Limnoria lignorum Rathke, 1799 Common name(s): Gribble
Synonyms:
Phylum Arthropoda  Class Malacostraca  Subclass Eumalacostraca  Superorder Peracarida  Order Isopoda  Suborder Flabellifera  Family Limnoriidae
	Limnoria lignorum, about 3 mm long on the left and 2.5 mm long on the right, found boring into wood on Rialto Beach.  These specimens are dried ones found within the cast-up driftwood.  Several lighter particles or pustules are clinging to the dorsal pleotelson of the individual on the left--a phenomenon seen on several individuals.
	(Photo by:  Dave Cowles)

Description:  As in other Flabelliferan isopods, the uropods are flattened and are lateral to the pleotelson rather than ventral or terminal.  The exopodites of the uropods do not arch over the pleotelson and pereopod 1 is not greatly enlarged.  This family is unusual for an isopod because the head (cephalon) is not attached broadly to the pereon so the head can rotate, and the uropods are unusually small.  The first thoracic (pereon) segment in Limnoria is about twice as long as the rest.  In this species the pleon has 4-5 visible pleonites plus the pleotelson.  The animal can roll up into a ball.  The uropods are small, and the exopodites of the uropods are somewhat clawlike.  The incisor process of the right mandible has file-like ridges, and the incisor process of the left mandible has rasplike scales (but the mandibles have no teeth).  The dorsal surface of the pleotelson has a small median ridge that splits into two ridges posteriorly but does not have 3 or 4 small tubercles.  Bores into wood.  Usually less than 4 mm long.

How to Distinguish from Similar Species:Limnoria algarum bores into kelp holdfasts. Limnoria tripunctata bores into wood but it has 3 small tubercles on the dorsal pleotelson instead of the bifurcating ridge.

Geographical Range:  In temperate and polar waters of the northern hemisphere.  From Kodiak Alaska to Point Arena, California on our coast.

Depth Range:  Intertidal to 20 m

Habitat:  Boring into wood

Biology/Natural History:  This species bores into wood near the intertidal zone, such as pilings and driftwood.  Its boring rate is about 1-3 cm/year, and it can eventually do extensive damage.  The burrows of this isopod are far smaller in diameter than that of the shipworms (bivalves which also bore into wood), being only about 1 mm in diameter (photo).  When digging, they jerk their head back and forward while turning slowly in the burrow.  They typically start to burrow into the wood at a shallow angle rather than digging straight in.  They swallow the wood chips, which pass through the gut as rapidly as 80 minutes.  Besides being of much smaller diameter, the burrows of gribbles are much shallower than those of shipworms, going only a centimeter or two into the wood.  Also, unlike shipworms, they often dig interconnections between their burrows (photo).  Although this species has several ciliated protozoan symbionts, some of which also produce cellulases, it is a very unusual animal because it appears to produce the cellulase enzymes necessary for digesting the cellulose in the wood itself.  The gut, where the wood is digested, appears to be sterile, with very few or no bacteria or protozoans within it.  One further problem with digesting cellulose from wood is that much of the cellulose is bound in lignin, which is very difficult to digest.  A related species (L. quadripunctata, see King et al., 2010) has high amounts of hemocyanin in its gut which may produce phenoloxidase.  The phenoloxidase may be used to break up lignin, plus may explain why bacteria seem absent from the gut since phenoloxidase is toxic to bacteria.  Various symbionts can be found on the outer surface, especially the pleopods, and may be important in providing nitrogen to the animal.
 

Females of this species carry around 22 large eggs in their marsupium or pouch between the front legs.  The eggs are nearly 1/4 the width of the body.   They do not hatch into a larval stage but instead hatch like small adults which begin burrowing.  Therefore gribble colonies in wood are quite localized and spread slowly across the wood surface as they reproduce and start more burrows.  Young and eggs can be found year-round, but peak breeding in the Salish Sea area is in April and May.  Not only do the young not normally swim, but they do not even seem to be chemically attracted to wood, as many other larvae are attracted to their appropriate substrates when settling.  Presumably they simply remain on the wood in which they are born and begin burrowing there by default.  Later in life, some adults do swim away, especially in the spring and summer.  During this migration phase they do respond positively to wood (see references in Cragg et al., 1999).  Their preference for wood seems to be enhanced by the presence of fungus or fungal metabolites on the surface or the scent of other gribbles.

References:

General References:
  Johnson and Snook, 1955
  Kozloff, 1993
  Ricketts et al., 1985

Scientific Articles:
Cragg, S.M., A.J. Pitman, and S.M. Henderson, 1999.  Developments in the understanding of the biology of marine wood boring crustaceans and in methods of controlling them.  International Biodeterioration and Biodegredation 43:4 pp 197-205

Elshanshoury, A.R., M.H. Mona, F.A. Shoukr, and A.M. Elbossery, 1994.  The enumeration and characterization of bacteria and fungi associated with marine wood boring isopods, and the ability of these microorganisms to digest cellulose and wod.  Marine Biology 119:3 pp 321-326

King, Andrew J., Simon M. Cragg, Yi Li, Jo Dymond, Matthew J. Guille, Dianna J. Bowles, Neil C. Bruce, Ian A. Graham, and Simon J. McQueen-Mason, 2010.  Molecular insight into lignocellulose digestion by a marine isopod in the absence of gut microbes.  Proceedings of the National Academy of Sciences 107:12 pp 5345-5350 (for a related species, L. quadripunctata)

Richards, B.R. and C.I. Belmore, 1976.  Occurrence of woodborer, Limnoria-lignorum (Rathke) at Amchitka Island, Alaska.  Journal of the Fisheries Research Board of Canada 33:7 pp 1642-1644
 
 

Web sites:

General Notes and Observations: Locations, abundances, unusual behaviors:

This is the piece of washed-up driftwood from which the Limnoria lignorum photographed above was extracted.  The ruler is included for scale.

A ventral view of another individual from the same piece of wood.
 

Authors and Editors of Page:
Dave Cowles (2015):  Created original page
CSS coding for page developed by Jonathan Cowles (2007)

Salish Sea Invertebrates web site provided courtesy of Walla Walla University