• Communication Channels
• visual
• acoustic

• Perception Channels
• visual
• tactile
• acoustic
• chemical

Food Habits

Cichlids are omnivorous with carnivorous preferences. Oscars are mostly bottom-feeders, which include a diet of snails, shrimp, insects, clams, and detritus that they suck up from the mud floor. Studies have shown the need for vitamin C due to a deficiency in oscars. Vitamin C is essential for the growth and overall health of fish. (Barlow, 2000; Nichols and Oftedal, 1998)

• Primary Diet
• carnivore
  • piscivore
  • eats eggs
  • insectivore
  • eats non-insect arthropods
  • molluscivore
  • vermivore
  • eats other marine invertebrates
• herbivore
  • frugivore
  • algivore
• omnivore
• planktivore
• detritivore

• Animal Foods
• fish
• insects
• mollusks
• zooplankton

• Plant Foods
• algae

• Other Foods
• detritus

Predation

Astronotus ocellatus has bright eye spots, called ocelli, at the base of the caudal fin. Winemiller (1990) examined whether the eyespots serve as a defense mechanism against fin-nipping piranhas, Serrasalmus and Pristobrycon. The main food source of these piranhas are the fins of fish. Fish that are victims of fin-nipping have a hindered growth and survival rate and are more susceptible to infections. Because the fish have to focus their energy on the regeneration of the fins, they don't have enough energy to focus on gonadal development and somatic growth. The eyespots of oscars are a mimic of the head; when searching for food in densely-vegetated areas, it looks as if the head is positioned upwards instead of the tail.

Consoli et al. (1991) tested the predacious behavior of oscars toward immature mosquitoes Aedes fluviatilis and freshwater snails Biomphalaria glabrata. There was a change in behavior based on the presence or absence of other non-living food. Astronotus ocellatus was a main predator of the immature mosquitoes, which the scientists saw as a potential mosquito control agent. (Consoli, et al., 1991; Winemiller, 1990)

• Known Predators

  • Pirhanas in the genus Pristobrycon and Serrasalmus

Ecosystem Roles

Astronotus ocellatus is a common subject when studying parasites due to the abundance and variety of species found within them. Neves et al. (2013) looked at the parasite variation depending on the two seasons, flood season (December to May) and drainage season (June to November). From a freshwater lake in northern Brazil, 202 oscars were examined and were found to have a total of 6,308,912 parasites classified in 11 taxa: Protozoa, Monogenea, and Digenea metacercariae. Of these, the most abundant taxa were protozoa, specifically, Ichthyophthirius multifiliis. The second most abundant were monogenea, followed by trematode metacercariae and nematode larvae.

There was a difference in the amount and prevalence of the parasite species between the two seasons. The protists and nematodes showed a greater abundance and prevalence in the flood season, whereas the monogeneans were greater in the drainage season. Neves et al. (2013) suggested that the protists showed the greatest abundance.

Overall, the oscars contained mainly ectoparasites, which is typical of fish found in lentic water. Oscars are very active in the parasite community because they are both definitive and intermediate hosts.

Kim et al. (2002) suggested that a monogenean Gussevia asota led to oscar deaths in aquaria. (Kim, et al., 2002; Neves, et al., 2013)

Economic Importance for Humans: Positive

Oscars are used to study a variety of topics such as diet, ecosystem roles, development, aggression, and behavior, due to their abundance in the freshwaters of South America. Although they are not native to south Florida, oscars are a popular imported game fish because of their aggressiveness and attraction to a variety of bait. Oscars are also a popular aquarium fish around the world. ("Florida Fish and Wildlife Conservation Commission", 1999; Barlow, 2000; Beeching, 1992; Fury and Morello, 1994; Nichols and Oftedal, 1998; ; Yan and Popper, 1993; do Carmo, et al., 2015)

• Positive Impacts
• pet trade
• ecotourism
• research and education

Economic Importance for Humans: Negative

In Florida, where oscars are an introduced sportfish, a 1993 report suggested limiting consumption of Oscars. These fish, among others, were contaminated with mercury. (Nico, et al., 2015)

• Negative Impacts
• injures humans
  • poisonous

Conservation Status

Astronotus ocellatus has not yet been assessed on the IUCN Red List, or the United States Endangered Species list, and is not protected by CITES. Oscars have been introduced in conservation areas and fisheries.

Oscars grow to be very large in aquaria and are often released into natural freshwaters when the owners no longer want them. This has increased their non-native geographic range. ("Florida Fish and Wildlife Conservation Commission", 1999; Fury and Morello, 1994; Nico, et al., 2015)

• IUCN Red List

  Not Evaluated

• US Federal List

  No special status

• CITES

  No special status

• State of Michigan List

  No special status

Contributors

Meredith Dowdy (author), Radford University - Fall 2015, Karen Powers (editor), Radford University, April Tingle (editor), Radford University, Cari Mcgregor (editor), Radford University, Zeb Pike (editor), Radford University, Jacob Vaught (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

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Barlow, G. 2000. The Cichlid Fishes. Cambridge, Massachusetts: Perseus Publishing.

