Isopods, commonly known as pill bugs or roly-polies, are crustaceans that have successfully adapted to life on land. Related to crabs and shrimp, they are frequently found scavenging in gardens and under leaf litter. The life cycle of these arthropods includes a distinct stage for their young, whose appearance is quite different from that of the adults. Understanding the look of a newborn isopod requires an examination of the specific phase they enter immediately following their birth.
The Manca Stage
The young of an isopod are not born as miniature versions of the adult; instead, they emerge into a post-larval developmental phase known as the manca stage. The female isopod carries her developing eggs in a specialized fluid-filled pouch located on her underside, called the marsupium.
The marsupium is formed by overlapping plates, or oostegites, which create a safe, moist chamber for the eggs to hatch. Once the eggs hatch within this pouch, the young remain there for a short period before venturing out into the environment. This emergence marks the beginning of the manca stage, which varies among species, sometimes lasting for several weeks through multiple molts.
Visual Characteristics of Newborn Isopods
Newborn isopods (mancas) are minute, making them barely visible to the unaided eye. They are typically around half a millimeter to just over a millimeter in width upon release from the pouch. They easily blend into their substrate.
The coloration of a newly emerged manca is typically very pale, often appearing translucent, white, or a light gray. This lack of pigmentation sometimes makes the internal organs, particularly the digestive tract, visible as a darker line running down the center of their body. Their exoskeleton is soft and delicate immediately after birth, requiring several molts to develop the harder, calcified structure of an adult.
Due to their small size, mancas remain hidden in protected, moist environments like burrows or under leaf litter. As they grow and pass through subsequent manca stages, their bodies begin to develop increased pigmentation and a more defined form.
Distinguishing Mancas from Adult Isopods
The most definitive anatomical difference between a manca and an adult isopod lies in the number of walking legs, or pereopods. Adult isopods possess seven pairs of jointed limbs attached to their thorax. In contrast, a manca is born with only six pairs of fully developed legs.
The manca stage is characterized by the absence or rudimentary nature of the final, seventh pair of pereopods. The segment of the thorax where this last pair of legs will eventually grow is either not fully formed or lacks a functional limb. This seventh pair of legs does not become functional until the isopod undergoes its first or second post-marsupial molt, at which point it transitions into the juvenile stage.
Mancas exhibit other structural differences compared to mature adults. They may have less developed antennae and often lack the full range of respiratory appendages, or pleopods, found on the adult abdomen. The coloration patterns that define many adult isopod species are also significantly less defined or entirely absent in the young manca.