Fish sperm, more commonly known as ‘milt,’ is the seminal fluid containing sperm. It can also refer to the testes or sperm sacs that hold the fluid. This fluid is a component in the reproductive life of most aquatic species and has applications in different human cultures.
The Role in Fish Reproduction
The biological purpose of milt is to fertilize eggs, often through external fertilization. During spawning, female fish release their eggs, known as roe, into the water. Males then release their milt, which appears as a milky, white cloud that drifts and mixes with the eggs.
This process is characteristic of many fish species, including salmon and cod. This method of reproduction is a numbers game, with environmental factors like water currents and predators presenting challenges to successful fertilization and embryo survival.
While external fertilization is common, it is not universal among fish. A smaller number of species, such as guppies and many sharks, practice internal fertilization. In these cases, the male uses a specialized fin or organ to deliver sperm directly into the female’s reproductive tract, much like mammals. This approach results in fewer offspring but provides a higher degree of certainty for the fertilization of each egg.
Physical and Biological Properties
Milt is a white, opaque fluid with a consistency that can range from watery to creamy. It consists of the male reproductive cells, the spermatozoa, suspended in a carrier liquid called seminal plasma. This plasma provides the spermatozoa with nutrients and a medium for movement.
Spermatozoa show diversity across different fish species. While mammalian sperm typically have a single tail, or flagellum, to propel them, the morphology in fish can vary. Some species possess sperm with two flagella, and others have sperm with no flagella, relying on water currents for transport to the egg.
Human and Commercial Applications
Beyond its reproductive function, milt is used in scientific conservation and is considered a culinary delicacy in several cultures. In Japan, cod milt, known as “shirako” (meaning “white children”), is a seasonal ingredient enjoyed during the winter months. It is known for its soft, creamy, and custard-like texture with a subtle, slightly sweet, and oceanic flavor. Common preparations include serving it raw with ponzu sauce, adding it to soups, grilling it, or frying it as tempura.
In the fields of aquaculture and conservation, milt is a resource for managing and preserving fish populations. In fish hatcheries, workers manually collect milt from male fish to fertilize eggs in a controlled setting. This practice allows for selective breeding to enhance traits in farmed fish, such as growth rate and disease resistance, improving food production efficiency.
The ability to freeze and store milt, a process known as cryopreservation, is a tool for conservation. Scientists can collect milt from endangered or threatened fish species and preserve it in liquid nitrogen for long-term storage. This genetic material can be used years later to fertilize eggs, helping to increase genetic diversity and support the recovery of vulnerable populations without needing to transport live fish.