Brood sac

Definition

A specialized internal or external pouch or chamber used for retaining, nourishing, or protecting developing offspring until they are ready for independent life. In arthropods, brood sacs occur in diverse forms: female isopods and amphipods possess ventral marsupial-like pouches formed by overlapping oostegites; pseudoscorpions carry eggs and early instars in a temporary brood sac attached to the genital opening; some arachnids and crustaceans use modified appendages or body folds to create enclosed chambers. The structure may provide gas exchange, osmoregulation, or defense against predators and desiccation.

Etymology

From Middle English 'brood' (offspring, hatchlings) and 'sac' (pouch, bag), referring to the reproductive function.

Example

In the pseudoscorpion Chelifer cancroides, the female produces a silken brood sac that attaches to her genital operculum; she carries 20–40 eggs and subsequent protonymphs for several weeks, actively cleaning the sac and preventing fungal infection until the young emerge as deutonymphs.

Synonyms

• marsupium (in some crustacean contexts)
• egg sac (when egg-bearing only)

Related Terms

• Ootheca
• oothecal gland
• marsupium
• ovisac
• pseudoscorpion
• isopod
• peracarid
• parental care
• viviparity
• ovoviviparity

Usage Notes

Distinguished from ootheca by being a soft, membranous, or expandable structure rather than a hardened case; distinguished from simple egg masses by active parental association and often physiological maintenance of offspring. Usage varies: 'brood pouch' is often preferred for crustacean oostegite chambers, while 'brood sac' dominates in arachnology and parasitology. In parasitic flatworms (e.g., Leucochloridium), the term refers to a diverticulum of the uterus containing developing larvae—this usage is anatomically analogous but phylogenetically distant from arthropod structures.