When a female crab reproduces, she releases a large mass of fertilized eggs that she carries externally until they hatch. This state is often informally described as being “in sponge” because the clustered eggs form a dense, spongy mass beneath her body. The mass’s appearance changes dramatically as the embryos develop, providing a clear timeline of the growth process.
The Visual Characteristics and Location of the Eggs
The eggs are tiny, individual spheres, often compared to poppy seeds, tightly packed into a substantial cluster. When first extruded, they are typically a vibrant color—such as bright orange, deep yellow, or red—depending on the species. This rich coloration comes from the high concentration of yolk, the primary food source for the developing embryo.
Each fertilized egg is microscopic, starting at roughly 260 micrometers in diameter. The entire egg mass is securely fastened to specialized, feather-like appendages called pleopods (or swimmerets), located under the female crab’s abdominal flap, or apron. She holds this flap tightly curled under her body, protecting the eggs from predators.
The mass can contain hundreds of thousands to several million eggs. The female uses her pleopods to keep the eggs clean and well-oxygenated by circulating water through the mass.
Changes in Appearance During Incubation
The most apparent change during incubation is a progressive shift in the color of the egg mass, which indicates embryonic maturity. As the embryos consume the initial yolk supply, the bright orange or yellow hue fades and transitions into darker shades. This marks the mid-incubation stage, where the eggs typically appear brown or a dull gray.
The darkening is caused by the development of the embryo’s internal structures. Specifically, the formation of eye spots, which contain dark pigment, makes the entire egg case appear darker. This signifies that the embryos are well into their growth phase and have begun to form recognizable features.
In the final stages, just before hatching, the egg mass takes on a dark gray or black appearance. The yolk has been nearly fully absorbed, and the fully formed larvae are visible within the egg membrane. The female will soon release the tiny, free-swimming larvae into the water column.
Understanding the “Berried” State and Egg Carrying
The term used to describe a female crab carrying an external egg mass is “berried” or ovigerous. This name is a historical reference to the resemblance of the clustered eggs to a bunch of berries. The berried state represents a significant period of maternal care.
The female actively manages the egg mass throughout the incubation period, which can last from a few days to several weeks depending on water temperature and species. She uses her pleopods to fan the eggs, a constant action that ensures a fresh supply of oxygenated water reaches every embryo. This aeration is necessary for the survival of the large egg cluster.
The maternal protection and constant care ensure a high survival rate for the developing offspring. Once the larvae are ready, the female typically releases them into the water. She often times the release with high tides or dark hours to maximize the dispersal and survival of the newly hatched larvae.