Barreleye (Spookfish) Overview

General Information

• Common Name: Barreleye, Spookfish
• Family: Opisthoproctidae
• Habitat: Deep-sea fish found in tropical to temperate waters of the Atlantic, Pacific, and Indian Oceans.
• Scientific Classification:
  • Domain: Eukaryota
  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Actinopterygii
  • Order: Argentiniformes
  • Family: Opisthoproctidae

Physical Characteristics

• Eyes: Barrel-shaped, tubular eyes directed upwards; can also gaze forward.
• Head: Transparent dome covering the eyes, which helps protect from nematocysts and collects light.
• Body Morphology:
  • Stout, deep-bodied forms.
  • Slender, elongated forms.
  • Intermediate fusiform forms.
• Coloration: Dark brown with large, silvery scales; some species are transparent.
• Luminosity: Presence of bioluminescent organs in some species due to symbiotic bacteria.

Eye Structure and Function

• Large lens and retina with high rod cell count.
• No cone cells present.
• High rhodopsin concentration for enhanced vision in low light.
• Some species use mirrors in eyes for image focusing (e.g., Dolichopteryx longipes).

Feeding and Adaptations

• Mouth: Small, toothless, terminal.
• Feeding Strategy: Use sensitive eyes for tracking zooplankton and small crustaceans.
• Vision: Adapted for binocular vision at the expense of lateral vision.
• Protective Head Dome: May act as a light refractor or accessory lens.

Reproduction

• Pelagic spawners: Eggs and sperm released into the water.
• Fertilized eggs are buoyant and planktonic.

Behavior and Ecology

• Habitats range from mesopelagic to bathypelagic zones (400–2,500 m deep).
• Typically solitary and do not undergo diel vertical migrations.
• Utilize counterillumination for predator avoidance.

Notable Species and Adaptations

• Macropinna microstoma: Transparent head membrane.
• Winteria telescopa: Eyes oriented more forward than other species.
• Dolichopteryx longipes: Unique use of mirrors in eyes.
• Monacoa grimaldii: Split from Opisthoproctus in 2016.

Unique Features

• Some species possess wing-like pectoral fins for station keeping in the water column.
• Presence of luminous organs, e.g., Dolichopteryx with luminous belly organs.
• Camouflage techniques such as counterillumination to blend into ambient light conditions.
• Symbiotic relationship with bioluminescent bacteria for light emission.

References and Further Reading

• Froese, Rainer, Pauly, Daniel (eds.). "Family Opisthoproctidae". FishBase.
• Various scientific articles and encyclopedia entries on mesopelagic fishes and ichthyology.

These notes provide a comprehensive overview of the barreleye fish, covering their classification, physical characteristics, adaptations, and ecological behavior.