A gribble is any of about 56 species of marine isopod from the family Limnoriidae. They are mostly pale white and small (1-4  mm long) crustaceans, although Limnoria stephenseni from subantarctic waters can reach 10 mm. The term gribble was originally assigned to the wood-boring species, especially the first species described from Norway by Rathke in 1799, Limnoria lignorum. The Limnoriidae are now known to include seaweed and seagrass borers, as well as wood borers. There are three genera, Paralimnoria (two species wood boring), Limnoria (about 28 species wood boring, 20 species algal boring, and 3 species seagrass boring) and Lynseia (3 species seagrass boring). Paralimnoria occurs in the tropics, and has the most plesiomorphic characters. Lynseia are so far known only from Australia, while Limnoria has species in most seas. Those gribbles able to bore into living marine plants are thought to have evolved from a wood (dead plant) boring species.

Gribbles bore into wood and plant material for ingestion as food. The cellulose of wood is digested, most likely with the aid of cellulases produced by the gribbles themselves. The most destructive species are Limnoria lignorum, L. tripunctata and L. quadripunctata. Due to dispersal while inhabiting wooden ships, it is uncertain where these three mentioned species originated. Limnoriidae are second only to the Teredinidae in the amount of destruction caused to marine timber structures such as jetties and piers. L. tripunctata is unusually tolerant of creosote, a preservative often used to protect timber piles, due to symbiosis with creosote-degrading bacteria. Gribbles bore the surface layers of wood, unlike the Teredinidae which attack more deeply. Their burrows are 1–2 mm diameter, may be several centimetres long, and have the burrow’s roof punctured with a series of smaller ventilation holes. Attacked wood can become spongy and friable.

Gribbles play an ecologically important role, by helping to degrade and recycle driftwood. Most seaweed boring gribbles attack holdfasts, and their activities can cause the seaweed to come adrift especially during storms.

For defence, gribbles can jam themselves within their burrows using their uropods, and block the tunnel with their rear disc-shaped segment, the pleotelson.

A number of crustaceans have evolved as commensals with Limnoriidae. Chelura are amphipods that inhabit the more severely attacked regions of gribble-attacked wood. Donsiella are tiny copepods that inhabit the brood pouch and body of Limnoriidae.

It has been suggested that the enzymes used by limnoriidae to break down wood may be useful for producing sugar from non-food biomass, such as wood or straw, in a sustainable way. This could then be used to produce alternative fuels.

Key to genera

See dichotomous key for instructions on using a key.
1: Body more than 6 times as long as wide. Pereopods (legs) 6 and 7 much longer than pereopod 5. Mandibular palp of at most 1 minute article. Maxillipedal palp of 1 or 3 articles. — Lynseia
1a: Body less than 5 times as long as wide. Pereopods 6 and 7 not much longer than pereopod 5. Mandibular palp of 0–3 articles; maxillipedal palp of 5 articles. — 2
2: Uropodal rami elongate, both with corneous apex. Antenna 1 flagellum of 5 articles. Pereopod 1 secondary unguis trifid. — Paralimnoria
2a: Uropodal exopod much shorter than endopod; only exopod with corneous apex. Antenna 1 flagellum with 4 or fewer articles. Pereopod 1 secondary unguis bifid, simple or sometimes with spinules. — Limnoria