Thoracocharax stellatus – Spotfin Hatchetfish
Thoracocharax stellatus was first described by Kner in 1858. It is commonly known as the Spotfin Hatchetfish, Silver Hatchetfish, and Platina Hatchetfish. However, some of these names are also used for other species and do not accurately reflect their appearance.
The genus name Thoracocharax originates from the Greek word “θώραξ” (thṓraks), meaning “breastplate” or “armor.” The term Charax, the type genus of the order Characiformes, comes from the Ancient Greek word “χάραξ” (chárax), meaning “a palisade of pointed sticks.” This designation refers to the fish’s tightly packed, sharp teeth. The specific epithet stellatus is Latin for “star-shaped” or “set with stars,” referring to the star-shaped markings found on the flank of this species.
The documents discovered suggest that there might be multiple species associated with the name Thoracocharax stellatus, as four distinct genetic lineages have been identified in various South American rivers. Additionally, two unnamed species have been described in a doctoral thesis. This evidence indicates the possibility of a species complex.
Synonym: Gastropelecus stellatus
Description
Thoracocharax stellatus, commonly known as the Spotfin Hatchetfish, has a characteristic hatchetfish shape, featuring a high, laterally flattened body with a well-defined keel on the pectoral fins. Its head and sides are silvery and shiny, often described as “platinum.” A notable feature is the dark spot on the dorsal fin, while the other fins are either transparent or light-colored. In the wild, these fish typically reach a maximum length of 6 to 8 centimeters, but in captivity, they can grow to between 7 and 8 centimeters. Their growth rate is generally slow.
The distinction between males and females can be challenging. Females often have rounder bodies, especially when carrying eggs.
Behavior and Temperament: Thoracocharax stellatus is a peaceful fish, but it can be shy. These fish thrive best in groups of at least five, as this provides them with a sense of security. They are surface dwellers, often swimming near the water’s surface and close to shoreline vegetation. While they are not aggressive towards other fish, they may struggle to compete for food with more assertive species.
Life expectancy: Life expectancy in the wild is unknown. In an aquarium, they can live up to 7 years.
Biotope
Thoracocharax stellatus is native to Venezuela, Colombia, Ecuador, Brazil, Peru, Bolivia, Paraguay, and Argentina. Within these countries, they are found primarily in the Amazon, Orinoco, Paraguay, and Tocantins-Araguaia river basins, particularly in areas with abundant riparian vegetation. They are riverine fish that live in the upper reaches of rivers, not in lakes or seas. They inhabit rivers such as the Tocantins, Amazon, Orinoco, and Paraguay.
Their natural habitat is defined by a rich presence of vegetation along the banks, which provides a food source in the form of land insects. The banks are lined with trees and shrubs, and the underwater substrate typically consists of sand or gravel, varying in composition.
Thoracocharax stellatus is a fish species that thrives in a tropical climate characterized by predictable patterns of water levels and river flows throughout the year. In the upper Rio Tocantins, where Thoracocharax stellatus is found, there is a distinct difference between high water and low water periods. The high water season occurs from November to April, while the low water season lasts from May to October. This seasonal variation in water levels plays a crucial role in the ecosystem, influencing the availability of food and habitat for the fish. Additionally, the region experiences seasonal flooding and droughts.
Diet
Diet in the Wild: Thoracocharax stellatus is primarily an insect-eater. Its diet consists almost entirely of insects, making up 99.6% of its intake, with the majority (87.6%) coming from terrestrial sources. The main food sources include ants, beetles, and mayflies. These fish primarily feed during dusk and dawn, often jumping out of the water to catch insects from the vegetation along the riverbank. Their diet is relatively specialized, showing little flexibility in switching to other food sources.
Aquarium Diet: In an aquarium, Thoracocharax stellatus should be given a varied diet that includes dry, frozen, and live foods. It’s essential to provide a protein-rich diet since they primarily consume insects in the wild. Suitable live foods include small insects like fruit flies and small crustaceans. Frozen options such as daphnia and artemia are also good choices. When it comes to dry foods, ensure they are of high quality and specifically designed for surface feeders. It’s best to feed small amounts several times a day to prevent the food from sinking to the bottom.
The Aquarium
For a group of five to six Thoracocharax stellatus, an aquarium of at least 80 centimeters is recommended. For larger groups, a larger aquarium is needed, for example, 120 centimeters or more. The length of the aquarium is more important than the height, because they are surface dwellers that need a lot of horizontal swimming space.