Beeching, S. 1995. Color pattern and inhibition of aggression in the cichlid fish Astronotus ocellatus. Journal of Fish Biology, 47/1: 50-58.

Beeching, S. 1997. Functional groups in the social behavior of a cichlid fish, the oscar, Astronotus ocellatus. Behavioural Processes, 39/1: 85-93.

Beeching, S. 1992. Visual assessment of relative body size in a cichlid fish, the oscar, Astronotus-ocellatus. Ethology, 90/3: 177-186.

Chellappa, S. 2003. Reproductive ecology of a neotropical cichlid fish, Cichla monoculus (Osteichthyes: Cichlidae). Brazilian Journal of Biology, 63/1: 17-26.

Consoli, R., C. Guimaraes, J. do Carmo, D. Soares, J. dos Santos. 1991. Astronotus ocellatus (Cichildae: Pisces) and Macropodus opercularis (Anabatidae: Pisces) as predetors of immature Aedes fluviatilis (Diptera: Culicidae) and Biomphalaria glabrata (Mollusca: Planorbidae). Memorias do Instituto Oswaldo Cruz, 86/4: 419-424.

Crumly, C. 1998. Cichlids. Pp. 200-204 in J Paxton, W Eschmeyer, eds. Encyclopedia of Fishes, Vol. 1998, 2 Edition. USA: Weldon Owen Pty Limited.

Feldberg, E., J. Porto, L. Bertollo. 2003. Chromosomal changes and adaptation of cichlid fishes during evolution. Pp. 285-308 in E Feldberg, J Porto, L Bertollo, eds. Fish Adaptation. Enfield – NH, USA: Science Publishers, Inc..

Fury, J., F. Morello. 1994. The contribution of an exotic fish, the oscar, to the sport fishery of the everglades water conservation area. Proceedings of the Southeastern Association of Fish and Wildlife Agencies, 48: 474-481.

Goldstein, R. 2015. "Oscar fish breeding" (On-line). FishChannel.com. Accessed October 16, 2015 at http://www.fishchannel.com/freshwater-aquariums/fish-breeding/oscar-fish.aspx.

Goncalves-de-Freitas, E., T. Mariguela. 2006. Social isolation and aggressiveness in the Amazonian juvenile fish Astronotus ocellatus. Brazilian Journal of Biology, 66/1b: 233-238.

Kim, J., C. Hayward, S. Joh, G. Heo. 2002. Parasitic infections in live freshwater tropical fishes imported to Korea. Diseases of Aquatic Organisms, 52: 169-173.

Kullander, S. 2003. Family Cichildae (Cichlids). Pp. 605-607 in R Reis, S Kullander, C Ferraris, eds. Check List of the Freshwater Fishes of South and Central America. Porto Alegre, Brazil: Edipucrs.

Meschiatti, A., M. Arcifa. 2002. Early life stages of fish and the relationships with zooplankton in a tropical Brazilian reservoir Lake Monte Alegre. Brazilian Journal of Biology, 62/1: 41-50.

Mills, D. 1993. Aquarium Fish. USA: Dorling Kindersley Limited.

Neves, L., F. Pereira, M. Tavares-Dias, J. Luque. 2013. Seasonal influence on the parasite fauna of a wild population of Astronotus ocellatus (Perciformes: Cichildae) from the Brazilian Amazon. Journal of Parasitology, 99/4: 718-721.

Nichols, D., O. Oftedal. 1998. Oscars, Astronotus ocellatus, have a dietary requirement for vitamin C. Journal of Nutrition, 128/10: 1745-1751.

Nico, L., P. Fuller, M. Neilson. 2015. "USGS Nonindigenous Aquatic Species Database" (On-line). Astronotus ocellatus. Accessed November 30, 2015 at http://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=436.

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Toffoli, D., I. Farias. 2012. Deep phylogenetic divergence and lack of taxonomic concordance in species of Astronotus (Cichildae). International Journal of Evolutionary Biology, 2012: 1-8.

Waltzek, T., A. Carroll, J. Grubich. 2001. Evaluating the use of ram and suction during prey capture by cichlid fishes. The Journal of Experimental Biology, 204: 3039-3051.

Winemiller, K. 1990. Caudal eyespots as deterrents against fin predation in the neotropical cichlid Astronotus-ocellatus. Copeia, 1990/3: 665-673.

Yan, H., A. Popper. 1993. Acoustic intensity discrimination by the cichlid fish Astronotus ocellatus (Cuvier). Journal of Comparative Physiology A, 173/3: 347-351.

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do Carmo, F., L. Makino, L. Vasquez, J. Fernandes, F. Valentin, L. Nakaghi. 2015. Induced reproduction and early development histology of oscar Astronotus ocellatus (Agassiz, 1831). Zygote, 23/2: 237-346.