If you’ve ever visited an aquarium or a reptile house at the zoo, you may have noticed some similarities between fish and reptiles. Though they come from very different evolutionary lineages, these cold-blooded vertebrates share a surprising number of traits.
If you’re short on time, here’s a quick answer to your question: fish and reptiles share traits like scales, breathing through gills (in some reptiles), laying eggs, and being ectothermic.
In this comprehensive guide, we’ll explore over a dozen anatomical, physiological, and behavioral traits that fish and reptiles have in common. Read on to learn more about the surprising connections between these very different classes of animals.
Scaly Skin
Scales are a defining feature of fish and reptiles. Both groups have evolved scales as a protective covering for their skin, though the scales are structured differently in each case. The development of scales has allowed fish and reptiles to thrive in aquatic and terrestrial environments.
Scales in Fish
Fish scales are formed from a protein called collagen and typically cover the entire body. There are two main types of fish scales:
- Placoid – Also known as dermal denticles, these scales have a tooth-like structure and are found on sharks and rays.
- Teleost – The most common type of fish scale with a smooth external layer of collagen and an inner layer of bone.
Fish scales provide several important functions including:
- Protection – The hard external plate protects against predators, parasites, and abrasions.
- Reduced friction – Their smooth and flexible structure allows fish to glide quickly through water.
- Coloration – Thin plates in the scales reflect light to create iridescent and metallic colors.
Scales in Reptiles
Reptilian scales are made of the protein keratin and are attached to the epidermis, or outer skin layer. There are four main types of reptile scales:
- Scutes – Large, bony plates providing armor-like protection seen in crocodiles and turtles.
- Tubercles – Smaller bumps and nodules found on the skin of chameleons and other lizards.
- Osteoderms – Irregular bony deposits forming scales underneath the skin in alligators and armadillos.
- Lepidotic – Thin, horny overlapping scales characteristic of snakes and other serpents.
Important functions of reptilian scales include:
- Protection – Hard external scales shield the body from physical damage and dehydration.
- Camouflage – Colored scales allow reptiles to blend into their environments.
- Thermoregulation – Regulating body temperature is aided by scales trapping heat or moisture close to the body.
Functions of Scales
While fish and reptile scales have some differences in structure and composition, they serve many of the same basic functions:
| Fish Scales | Reptile Scales |
|---|---|
| External protection | External protection |
| Streamlined movement in water | Prevent water loss on land |
| Reflect light | Camouflage colors |
| — | Insulate and regulate temperature |
Research indicates over 70% of fish species and 80% of reptile species rely on scales for defense, lubrication, thermoregulation and other essential functions (ANU, AMNH). Scales enable these creatures to survive and thrive in a variety of harsh conditions.
Over millions of years, these protective dermal structures have evolved to perfectly suit the lifestyles of these amazing aquatic and terrestrial animals.
Gills
How Gills Work
Gills are respiratory organs that allow fish and some amphibians to breathe underwater. They are composed of filaments that maximize surface area for gas exchange with the surrounding water. As water passes over the gills, dissolved oxygen moves across the gill surface and enters the bloodstream, while carbon dioxide exits the blood and enters the water.
This countercurrent exchange allows efficient oxygen uptake.
Gills are located on either side of the pharynx. Most fish have four gill arches on each side, each containing two rows of gill filaments. Water enters through the mouth, flows across the gills, and exits via openings behind the eyes and along the underside of the body.
This unidirectional water flow provides continuous oxygenation.
Gills in Fish
All fish use gills to breathe. The gills are composed of filaments containing a dense network of blood vessels. When water flows over them, the blood takes up oxygen and releases carbon dioxide. Fish can regulate water flow for optimal gas exchange by altering the gaps between filaments or opening/closing flaps over the gills.
Cartilaginous fish like sharks have separate gill slits on each side of the head that allow water to flow directly over the gills. In bony fish, the gills are protected under an operculum or gill cover. As the mouth opens and closes, the operculum pumps water over the gills efficiently.
The high surface area and thin membranes of gill filaments allow rapid gas exchange with the surrounding water. This allows fish to extract the oxygen they need even when dissolved oxygen is low. Some fish can also gulp air at the surface to supplement oxygen uptake via the gills when needed.
Gills in Larval Amphibians and Lungfish
While adult amphibians breathe with lungs, the larvae (tadpoles) use external gills. These feathery filaments allow tadpoles to extract oxygen from water and are located in tufts behind the head. In some salamanders, external gills may be retained into adulthood.
Lungfish are a primitive group of fish that have both gills and lungs. When water levels fall and their habitat dries up, lungfish can survive by breathing air. Their gills become nonfunctional out of water.
Lungfish demonstrate an evolutionary adaptation linking fish physiology to that of tetrapods (four-limbed vertebrates).
Laying Eggs
Fish Egg Types
Fish produce several types of eggs depending on species. The most common are adhesive eggs, which stick to plants or rocks, and demersal eggs, which sink to the bottom substrate. Pelagic eggs float in the water column until hatching.
Some fish species like bettas and gouramis build bubble nests to hold their eggs.
The number of eggs laid by fish varies greatly – from just a couple to several million! Tiny guppies may produce less than 50 eggs at once. Meanwhile, the ocean sunfish lays up to 300 million eggs at one time. Fish eggs come in different sizes, shapes and colors too.
Salmon eggs are large orange spheres. Eel eggs look like transparent leaves. The eggs of catfish often cling together in a large mass.
Reptile Egg Types
Most reptiles lay amniotic eggs on land which have protective membranes and shells. Turtles, crocodiles, tuataras, lizards, and snakes all lay this type of egg. The shells allow gas exchange while preventing water loss. Some species bury their eggs while others guard them.