Setup: For an ideal Amazon biotope-style aquarium, the setup should include a sandy bottom, floating plants to provide a sense of security for the fish and to prevent them from jumping out of the water, as well as driftwood and branches for added decor. A solid lid is essential since these fish are excellent jumpers. It’s recommended to plant the back wall with suitable aquatic plants. A substrate of fine sand works best for this type of aquarium.
Water parameters: The ideal water parameters are a temperature between 23-29°C, a pH of 6.3-7.4 and a hardness of 3-15°dH. Regular water changes (approximately 25% per week) are essential for maintaining good water quality.
Compatibility: Thoracocharax stellatus is a peaceful fish that coexists well with other calm surface and midwater dwellers. Suitable tank mates include certain tetra species, small Corydoras catfish, and specific Loricariidae catfish. It is important to avoid keeping aggressive or larger fish that may threaten or intimidate Thoracocharax stellatus during feeding times.
Breeding
For breeding, it is best to use a separate breeding tank with a thin layer of gravel as a substrate. The water parameters should be soft, slightly acidic and moderately elevated temperature. Addition of peat to the water is recommended to darken and soften the water. A dimly lit environment and floating plants are essential. A small air-driven sponge filter can be used. It is advisable to use a small group of four to six fish. The breeding fish should be conditioned with a nutritious diet of small insects, crustaceans and mosquito larvae.
The eggs are laid open and exposed on the bottom. The parents must be removed after spawn to prevent cannibalism. The eggs are about 1 mm in diameter.
The eggs hatch after about 36-48 hours. The fry begin to swim freely within 1-2 days. The first food consists of infusoria, paramecium, or ground fish food. Later, newly hatched artemia or microworms can be given.
Particularities
Some interesting details about Thoracocharax stellatus not covered in the previous chapters:
- Species Complex: Evidence suggests that Thoracocharax stellatus is not a single species, but rather a species complex that includes several genetically distinct lineages. At least four distinct lineages have been identified in various rivers across South America, along with two additional unnamed species described in a doctoral thesis. This indicates a significant level of genetic diversity within the species, which may necessitate a revision of its taxonomy.
- Food Specialization: This fish species is a specialist in terrestrial insects and has limited dietary flexibility. This specialization makes it vulnerable to changes in its habitat, such as those caused by dam construction. A study in the upper Rio Tocantins found that the population of Thoracocharax stellatus significantly declined after a dam was built, likely because it could not adapt to the altered food availability. The construction of the dam reduced the land-water ecotope, which in turn affected the availability of terrestrial insects. Despite these environmental changes, the diet of the remaining fish did not significantly change.
- Jumping ability: The fish has a remarkable ability to jump out of the water. This ability is related to the highly developed pectoral fins and pectoral girdle muscles. Although they are capable of long jumps, they do not do this often, and especially not in aquaria. Jumping seems to be a response to threats, rather than a primary method of catching prey.
- Food selectivity: Although the fish is primarily insectivorous, there is no evidence of selectivity at the level of specific insect taxa. The high frequency of ants and beetles in stomach contents may be related to slower digestion of their hard exoskeletons.
- Behaviour at dusk and dawn: The highest feeding activity occurs at dawn and dusk. This may be related to the activity patterns of the prey and the reduced visibility of the fish at dusk, which reduces the risk of predation.
These details highlight the complexity of the ecology and behavior of Thoracocharax stellatus, and the need for further research to understand the species better.
Conclusion
Thoracocharax stellatus, commonly known as the Spotfin Hatchetfish, is a captivating species with a unique appearance and interesting behavior. While these fish are generally peaceful and relatively easy to care for, successful keeping requires a well-established aquarium with the right water conditions and enough space for a group of them. Spotfin Hatchetfish prefer to inhabit the water’s surface and have a tendency to jump; therefore, it is essential to have a well-sealed aquarium with floating plants. They grow slowly, and there is limited information about breeding, which makes them less suitable for beginners. Additionally, their specialized diet and vulnerability to changes in their habitat underscore the importance of a careful approach when caring for them.
The species complex of Thoracocharax stellatus, along with the limited information available on their captive breeding, highlights the need for further research. These fish possess impressive abilities, such as jumping out of the water and exhibiting crepuscular behavior, which are quite unusual. While captive breeding is achievable, it requires a specific approach and presents challenges. Overall, Thoracocharax stellatus is a fascinating fish, but they are best suited for more experienced aquarists.
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Author
John de Lange
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