The Trouble with Gribbles

Co-written with Dr. James T. Carlton, a knower of maritime history and invasive species expert

What could not be accomplished by the brute force of any marine animal, and might baffle even the ingenuity and power of man himself, is yet quietly accomplished by the gradual but steady operations of a tiny crab.

John Coldstream, 1934

Gribbles are small marine invertebrates (hardly 1/8 of an inch, or the size of a grain of rice) responsible for the destruction of vast amounts of wood in the ocean. On the one hand – recycling!  yea! On the other hand, if the destruction is of wood we value (pilings, docks, wharves, boats), that’s a problem — and sometimes a really big problem, but we’ll discuss that later.

Gribbles are crustaceans, which include such well-known animals as crabs, lobsters, and shrimps. Specifically, gribbles are isopods, which we’re familiar with in our backyards as pill bugs, roly-polys, or potato bugs (in England, they’re called wood lice). These terrestrial isopod species consume detritus; they aren’t wood borers.

Despite the destructive nature of gribble “infestations”, these smol beans are at the “I just can’t even” level of cuteness.

There are male and female gribbles. After reproduction, females brood the young in a special pouch on her belly. Once the gribble babies fully develop, they are released from the pouch into their woody world. Both baby and adult gribbles are active swimmers – which is one way they colonize fresh, new wood.

A little history and some taxonomy

Gribbles were likely well-known to ancient sea-farers (there are records from ancient Greece of animals that destroy ships and evidence that ancient Egyptians took measures to replace bored ship wood), but it wasn’t until 1799 when Jens Rathke, a Norwegian naturalist, illustrated and described Cymothoa lignorum. In 1811, they were discovered to have seriously damaged wood from the temporary railway constructed in 1807 to service the famous Bell Rock Lighthouse in Scotland, built by the engineer Robert “Big Bob”* Stevenson.

* we might or might not have made this up

So destructive to timber is this small insect, that the Norway logs, laid down to support the temporary railways in 1807, when lifted in 1811, were found to have been reduced by its ravages from 10 inches square to 7 inches, or at the rate of about an inch in the year.

Robert Stevenson, 1824

Stevenson did some gribble-gathering for his buddy William Elford Leach, who, as email and smart phones had not yet been invented, did not know of Rathke’s account, so named and described Limnoria terebrans in 1815. Both Cymothoa lignorum Rathke, 1799 and Limnoria terebrans Leach, 1815 refer to the same species, now called Limnoria lignorum (Rathke, 1799).

For over 100 years, scientists thought Limnoria lignorum was the only gribble species. It wasn’t until the 1880s that scientists realized that there were other species in the genus. In the last 70 years or so, over 50 species of Limnoria have been described! Most of them bore into wood — some bore into seagrass roots and other substrates.

What do they actually do, and what’s their impact?

Since they gnaw and chew and nibble from the outside, the outer layers of wood steadily fall off, a process that is sped up by crashing waves and rubbing against rocks or other things (in mobile wooden structures), and as a result the tiny crustaceans bore deeper. When gribbles bore into wooden pilings, they produce a classic hour-glass shape which one can see from the shore at low tide under old wharves.

Gribbles,

• destroy wooden structures in U. S. ports and harbors, including North Carolina, causing major economic damage.  Gribbles and other marine borers (which we talk about below) cause over $100,000,000 in damage annually!
• destroy wooden ships and shipwrecks, and thus have had an important impact on nautical archaeology and our maritime heritage
  • for example, scientists found a sediment-covered ship, believed to have been abandoned by Byzantine sailors around 610-695 AD, in the Harbor of Theodosius, Constantinople (now Istanbul) that had evidence of gribble damage
  • for most sunken ships that weren’t covered (and thus preserved) by sediment, other types of remains like pottery and metals are likely all that is left to study
• have been transported by wooden ships for centuries, such that there have been many gribble species invasions, introducing gribbles to regions where they never occurred historically in ports and harbors on continents all over the world, including remote islands like Hawaii and the Galapagos.  If you were to look, you might even find Pacific ocean gribble species boring into mangrove trees on the Atlantic coast of Florida!

Gribb-lit (an Interlude)

Gribbles: one of many marine wood-munchers!

Most wood doesn’t last long in the ocean because, in addition to gribbles, there are many other animals that eat wood:

• Shipworms, which are bivalve mollusks in the genus Teredo — they bore from the “inside out,” rather than from the “outside in” like gribbles. There are many species of shipworms all over the world.
• Pholads, called piddocks, which are also bivalve mollusks.
• Wood-boring pill-bugs, or Sphaeroma species, a major wood-boring one being Sphaeroma terebrans (one of its synonyms is Sphaeroma destructor !). Native to the Indian Ocean, Sphaeroma terebrans is very common in Florida and south — and will be coming to North Carolina with global warming!

The destructive Teredo, like the lion, has his jackal — the Limnoria terebrans, or gribble worm…The gribble finds some little space, bores in and destroys the wood around. The Teredo then finds an entry and destruction follows.

Francis Buckland, 1861

How do we manage to control Gribbles and other marine borers?

Written records of ways to protect against marine wood-borer damage extend back to the time of ancient Greece. We have no idea which borers they were – everything was called a worm! In the past, many different ways of protecting ship hulls (called sheathing) and wood pilings for piers, bridges, etc. were tried, but most didn’t really work. Creosote (“obtained from high temperature distillation of coal tar” – EPA.gov) seems to do the trick if used correctly. In the United States, creosote treated wood was first patented in 1838 to protect wood from marine borers.

Bonus fun fact! Only 2 references to the Limnoriacide Company have been made since 1904: 1) somewhere in the 904 typewritten pages of Jim’s dissertation and 2) this blog post.

A warming world – good news for Gribbles?

More and more gribble species will move towards the poles in both hemispheres with a warming ocean brought about by climate change. This means that:

• with the shorter and warmer winters in colder temperate latitudes, like ticks, gribble populations will suffer less winter mortality and boring activity seasons will be longer and longer, and
• gribble species could be introduced to new locations too cold for them in the past

Both points above are bad news for any human-made wooden structures in the ocean that previously didn’t have any trouble with gribbles!

Benefits of Gribbles

It’s not all bad though, it’s possible humans can harness the awesome power of the gribble! Most animals are simply unable to break down the complex carbs of wood, called lignin, without the help of microbes – even cows have multiple stomachs to deal with their diet of grass! Gribbles are unique in that they have special enzymes (and NO gut microbes) to break down lignin for energy – if we can figure out how to synthesize those enzymes, they could help us turn waste into more sustainable biofuels!

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A final note

And perhaps after reading all this you wonder, “BUT WHY ARE THEY CALLED GRIBBLES?” – most etymological suggestions point to gribble probably being derived from “grub,” since they look like little insect grubs.