There are a few exceptions, like some snakes and lizards which give live birth rather than laying eggs. And certain sea turtles have soft-shelled eggs optimized for sandy nests. Most reptiles do not provide parental care, but some snake and crocodile mothers may protect the nests and help the new hatchlings.
Parental Care
The majority of fish provide no care for their eggs or offspring. But some species exhibit fascinating parental behavior. For example, the male stickleback fish fans his nest and protects the fertilized eggs until they hatch.
Cichlids play homemaker too – digging pits for the female to lay eggs and guarding them ferociously.
Mouthbrooders like tilapia grab their eggs in their mouth to incubate them and may keep the fry in their mouths for days after hatching. Some catfish even incubate eggs in their mouth and let the babies swim back inside when danger lurks!
| Fish | Reptiles |
| Most species provide no parental care | Most species provide no parental care |
| Some build nests or mouths to incubate eggs | Some species guard or bury their eggs |
| A few protect eggs and newly hatched offspring | Very few species protect eggs or new hatchlings |
Ectothermy
What is Ectothermy?
Ectothermy refers to organisms that rely primarily on external heat sources to regulate their body temperature. Both fish and reptiles exhibit ectothermy, meaning they depend on their environments to maintain optimal body temps.
Ectothermy in Fish
As ectotherms, fish body temperatures match closely with the water temperatures in their habitats. Factors like weather, seasons, depth of water all impact the external temperature. Most fish species thrive in water ranging from 65°F to 85°F.
There are pros and cons to ectothermy for fish. An advantage is not needing to generate their own body heat, which conserves calories. But if the water temperature dips too low or high, it impairs functions like digestion and oxygen circulation.
Ectothermy in Reptiles
Reptiles also depend on ambient heat from the sun and atmosphere. They rely on behavioral adaptations like basking to maintain ideal body temperatures. When their environment gets too cold, reptiles become sluggish as chemical reactions in their bodies slow.
An ectothermic reptile benefits by requiring relatively little energy to thermoregulate. But the disadvantage is vulnerability in very hot or cold surroundings that are suboptimal for biological processes.
Advantages and Disadvantages of Ectothermy
Key perks of ectothermy in fish and reptiles include:
- Conserving calories rather than producing internal heat
- Thriving in moist environments with mild ambient temperatures
Downsides of ectothermy can include:
| Fish | Reptiles |
|---|---|
| Vulnerability to changes in water temps that impair oxygen circulation and digestion | Sluggishness and torpor in surroundings that are too hot or cold |
With climate change shifting weather patterns, ectothermic animals like fish and reptiles may face increasing challenges from temperature swings in their native environments. Wildlife conservation efforts to protect their habitats could help species that rely heavily on external temperatures to thrive.
Camouflage
Camouflage in Fish
Fish employ a variety of clever camouflage strategies to avoid detection by predators. Some fish blend in with their surroundings by having colors and patterns that match the seabed or coral reef. The leafy seadragon, for example, has leaf-like protrusions all over its body that allow it to disappear against seaweed.
Other fish like flatfish can rapidly change color to match the sea floor beneath them. Some fish even coat themselves with sand or mud to conceal their silhouette and smell.
Fish also use counterillumination to camouflage themselves. Many midwater fish have bioluminescent organs that emit light. This light production serves to match the intensity and wavelength of downwelling light, effectively cancelling out the fish’s silhouette when viewed from below.
For example, the cookiecutter shark has a bioluminescent strip that runs along its belly, disguising its body against the brighter water surface when viewed from the depths.
Camouflage in Reptiles
Reptiles are masters of camouflage and use a variety of strategies to avoid detection. Many reptiles rely on crypsis, or having colors and patterns that allow them to blend into their surroundings. For example, the Gaboon viper has patchy brown and black scales that resemble leaf litter, allowing it to disappear against the forest floor.
Chameleons are famous for their ability to change color to match their environment. Some chameleons can shift through a wide range of colors while others may change only between two or three hues that match their habitat.
Disruptive coloration is another camouflage tactic used by reptiles. Animals with disruptive patterns use strongly contrasting markings that help break up their outline. A great example is the banded sea krait, which has bold black bands on a pale blue background that make it hard to detect against coral environments.
Reptiles may also disguise their shape with protuberances and skin flaps that distort their recognizable form. Horned lizards, for instance, have body spines and head horns that help them blend into thorny desert scrub.
Common Camouflage Strategies
While fish and reptiles have evolved camouflage independently, there are some key strategies shared across these diverse taxa. Crypsis, or having a color and pattern that matches the background environment, is a camouflage tactic used by both groups.
Disruptive coloration is another common approach seen in striped fish and banded snakes that obscures the animal’s distinct outline. Counter-shading, where the animal is darker on top and lighter on the underside, is an effective strategy to disguise 3D shape that fish and reptiles both utilize.
Flexible camouflage adaptations like color changing with mood or surroundings are also seen in chameleons, flatfish, and other species.
Both fish and reptiles also make use of decoys and lures to avoid detection or attack. For example, some fish species like the alligator pipefish have caudal lures that mimic small fish, diverting attacks to their less vulnerable tails.
Snakes like the Western Hognose have enlarged scales on their heads that can flare to appear like menacing eyes, frightening off potential predators. Avoiding detection entirely is often the best camouflage strategy, and nature has produced many ingenious solutions in fish and reptiles.
Conclusion
While fish and reptiles evolved from very different lineages, convergent evolution has led to many shared characteristics between these cold-blooded vertebrates. Studying the common traits of scales, gills, egg-laying, ectothermy, and camouflage provides fascinating insight into how evolution shapes life to thrive in similar aquatic and terrestrial environments.
The next time you visit an aquarium or reptile house, look for the not-so-obvious connections between fish and reptiles reflected in their scales, eggs, and adaptations for life in the water and on land.
