Comparative Anatomy: Frog Reproductive System Vs. Fish Reproductive System

Sharks, a type of marine cartilaginous fish, are also referred to as Scoliodon. They are poikilothermic, meaning their body temperature fluctuates with their environment. Frogs, on the other hand, are amphibious creatures and similarly classified as poikilothermic. Both sharks and frogs are categorized under the group Anamniota, which comprises animals that do not produce amniotic eggs.

The reproductive system of both sharks and frogs consists of the gonads and genital ducts, which are crucial for reproduction. Gonads, the primary reproductive organs, are known as testes or spermaries in males, and ovaries in females. These gonads develop from mesodermal tissue and originate from a pair of genital ridges in the embryo. These genital ridges are formed by folds in the coelomic epithelium, located along the medial sides of the embryonic kidneys. Upon full development, the gonads are suspended from the dorsal body wall by double folds of the peritoneum—called the mesorchium in males for the testes and the mesovarium in females for the ovaries. These gonads have dual functions: they produce sex cells, or gametes, such as sperm in males and ova in females, and they also secrete essential reproductive hormones.

Genital ducts, responsible for transporting these gametes, are referred to as vasa deferentia in males and oviducts in females. There exists a close relationship between the ureters and genital ducts, particularly in males, leading to the combined term urinogenital system, which refers to both the excretory and reproductive systems working in conjunction.

Comparative Anatomy: Male Reproductive System of Sharks and Frogs

In male sharks, the testes are long and ribbon-like, attached to the kidneys anteriorly. They are also connected to the rectal gland via epigonal organs, which are absent in frogs. Conversely, the male frog has smaller, rod-shaped testes, attached to the kidneys by a structure known as the mesorchium.

Both animals possess vasa efferentia, but in sharks, these ducts exit from the anterior end of the testes and open into the wolffian duct. In frogs, the vasa efferentia exit along the inner border of the testes and pass through the kidney, opening into Bidder's canal, which then drains into the wolffian duct. The wolffian ducts in sharks are specifically genital ducts, whereas in frogs, they function as both urinary and genital ducts, giving them the name urinogenital ducts.

A striking difference is observed in their copulatory apparatus. Sharks have specialized structures such as siphons and claspers for internal fertilization, while frogs lack these features. In frogs, fertilization is external; sperm is released over the eggs in freshwater, a process known as spawning, whereas sharks release sperm directly into the female reproductive tract.

Comparative Anatomy: Female Reproductive System of Sharks and Frogs

In female sharks, the ovaries are relatively small and located near the base of the liver, connected to the rectal gland by long epigonal organs, which are absent in frogs. The frog's ovaries, in contrast, are larger, lobed sacs that lie on the ventral surface of the kidneys. The oviducts in sharks are long but not convoluted, while those in frogs are highly convoluted and much longer.

A significant feature in the female shark is the presence of a shell gland in the oviducts, which is absent in frogs. The shell gland plays a role in forming protective layers around the eggs. In sharks, the oviducts expand into large uteri, whereas in frogs, they form smaller structures called ovisacs.

Additionally, while sharks have a singular median sac known as the vagina, which opens into the cloaca, frogs have no vagina, and each oviduct opens directly into the cloaca. Fertilization in sharks is internal, negating the need for external egg release, while frogs release their eggs in water where external fertilization occurs.