The question of whether female fish experience a menstrual cycle, or a “period,” has a clear answer: generally, no. Menstruation is defined as the cyclical shedding of the uterine lining, called the endometrium, which occurs in humans, some primates, and a few other mammals when a pregnancy does not happen. Fish do not have this process, though they do have complex and regular reproductive cycles. Their biological strategy for reproduction is fundamentally different from that of menstruating mammals, eliminating the need for a bloody discharge.
The Physiological Difference
The core reason fish do not menstruate lies in the anatomical differences between their reproductive system and that of mammals. Mammals possess a uterus, which develops a specialized, nutrient-rich lining designed to support a developing embryo after implantation. When no pregnancy occurs, the body sheds this thickened lining, resulting in the menstrual flow.
Female fish, which are overwhelmingly oviparous (egg-laying), lack a uterus and an endometrial lining altogether. Their eggs develop in ovaries which, in bony fish, often form a hollow space that connects to the outside via a small duct. Since fish eggs are typically released outside the body for fertilization, their reproductive tract is not designed to support internal embryonic development, meaning there is no thickened lining to slough off.
How Fish Manage Their Reproductive Cycles
Fish have annual or seasonal reproductive cycles timed by environmental factors like water temperature and daylight hours. The most common reproductive strategy is spawning, where a large group of adults releases eggs and sperm into the water column simultaneously for external fertilization. This process involves the female releasing eggs, which are then fertilized by the male’s milt in the water, without any internal shedding of tissue.
When environmental conditions are not right for reproduction, or if the fish does not spawn all its eggs, the female body deals with unreleased eggs efficiently. Instead of shedding tissue externally, the female fish reabsorbs the unfertilized eggs, a process known as atresia. This allows the female to recycle the proteins and fats back into her own body, conserving energy and nutrients while managing the reproductive cycle without external discharge.
Live-Bearing Fish and Viviparity
A small minority of fish are viviparous, meaning they give birth to live young rather than laying eggs. This group includes species like guppies and mollies, as well as some sharks and rays. These live-bearing species engage in internal fertilization, and the developing embryos are carried inside the female’s body.
Even with internal gestation, this process does not lead to menstruation. Some viviparous fish, classified as hemotrophic, develop structures analogous to a placenta to nourish their young, but this structure is not the same as the mammalian endometrium. The reproductive cycle in these fish is a continuous preparation for the next batch of young, lacking the cyclical shedding of a uterine lining that characterizes a true period.