Asked by: Anonymous
It’s the sperm and eggs that give it away. Biologically, the male sex is always that which produces smaller reproductive cells (sperm), usually adapted to be more mobile. Females produce larger ones (eggs), usually containing nutrient to sustain the early embryo. Anatomical inspections of fish easily reveal those with viable ovaries that can produce nutrient-rich egg cells.
In seahorses and related species such as pipefish, it can be shown that during copulation these females pass their unfertilized eggs to the sperm-producing males, where they are fertilized and then brooded. Egg transfer is easy to see in some pipefish, where males carry their eggs stuck on the outside of their body. It’s not quite so obvious in male seahorses, which have a sealable brood pouch into which the females inject eggs using a special ovipositor.