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The Atlantic threadfin (Polydactylus octonemus) is a species of ray-finned fish belonging to the family Polynemidae, the threadfins. It is characterized by its distinctive pectoral fins, which are divided into two sections: an upper section with rays connected by a membrane, and a lower section consisting of several free, filamentous rays. These free rays are sensory organs used to locate food in the substrate. This species is typically found in shallow, coastal waters over sandy or muddy bottoms.
Physical Characteristics
- Body shape
- elongated
- Coloration
- Silvery-gray to yellowish-brown on the back, fading to white or silvery on the belly. Fins are typically dusky or yellowish. No prominent markings.
- Scale type
- ctenoid
- Mouth position
- subterminal
- Distinctive features
- Eight free, filamentous rays on the lower part of each pectoral fin, which are significantly longer than the upper rays., Snout projects slightly beyond the mouth., Large eyes.
- Dorsal fin
- Two dorsal fins; the first is spiny with 8 spines, the second is soft-rayed with 1 spine and 12-14 soft rays.
- Tail fin
- Deeply forked caudal fin.
- Pectoral fin
- Divided into two parts: an upper part with 14-16 rays connected by a membrane, and a lower part with 8 long, free, filamentous rays.
- Distinctive fins
- The eight free pectoral fin rays are the most distinguishing feature, used for sensory exploration of the substrate.
Identification Tips
Pectoral Fin Rays
Count the free, filamentous rays on the lower part of the pectoral fin. Polydactylus octonemus consistently has eight such rays, which are notably long.
Snout
Observe the snout, which protrudes slightly beyond the mouth, a characteristic of many threadfin species.
Body Coloration
Note the generally silvery to yellowish-brown body color, lacking prominent bars or spots.
Habitat
- Water type
- saltwater
- Depth
- Typically found in shallow coastal waters, from the surf zone to depths of about 70 meters (230 feet).
- Environment
- Prefers sandy or muddy bottoms, estuaries, and nearshore areas. Often found in turbid waters.
- Geographic range
- Western Atlantic Ocean, from North Carolina (USA) south along the U.S. coast, throughout the Gulf of Mexico, and along the coast of Central and South America to Brazil.
Aquarium Information
- Care level: advanced
- Temperament: peaceful
- Diet: carnivore
- Minimum tank size: 200 gallons
- Temperature: 72-78°F (22-26°C)
- pH level: 8.1-8.4
- Hardness: 8-12 dKH
- Compatibility: Generally peaceful with other non-aggressive, similarly sized fish. Avoid housing with very small fish or aggressive species.
- Special requirements: Requires a large tank with a soft sand substrate to allow for its natural foraging behavior using its free pectoral fin rays. Good water quality and strong filtration are essential. Can be sensitive to poor water conditions.
Wild Information
- Conservation status: least concern
- Seasonality: More abundant in warmer months, particularly during spring and summer in its northern range.
- Market price: Low to moderate
- Commercial importance: low
- Taste: Mild, slightly sweet
- Texture: Flaky, white meat
- Cooking methods: Frying, Baking, Grilling, Steaming
- Nutritional value: Good source of lean protein and omega-3 fatty acids.
Key Facts
Sensory 'Fingers'behavior
The Atlantic threadfin uses its eight free pectoral fin rays as highly sensitive sensory organs to 'feel' for prey, such as crustaceans and small fish, buried in the sand or mud.
Bottom Feederbehavior
This species is primarily a benthic feeder, constantly probing the substrate with its specialized fin rays to locate food.
Migratory Habitsbehavior
Atlantic threadfins are known to undertake seasonal migrations, moving into shallower, warmer waters during spring and summer and retreating to deeper, offshore waters in winter.
Safety Information
Safe- Safety level: safe
- Venomous: No
- Edibility: edible
- Handling notes: No specific handling precautions are required beyond standard fish handling practices to avoid fin pricks. The free pectoral rays are not venomous.
Similar Species
Atlantic Croaker
Micropogonias undulatus
Atlantic croaker has barbels on its chin and lacks the free pectoral fin rays characteristic of threadfins. Its body shape is also generally deeper.
Other Threadfin Species
Various Polydactylus spp.
Other threadfin species in the genus Polydactylus may have a different number of free pectoral fin rays (e.g., P. virginicus has 7, P. sexfilis has 6). Careful counting of these rays is crucial for accurate identification.
Scientific Classification
- Kingdom
- Animalia
- Phylum
- Chordata
- Class
- Actinopterygii
- Order
- Carangiformes
- Family
- Polynemidae
- Genus
- Polydactylus
- Species
- Polydactylus octonemus
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