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Pholis gunnellus, commonly known as the Butterfish or Rock Gunnel, is a small, elongated marine fish belonging to the family Pholidae. It is characterized by its eel-like body, smooth skin, and a distinctive row of ocellated spots along its dorsal fin. This species is well-adapted to life in rocky intertidal and subtidal zones, where its cryptic coloration and behavior allow it to hide effectively among seaweed and under stones. It is a fascinating subject for ecological studies due to its parental care behavior and its role as a prey item for larger fish and seabirds.
Physical Characteristics
- Body shape
- eel-like
- Coloration
- Highly variable, ranging from yellowish-brown to reddish-brown or olive-green, often mottled or barred to blend with its surroundings. The ventral side is typically paler. The most distinctive feature is a series of 9-16 (usually 9-13) black, white-ringed ocelli (eye-spots) along the base of the dorsal fin.
- Scale type
- none
- Mouth position
- terminal
- Distinctive features
- Eel-like, elongated body, Smooth, scaleless skin (or very small, embedded scales), Long dorsal fin extending from head to tail, Series of black, white-ringed ocelli along the dorsal fin base, Small, rounded pectoral fins, Pelvic fins are greatly reduced or absent
- Dorsal fin
- Very long, continuous dorsal fin extending from just behind the head to the caudal fin, composed entirely of flexible spines (typically 75-90).
- Tail fin
- Small, rounded caudal fin, confluent with the dorsal and anal fins.
- Pectoral fin
- Small and rounded, located just behind the operculum.
- Distinctive fins
- Anal fin is also long, extending from mid-body to the caudal fin, composed of 1-2 spines and 39-47 soft rays. Pelvic fins are either absent or represented by a single, minute spine.
Identification Tips
Body Shape
Look for a very slender, eel-like body, not flattened like a flatfish or robust like a cod.
Dorsal Fin Spots
The most reliable identifier is the row of distinct, black, white-ringed spots (ocelli) along the base of the dorsal fin. Count them; there are typically 9-13.
Skin Texture
The skin appears smooth and slimy, lacking prominent scales.
Fin Structure
Note the continuous dorsal and anal fins that merge with the caudal fin, and the very small, rounded pectoral fins.
Habitat
- Water type
- saltwater
- Depth
- Intertidal zone down to approximately 100 meters (330 feet), most commonly found in shallow waters.
- Environment
- Rocky shores, tide pools, under stones, among seaweed (especially kelp and Fucus spp.), and in crevices. Prefers areas with good cover and shelter from strong currents.
- Geographic range
- North Atlantic Ocean, including the coasts of Europe (from the Bay of Biscay north to the Barents Sea, including the British Isles, Iceland, and Scandinavia) and North America (from Labrador south to Delaware).
Aquarium Information
- Care level: intermediate
- Temperament: peaceful
- Diet: carnivore
- Minimum tank size: 30 gallons
- Temperature: 4-15°C (39-59°F)
- pH level: 7.8-8.4
- Hardness: 1.020-1.025 SG
- Compatibility: Generally compatible with other small, peaceful, cold-water marine species. Avoid housing with aggressive or much larger fish that might prey on them.
- Special requirements: Requires a well-established cold-water marine aquarium with plenty of rockwork, crevices, and hiding spots. Good water circulation and filtration are essential. They are sensitive to elevated temperatures.
Wild Information
- Conservation status: least concern
- Seasonality: Present year-round in its habitat, but more easily observed during low tide in intertidal zones.
- Market price: Not typically sold for human consumption; occasionally used as bait.
- Commercial importance: low
- Taste: Not commonly consumed by humans.
- Texture: Not commonly consumed by humans.
- Nutritional value: Not commonly consumed by humans.
Key Facts
Parental Carebehavior
Female Butterfish lay their eggs in a compact mass, often under rocks or in empty shells, and both parents (though primarily the male) guard and aerate the eggs until they hatch. This is a relatively uncommon behavior among marine fish.
Slippery Skininteresting
The Butterfish's common name comes from its extremely slippery, mucous-covered skin, which helps it to wriggle into tight spaces and evade predators.
Nine-eyesinteresting
Another common name, 'Nine-eyes,' refers to the distinctive row of ocellated spots along its dorsal fin, which typically number between 9 and 13.
Tide Pool Residentbehavior
Butterfish are frequently found in tide pools, where they can survive periods of low tide by hiding under rocks or in seaweed, often breathing air directly from the surface if necessary.
Safety Information
Safe- Safety level: safe
- Venomous: No
- Edibility: not recommended
- Handling notes: Butterfish are harmless to humans. They are very slippery due to a protective mucous layer, which aids in their ability to squeeze into tight crevices. Handle gently if observed in tide pools and return to their habitat quickly.
Similar Species
Snake Blenny
Lumpenus lampretaeformis
The Snake Blenny has a more pointed snout, lacks the distinctive ocelli on the dorsal fin, and typically has a more uniform coloration without prominent markings. Its dorsal fin is also composed of both spines and soft rays, unlike the Butterfish's all-spiny dorsal fin.
Yarrell's Blenny
Chirolophis ascanii
Yarrell's Blenny has a more robust head with fleshy cirri (tentacles) on the head and nape, which are absent in the Butterfish. It also lacks the prominent ocelli on the dorsal fin and has a different fin ray count.
Shanny
Lipophrys pholis
The Shanny is a true blenny with a more blunt head, prominent eyes, and often has a single, branched tentacle above each eye. Its dorsal fin is notched and does not have the ocellated spots characteristic of the Butterfish. The Shanny also has scales, though they can be small.
Scientific Classification
- Kingdom
- Animalia
- Phylum
- Chordata
- Class
- Actinopterygii
- Order
- Perciformes
- Family
- Pholidae
- Genus
- Pholis
- Species
- Pholis gunnellus
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