The Gribble Worm
Owners of wooden boats kept within Hamble Harbour should be aware of the Gribble Worm and how it may effect the wooden structure of the hull underwater. There has recently been severe damage to a wooden hulled motor fishing vessel moored in the Harbour, rendering the vessel more or less useless owing to the damage done to the hull by this worm.
The worm likes to attack wood that is in free flowing water and not if the hull sits in the mud. It is not really understood yet if frequency of use of the vessel reduces the effects of the problem. It is understood that anti-fouling paint does have little effect only slightly restricting the effects of this wood boring worm.
As well as the MFV in Hamble being attacked the pier at Yarmouth has had to have its wooden piles replaced at great cost. A number of other structures in the Solent have had to have attention because of the damage caused by the Gribble Worm.
So, what is the gribble?
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 10mm.
The term gribble was originally assigned to the wood-boring species, especially the first species described from Norway by Rathke in 1799, Limnoria lignorum. 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).
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.
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 used to protect timber piles before its use became illegal) due to symbiosis with creosote-degrading bacteria.
Gribbles bore the surface layers of wood, unlike the Teredinidae which attack more deeply. Their burrows are 1-2mm diameter, may be several centimeters long, and have the burrow’s roof punctured with a series of smaller ventilation holes. Attacked wood can become spongy and friable.
For defense, gribbles can jam themselves within their burrows using their uropods, and block the tunnel with their rear disc-shaped segment, the pleotelson.
As an example of the damage that can be done to wood these photographs show the damage that the Gribble has done to a wooden slipway on the Medina.
The whole thing has been nibbled away from the inside. What appeared to be sound wood turned out to have the texture and strength of cheese.
Looks like the bearers are gone too.
Gribbles are marine isopods (related to woodlice) which have the ability to eat timber - and certainly enjoyed this slipway. They favour the softer wood, not surprisingly, and so they had a good go at the larch planks, slightly less on the hardwood bearers, and luckily couldn't make an impression on the old greenheart piles which won't need replacing this time. A characteristic of gribble attack is that the crustaceans don't care for the sunlight and dry timber that is found on the exposed surfaces, so the wood they prefer to eat is hidden away. Hence the nasty surprise when the broken boards were lifted.
- R.J. Menzies. The marine borer family Limnoriidae (Crustacea, Isopoda). Bulletin of Marine Science of the Gulf and Caribbean. 1957 7: 101-200.
- L.J. Cookson. Australasian species of the Limnoriidae (Crustacea: Isopoda). Memoirs of the Museum of Victoria 1991 52: 137-262.
- L.J. Cookson and G.C.B. Poore. New species of Lynseia and transfer of the genus to Limnoriidae (Crustacea: Isopoda). Memoirs of the Museum of Victoria 1994 54: 179-189.