Anglerfish are a fascinating group of deep-sea fish best known for the bioluminescent “fishing rods” that jut from their heads. These glowing lures attract prey in the darkness of the deep ocean. But can anglerfish actually turn their lights on and off? Or are they constantly glowing? The answer depends on the species.
Some anglerfish like the black devil anglerfish and certain ceratioids can control their bioluminescence and turn it off completely when not hunting. Other species like the goosefish seem to have constant light production. Research suggests that continuous glowing may aid some anglerfish in mate location and identification of their own species. But controllable bioluminescence is likely the ancestral condition as it provides an energetic advantage.
Overview of Anglerfish Bioluminescence
Bioluminescence, or the production of visible light by living organisms, is very common among deep-sea animals. Many fishes, crustaceans, and cephalopods have evolved light-generating photophores to attract prey, communicate, or camouflage themselves against residual downwelling light.
In anglerfish, bioluminescence is used primarily for predation. The esca or illicium (“fishing rod”) that protrudes above their mouths contains symbiotic bioluminescent bacteria. This lighted lure attracts curious fish and squid, which the anglerfish quickly inhales when in range. Different anglerfish groups have varied illicium morphologies with rods, tassels, or complex fleshy limbs tipped with light organs.
| Anglerfish Group | Bioluminescent Adaptations |
|---|---|
| Ceratioids (e.g. Ceratias, Cryptopsaras) | Esca at end of short illicium, may have photophore “bait” below |
| Melanocetids (e.g. Melanocetus) | Longer tapered illicium with terminal esca |
| Gigantactinids (e.g. Gigantactis) | Bushy illicium filled with bioluminescent bacteria |
| Lophiids (e.g. goosefish) | Dangling illicium with leaf-like esca |
This bioluminescence is intrinsic to anglerfish and produced either by the fish themselves or their symbiotic bacteria. It differs from extrinsic bioluminescence produced by symbiotic crustaceans or other organisms attached directly to the anglerfish’s body.
Species with Controllable Bioluminescence
Some anglerfish can completely shut off their bioluminescence at will when not actively hunting. This ability has been documented in:
– Certain ceratioids like the warty anglerfish (Ceratias uranoscopus)
– The black devil anglerfish (Melanocetus johnsonii)
– Other melanocetids like the Pacific footballfish (Himantolophus sagamius)
These species have well-developed intrinsic light organs and photophore systems that they likely control via musculature and chromatophore pigment movements. When the light organs are occluded or photophore outputs diminished, bioluminescence ceases.
Being able to turn off bioluminescence helps ceratioids and melanocetids conserve energy. Producing light requires substantial energy input, whether directly from the fish or from its symbionts. One study found that bioluminescence increases anglerfish oxygen consumption over 50% higher compared to non-luminous periods. So the ability to switch bioluminescence on and off selectively has clear energetic benefits.
Species with Constant Bioluminescence
Other anglerfish appear incapable of fully extinguishing their bioluminescence and instead glow persistently. This includes:
– Goosefish (Lophiidae)
– Some antenna codlets (Bathylychnops)
– Haplophryne mollis
– Many female linophrynids and ceratioids once sexually mature
Constant bioluminescence may aid these fish in attracting mates and recognizing their own species. Goosefish in particular use their conspicuous lures to help locate mates in the open ocean. Their continuous glowing makes them visible from longer distances for courtship. Some ceratioids also maintain permanent post-maturation glows that likely function for species recognition and mate finding in the lightless depths.
But constant bioluminescence comes at an energetic cost, indicating tradeoffs between mate finding and energy conservation. It suggests that controllable bioluminescence evolved in certain groups to optimize hunting efficiency by minimizing light production outside feeding periods.
Mechanisms of Bioluminescence Control
In species with controllable bioluminescence, how do anglerfish actually turn their lights on and off? Research points to a few possible mechanisms:
– Light organ occlusion – Anglerfish appear capable of mechanically covering or exposing their bioluminescent esca and photophores using flaps of skin and tissue. Occluding the light organs cuts off bioluminescence.
– Photophore crystal positioning – Some photophores contain movable crystals that can change the direction and intensity of emitted light. Adjusting the crystals filters and focuses bioluminescence.
– Symbiont density changes – The symbiotic bacteria that produce bioluminescence multiply and grow more dense when anglerfish are feeding. Reducing symbiont populations may dim bioluminescence.
– Intrinsic photophore control – Similar to fireflies controlling their bioluminescent lanterns, anglerfish likely have intrinsic musculature or chromatophore control over light organ output and can dim or brighten accordingly.
Being able to actively modulate bioluminescence using these mechanisms enables anglerfish like ceratioids and melanocetids to turn their lights fully off when not hunting. This provides major energetic savings in the constantly food-scarce deep ocean.
Evolution of Bioluminescence in Anglerfish
The fact that certain anglerfish groups have controlled bioluminescence while others have permanent glows provides insight into the evolution of this trait.
Bioluminescence first evolved in smaller anglerfish like ceratioids that used brief glowing bursts to attract prey. Being able to turn bioluminescence on and off selectively provided major energetic benefits that drove this trait’s evolution.
Later, larger species adapted permanent bioluminescence for additional functions like mating. The energetic tradeoffs were outweighed by the benefits of increased mate finding and species recognition.
Controllable bioluminescence is also thought to be the ancestral state in anglerfish. The capacity for bioluminescence control only diverged later in certain lineages where glowing became co-opted for multiple survival and reproductive purposes.
Conclusion
Anglerfish display an intriguing diversity of bioluminescent adaptations. Some species like ceratioids and melanocetids can fully control their bioluminescence, turning it off when not hunting to conserve energy. Other groups like goosefish exhibit constant glows that aid in mate seeking and species identification but have higher energetic costs. The capacity to switch bioluminescence on and off selectively appears to be the ancestral state in anglerfish, evolving for efficient prey capture. Understanding this diversity provides insight into how bioluminescence evolved for varied functions across different deep-sea environments.
