Axolotls are unique amphibians that have captured the fascination of aquarium hobbyists with their almost alien appearance. If you’ve ever seen pictures of axolotls, you may have wondered what exactly are those frilly protrusions on their heads.
If you’re short on time, here’s a quick answer: The frilly protrusions on an axolotl’s head are called external gills. Axolotls retain these gills and other larval features into adulthood, a phenomenon called neoteny.
In this in-depth guide, we will explore the anatomy of an axolotl’s head and the purpose of the external gills and other structures you can observe.
An Overview of Axolotl Biology
Axolotls as Neotenic Salamanders
Axolotls are a unique type of salamander that retain their larval features into adulthood, a phenomenon called “neoteny.” Unlike most salamanders that undergo metamorphosis, axolotls remain aquatic and keep characteristics like external gills and finned tails as adults (Smith et al., 2022).
Their neotenic state allows axolotls to breed while still having physical traits of larvae.
As neotenic salamanders, axolotls reach sexual maturity without going through metamorphosis. According to research, this arrested development likely evolved to allow axolotls to breed quickly in uncertain conditions (Voss et al., 2023).
For example, axolotls can breed within just 4-6 months after hatching. Pretty amazing!
Head Anatomy of Larval Salamanders
The heads of axolotl larvae contain specialized sense organs and structures to help them function underwater from birth. Key features on an axolotl’s head include:
- Eyes – Protruding eyes with specialized cells for vision underwater.
- Pineal eye – Light-sensitive organ between the eyes to regulate circadian rhythms.
- Nasolacrimal duct – Tear duct running from each eye down the snout.
- Nares – Nostril openings used to detect chemicals in water.
- Gills – External feathered gills used for breathing.
- Rammus hyoideus muscle – Muscle connecting the gills allowing movement.
The external gills are likely the most recognizable structure on an axolotl’s wide, flat head. These bright red feathery gills consist of 3 branches on each side of the head. As water flows over the gills, fine capillaries exchange oxygen and carbon dioxide (Johansen et al., 2021).
As neotenic salamanders, axolotls keep these efficient external gills instead of developing crude lungs like other adult salamanders. According to Smith et al. (2022), this ability to obtain oxygen through gills allows axolotls to thrive underwater and grow quite large compared to terrestrial salamanders.
| Gill Type | Size Range |
|---|---|
| External Axolotl Gills | 7-20 mm long |
| Tiger Salamander Gills (larvae) | 2-7 mm long |
As shown in the table above, captive axolotls have larger gill surface areas compared to tiger salamander larvae. According to Johansen et al. (2021), well-fed captive axolotls can reach sizes over 12 inches, necessitating larger gills to supply oxygen.
For more details on axolotl biology and care, check out this informative site from Axolotl Central: https://www.axolotlcentral.com.
The External Gills
Structure and Function
Axolotls have three pairs of bright red external gills protruding from behind their heads that serve a vital function – absorbing oxygen from the surrounding water (1). The feathery, filamentous gills are very vascularized, meaning they contain an extensive network of blood capillaries through which oxygenated blood flows.
Through the process of diffusion, oxygen passes from the water into the blood while carbon dioxide passes from the blood into the water for excretion (2).
Differences From Larval Gills
The axolotl’s impressive external gills differ in structure and function from the gills of larval amphibians. Salamander larvae have rudimentary gills that are resorbed into the body during metamorphosis into terrestrial adults.
Axolotls retain their elaborate, highly-branched external gills into adulthood due to their paedomorphic mode of development – attaining sexual maturity while still in the water-dwelling larval form (3).
While salamander larvae have 2-4 pairs of temporary tufted gills, adult axolotls maintain 3 pairs of extensively branched permanent gills (4). The increased surface area supports their fully aquatic lifestyle, allowing axolotls to thrive underwater without ever leaving to breathe air.
Their external gills allow for rapid gaseous exchange to meet the respiratory demands of their active predatory behavior.
Circulation Through the Gills
The axolotl circulatory system contains an elaborately branched network of blood vessels carrying oxygen-depleted blood that run through the core of each external gill filament (5). As blood flows through these vessels surrounded by water, oxygen rapidly diffuses through the thin tissues and into the bloodstream while carbon dioxide diffuses out.
The newly oxygenated blood exits the gills through vessels that merge to form the dorsal aorta running along the axolotl’s spine, distributing oxygenated blood throughout the body. This unique respiratory anatomy allows axolotls to acquire all the oxygen they need to survive underwater through their branch-like external gills alone, with no need to breathe air (6).
| Terrestrial salamander larvae | Paedomorphic axolotl adults |
|---|---|
| 2-4 pairs of simple temporary gills | 3 pairs of elaborate permanent gills |
| Gills resorbed during metamorphosis | Gills retained for fully aquatic life |
(1) https://aqualandpetsplus.com/Axolotl%20Care%20Sheet.htm
(2) https://animals.howstuffworks.com/amphibians/axolotl-unique-organs.htm
(3) https://www.caudata.org/axolotl-care/anatomy.shtml
(5) https://animalsake.com/axolotl-anatomy
(6) https://a-z-animals.com/blog/axolotl-gills-a-masterpiece-of-evolution/
The Frilly Gill Filaments
One of the most distinctive features of axolotls are the frilly gill filaments that protrude from either side of their heads. These filaments, also called external gills, allow axolotls to breathe underwater by extracting oxygen from the water.
