Vertebrates: Comprehensive Guide to Fish, Amphibians, and Reptiles

Posted on Sep 24, 2024 in Biology

Vertebrates

What are Vertebrates?

Vertebrates are metazoans with a spine, an articulated bone structure that protects the spinal cord. The term “spine” is most appropriate for bipedal vertebrates like humans and birds, but it’s also commonly used for fish and quadrupedal vertebrates.

All vertebrates have a closed circulatory system, meaning blood always circulates within vessels (arteries, veins, and capillaries). There are about 50,000 known vertebrate species, categorized into five groups: fish, amphibians, reptiles, birds, and mammals.

Fish

Fish are aquatic vertebrates with fins for movement. Their skin can be covered in scales (like sardines), dermal denticles (like sharks), or be scaleless (like lampreys). They breathe through gills, have a variable internal temperature, and usually reproduce by laying eggs that cannot withstand desiccation. Fish scales are covered by a transparent, mucous-secreting skin that helps them stay hydrated in water. Fish are the oldest known vertebrates, appearing in the Paleozoic Era about 500 million years ago.

External Morphology

Fish bodies have three main parts:

• Head: Contains two nostrils, a mouth, and two eyes without eyelids.
• Trunk: Features four pairs of fins (two pectoral and two ventral) and two odd fins (dorsal and anal). It also has a lateral line, a row of nerve endings that detect vibrations in the water.
• Tail: Extends from the cloaca to the end, where the caudal fin is located. The cloaca is a small cavity that opens to the outside and houses the genital, excretory, and anal orifices.

Internal Morphology

Fish have a backbone or spine (bony or cartilaginous) that protects the spinal cord. They have a ventral heart with one atrium and one ventricle, resulting in a single circulatory system where blood passes through the heart only once per cycle. Many fish have a swim bladder that can inflate and deflate, aiding in buoyancy.

Reproduction

Male fish have two testes, and females have two ovaries. Most species are oviparous, meaning they lay eggs. Both parents release gametes into the water, where fertilization occurs externally. The embryo develops within the egg, and after hatching, a young fish called a fry emerges. Some species are ovoviviparous, meaning fertilization is internal, and the eggs hatch inside the mother (e.g., Gambusia and guppies). There are also viviparous species where the embryo receives nourishment from the mother’s blood (e.g., some sharks and rays).

Amphibians

Amphibians appeared about 370 million years ago, with some reaching over three feet in length. Their pulmonary respiration allowed them to be the first vertebrates to colonize land. However, their simple lungs require supplemental cutaneous respiration (breathing through their skin). This necessitates moist, naked skin, restricting them to humid environments. They also require water for external fertilization and reproduction. Consequently, amphibians can live out of water but only in damp places with occasional rainfall, making them not entirely independent of aquatic environments.

External Morphology

Some amphibians, like frogs and toads, have two body parts (head and trunk), while others, like salamanders and newts, have three (head, trunk, and tail).

• Head: Has two eyes with eyelids (upper, lower, and a transparent nictitating membrane that covers the eye when submerged), a tympanic membrane on each side, a forked tongue, and a mouth with weak, uniform teeth.
• Trunk: Features four limbs. The forelimbs have four fingers, and the hind limbs have five in many species, often connected by interdigital membranes for swimming.

Internal Anatomy

Amphibians have two simple lungs (pulmonary respiration) and supplement this with oxygen uptake through their thin, moist skin (cutaneous respiration). They secrete mucous to keep their skin moist and sometimes produce toxins to deter predators. Their heart has two atria (one receiving blood from the lungs and the other from the rest of the body) and one ventricle where the blood mixes. This is an incomplete double circulatory system, meaning blood passes through the heart twice per cycle, but oxygenated and deoxygenated blood are not completely separated.

Reproduction

Fertilization is external. The fertilized eggs are deposited in water and encased in a mucous substance. The eggs have a permeable membrane that dries out in the air. After 2-3 months, larvae called tadpoles hatch. They breathe through gills, lack limbs, and have a tail for movement. They undergo metamorphosis, where the tail shrinks, limbs develop, gills disappear, and lungs form, allowing them to leave the water.

Reptiles

Reptiles evolved from amphibians about 300 million years ago. Their efficient pulmonary respiration and waterproof epidermis allowed them to colonize dry environments. Internal fertilization and eggs with a waterproof shell further enabled them to inhabit even deserts. They once dominated the planet (land, sea, and air) and reached gigantic sizes unmatched by any other terrestrial group. However, most species disappeared abruptly about 60 million years ago, possibly due to a large meteorite impact that triggered massive volcanic activity. The resulting ash layer may have blocked sunlight, causing a temperature drop that wiped out most reptiles, which are ectothermic (cannot regulate their internal temperature). Other theories suggest that reptiles might have been able to control their temperature, and their extinction was caused by food scarcity due to the lack of sunlight killing off plants.

Reptiles are vertebrates with bodies covered in horny epidermal scales (made of the same substance as our fingernails). They have lungs, breathe air, reproduce by internal fertilization, and lay eggs with a calcareous shell that can withstand dry environments.

External Morphology

Reptiles have three body parts: head, trunk, and tail.

• Head: Contains two nostrils, a mouth with teeth (sometimes venomous in snakes) or a horny beak (turtles), and two eyes with eyelids.
• Trunk: Features four limbs ending in five separate fingers (lizards), webbed fingers (European pond turtle), or flippers (leatherback turtle). Some reptiles, like snakes, lack limbs. Their limbs are articulated laterally, causing them to drag their bellies along the ground when walking (reptilian locomotion). They can only walk upright for short periods. Many ancient reptiles could walk on four legs (e.g., Diplodocus and Stegosaurus) or two legs (e.g., Tyrannosaurus).
• Tail: Extends from the cloaca to the end.

Internal Anatomy

The reptile skeleton has ribs connected to the spine and sternum, forming a ribcage, except in snakes, which lack a sternum. Their circulatory system is similar to amphibians (incomplete double circulation) except for crocodiles, which have two ventricles like birds and mammals (complete double circulation).

Reproduction

Male reptiles have a penis or copulatory organ. Fertilization is internal. Because they are ectothermic, they do not incubate their eggs but bury them. Development is direct, meaning there are no larval stages.