If you’ve ever gone fishing and caught a fish to eat, you may have noticed some strange protrusions on its underside. No, those aren’t nipples – fish do not have mammary glands or anything resembling nipples.
But read on to understand the full story behind some of those odd lumps and bumps on a fish’s underside.
If you’re short on time, here’s a quick answer to your question: Fish do not have nipples or mammary glands. However, some species of fish do have small protrusions on their underside that may look similar to nipples but serve other purposes.
Fish Reproductive Anatomy
Eggs and Milt
Fish reproduce sexually through the release of eggs and sperm, also known as milt. The reproductive organs of female fish are called ovaries. The ovaries produce eggs, also known as ova or roe, which contain all the genetic material to produce a new fish.
When a female fish is ready to spawn, the eggs are released from the ovaries into the body cavity and then exit the body through the vent or gonopore. The number of eggs produced by a female varies greatly between fish species.
Some species may only produce a few hundred eggs at a time, while other species can produce hundreds of thousands of eggs during a single spawning event!
Male fish have testes that produce sperm or milt. The milt contains spermatozoa, the male reproductive cells that fertilize the eggs. When it’s time to spawn, the milt is released through the vent and combines with the eggs that have been released by the female.
This fertilization usually takes place externally in the water. The eggs then continue their development and hatch into larval fish and eventually into mature, independent fish.
Papilla
Many fish species have specialized structures called papillae around their vents which aid in the release of eggs and milt. Papillae are small, nipple-like protrusions. In males, the papillae help guide the milt toward the eggs during spawning.
In females, the papillae provide an outlet for the eggs to flow out of the body. The shape and number of papillae vary between different fish species. For example, salmon and trout usually have around 6-12 pointed, cone-shaped papillae around their vents.
Purpose of Peculiar Protrusions
Sensory Organs
Fish have various protrusions on their bodies that serve important sensory functions. Many fish have taste buds distributed across their skin, allowing them to detect chemicals in the water. Barbels are whisker-like projections near the mouth that contain taste buds and help fish locate food.
Some fish also have lines of neuromasts, sensory receptors that detect water movements, running along their bodies and heads. These lateral lines help fish navigate their environments and detect predators or prey.
Other protrusions specifically enhance vision or olfaction. Some bottom-dwelling fish have upward-pointing eyes with eyebrows to protect them from sediment. Tubular nostrils help fish smell underwater. Certain ocean fish even have forehead olfactory organs called nares.
Overall, the bizarre bumps, whiskers, and nozzles on many fish provide them with critical sensory information to survive in aquatic habitats.
Increase Surface Area
Many protrustions and elaborate fin shapes in fish help maximize surface area. Greater surface area improves respiration, allowing more oxygen to diffuse into the bloodstream through the gills. More surface area also enhances ion exchange and waste removal.
Consequently, fish have evolved a diversity of wing-like fins, trailing filaments, and other structures to expand surface area and meet their physiological needs.
In addition, greater surface area improves propulsion and maneuverability. Long, flowing fins generate thrust and allow quick turns and stops. Other flattened or ridged fins function like airplane wings, providing lift.
So fish protrusions are not just sensory tools but also enhance swimming performance. Interestingly, juvenile fish have proportionally larger fins and tails compared to adults, providing more surface area to meet their increased oxygen demands as they grow.
Mammary Glands and Nipples in Mammals
Nourishing Young
Mammals have evolved specialized mammary glands to nourish their young with milk. The mammary glands produce milk which contains proteins, fats, vitamins and minerals essential for the growth and development of offspring.
This is an amazing evolutionary adaptation that allows mammals to provide ideal nutrition to their vulnerable babies.
In placental mammals like humans, milk production starts during pregnancy in response to reproductive hormones. After birth, stimulation of the nipple triggers the release of oxytocin which causes cells in the mammary glands to squeeze milk out through ducts in the nipple.
This allows the newborn to suckle and be rewarded with the perfect baby food – breastmilk!
The nutrients in breastmilk support brain development and strength the newborn’s immune system. For example, unique sugars called oligosaccharides act like a probiotic, feeding good gut bacteria in the baby. No infant formula can replicate all the health benefits of nursing directly from mom.
Breastfeeding helps explain why mammals have been so successful at raising their offspring.