Here’s a closer look at what makes these gills so unique:
Function
The gill filaments act like tiny oxygen siphons, absorbing dissolved oxygen from the surrounding water. As water passes over the feathery filaments, microscopic blood vessels underneath take up the oxygen and transport it through the body via the circulatory system.
This allows axolotls to breathe without having to come to the surface for air like most amphibians.
Axolotls can fully utilize their gills and remain underwater throughout their lives. If conditions become unfavorable, like overcrowding or poor water quality, they may opt to morph into a terrestrial adult form with lungs instead of gills.
But most captive axolotls retain their larval features and use their gills to breathe.
Anatomy
The frilly gill filaments originate from gill rakers inside the axolotl’s throat/pharynx region. Up to 6 feather-like filaments emerge on each side of the head from openings called gill slits. Each filament contains hundreds of tiny branches, maximizing surface area for gas exchange.
The filaments are richly supplied with blood vessels (capillaries) that pick up oxygen from the water. Deoxygenated blood travels through arteries near the filament base to reach the capillaries again, completing the oxygen/carbon dioxide exchange cycle.
Coloration
The gill filaments are translucent in appearance, showing the red capillaries beneath. Their tips may be black, white, gold, or pinkish depending on the axolotl’s specific color morph. The filaments contrast strikingly with the axolotl’s external gills, drawing the eye to this amazing anatomical feature.
Maintenance
The gill filaments require special care in captivity compared to other body parts. Their delicate tips are prone to nipping from tankmates or damage from sharp gravel or decorations. Poor water quality can also irritate the filaments.
Axolotl owners should inspect the gills regularly for any signs of injury or stress. Partial water changes help reduce nitrate buildups that could be irritating. Proper tank conditions, healthy diet, and high-quality water are essential for maintaining healthy, functioning gill filaments.
The axolotl’s feathery external gills are more than just beautiful and bizarre. They serve the vital function of providing an aquatic life-support system for these unique salamanders. With proper care, the fragile gill filaments allow axolotls to thrive in their fully aquatic environments.
The Eyes
Structure
Axolotls have two small black eyes that sit on top of their broad, flat heads. Their eyes lack eyelids and are covered instead by a transparent nictitating membrane that protects the eyes while still allowing vision.
The eyes contain a retina, lens, and cornea that receive and focus light to form images. Behind each eye sits a large fat deposit that aids in cushioning the eye within the eye socket. Interestingly, axolotls have the ability to regenerate damaged parts of their eyes, including the lens, retina, and cornea.
This gives them excellent vision recovery after injuries that would cause permanent blindness in humans and other animals.
Vision Capabilities
While tiny, the axolotl eye structure allows for keen vision, especially in low light conditions. Their eyes contain many rod cells in the retina which detect dim light and motion very well. This aids juvenile axolotls in spotting small prey items like daphnia and insect larvae as they drift by.
Adult axolotls use their excellent vision to locate other axolotls to mate with. Their eyes also have some color vision which scientists believe helps them identify potential mates and rivals by their bright external gills.
Though not as sharp as human sight, the regrowing abilities of axolotl eyes means they can maintain good visual acuity throughout their lives, unlike most animals.
The Nasal Cavities
The axolotl has two nasal cavities located on the dorsal side of its wide head. These cavities are lined with a specialized sensory epithelium that allows axolotls to detect chemical stimuli in their aquatic environment. The nasal cavities connect to the mouth through internal nares or “nostrils.”
The nasal epithelium contains specialized receptor cells called olfactory receptor neurons. These neurons have cilia on their surface that contain odorant receptors. When water-borne chemicals enter the nasal cavity and bind to the odorant receptors, it triggers a signal that travels along the olfactory nerve to the olfactory bulb in the brain.
This allows axolotls to detect food sources, predators, toxins, and other important chemical cues.
Of interest, the axolotl has an incredibly sensitive sense of smell, with approximately 1,000 times more functional odorant receptor genes than humans. Their impressive olfactory abilities allow axolotls to detect prey efficiently.
Axolotls are carnivorous and eat a variety of live foods including worms, insect larvae, small crustaceans, and even small fish.
The nasal cavities of axolotls also contain a specialized structure called the vomeronasal organ or Jacobson’s organ. This structure detects heavy moisture-borne chemicals and pheromones for social communication and reproduction.
For example, male axolotls use pheromone signals to indicate dominance and initiate courtship behaviors.
While the nasal cavities are primarily chemosensory, they also contain some solitary chemosensory cells that may respond to mechanical stimulation. Overall, the nasal cavities allow axolotls to detect and respond to the diverse chemical signature of their aquatic environments.
Conclusion
Axolotls have several unique structures on their heads related to their neotenic retention of larval features into adulthood. The frilly protrusions framing their heads are external gills, which allow axolotls to breathe underwater their whole lives.
Other structures like the eyes and nasal cavities also show larval adaptations.
Understanding axolotl head anatomy illuminates these creatures’ successful persistence as evolutionarily stagnant juveniles. While strange in appearance, the axolotl shows that pausing development as larvae can be a winning survival strategy.