Evolution of Lactation
Lactation is estimated to have first evolved over 200 million years ago in primitive therapsid mammals. Prior to this, newborn reptiles were on their own to hunt or forage. Milk provided a protective nutritional head start to help keep pace with mammals’ distinctive live births of very underdeveloped young.
Over the course of mammalian evolution, variations in milk composition allowed species to adapt to specialized diets and habitats. For example, sea lion milk has 10 times more fat than cow milk since sea lions need blubber for warmth in marine environments.
Kangaroo milk is very low in fat but high in natural sugars to fuel the energizer-bunny metabolism of joeys. The diverse milks showcased in mammals today are a testament to eons of natural selection at work for parenting success!
Why Fish Don’t Have Nipples
Eggs Fertilized Outside Body
Unlike mammals, most fish do not become pregnant or nourish developing embryos inside their bodies. Instead, female fish release eggs into the water, external to their bodies, where male fish release milt to fertilize them.
This unique form of reproduction, called spawning, eliminates the need for intricate delivery systems like the mammalian placenta or nipples to provide nutrients to developing young.
During the spawning process, the fish ovaries release anywhere from a few to millions of eggs, depending on the species. These eggs contain nutrients like lipids, proteins, and mRNA to support initial cell divisions after fertilization [1].
But since embryonic development happens externally, specialized mammary glands and nipples used for nursing are irrelevant for most fish species.
Independent Young
Most baby fish hatch from eggs as fully formed, self-sufficient organisms. They possess a complete set of organs at birth and have no need to suckle milk. This contrasts sharply with newborn mammals, who are relatively underdeveloped and require devoted maternal care and milk nourishment for survival outside the womb.
For example, seahorses exhibit a unique form of fish reproduction in which the male carries developing young in a ventral brood pouch. Though the pouch provides protection, the baby seahorses still hatch as fully independent organisms, negating any need for nipples or mammary glands [2].
The only fish possessions resembling nipples may be the mammary-like” brood pouches of male seahorses and pipefish used for protection rather than nourishment.
| Fish Type | Number of Eggs Produced |
| Cod | 9 million |
| Salmon | up to 14,000 |
| Tuna | 10 million |
In the end, nipples and complex nourishing systems seen in mammals are simply not evolutionarily advantageous for fish. Their independent offspring and external fertilization eliminate the need. So while fish may share other anatomical structures with mammals, nipples remain a uniquely mammalian feature.
Oddities and Exceptions
Male Pregnancy
While female fish are typically the ones to carry eggs and offspring, there are some interesting exceptions where the male fish actually becomes “pregnant”! This phenomenon is seen in sea horses and pipefishes (Syngnathidae family).
In these species, the female deposits her eggs into a specialized brood pouch on the underside of the male. The male then fertilizes the eggs internally and carries them until they hatch or give birth to live young, in some cases even providing nutrients to the developing embryos via a placenta-like connection.
According to one study, the evolution of male pregnancy in these species may be related to their unusual body shape and lifestyle which limits the female’s ability to carry eggs or internal fertilization.
By having the male carry the eggs, it ensures the survival and healthy development of the offspring. Truly a fascinating example of reproductive diversity in the fish world!
Mammary Gland-Like Structures
Most fish do not have nipples or breast structures. However, there are a couple of odd cases where fish have evolved glandular structures superficially similar to mammary glands:
- Some deep sea fish in the Ophidiidae family develop temporary papilla-like structures on their skin that ooze nutritious fluids. These attract and nourish their larvae which feed on the secretions.
- The Surfperch (Embiotocidae) developed specialized skin gland cells during spawning season that secrete nutrients. Young embryos absorb these secretions from specialized ovipositors while still enclosed in the egg capsule.
While intriguing, these cannot strictly be considered “nipples” or “breasts” evolutionarily since they likely evolved independently rather than being homologous to mammary glands. Still, it suggests the evolutionary pressure to provide post-spawning parental care even in typically egg-laying animals like fish.
Conclusion
While fish certainly don’t have nipples or mammary glands, they have evolved other fascinating ways to reproduce and nourish their young without that mammalian equipment. The next time you catch an odd-looking fish, take a closer look – those strange lumps just might have an important reproductive purpose after all!
