It was cold, windy, and overcast along the shore, with forecasters warning of an approaching Nor’easter. I saw a line of pelicans, a single Ruddy Turnstone, a single Sanderling, and a handful of people. But there were hundreds, perhaps thousands, of Sea Slaters (Ligeria exotica).
I’ve glimpsed a single Sea Slater from time to time, but never so many in one location. I stopped to watch as they foraged in groups and vanished like crabs at the slightest movement.
Although these 2-4 cm creatures resemble swarming roaches from a distance, they are actually crustaceans that feed on algae and decaying plant and animal debris. They also eat their own feces in order to recycle the copper in their diet. While our blood is iron-based, that of marine crustaceans is copper-based.
However, Sea Slaters don’t urinate! Instead, they discharge ammonia through their shells. This is why they taste like strong urine to humans.
The habitat on this particular beach was perfect for Sea Slaters. Huge boulders had been piled to protect the shore, providing hiding crevices and preserving pockets of moisture. These animals dry out easily and can only survive in damp areas, although they aren’t picky about the salinity of the water.
They scramble to hiding places when movement is detected. They are hunted by crabs, birds, lizards, small mammals, and fishermen seeking bait. Because they provide food to so many, they help maintain a diversity of species.
On this beach, mats of bamboo-like grass had collected around the stones, adding additional hiding areas and preserving moisture between the layered stalks. And as the grass breaks down, it provides the Sea Slaters an unending feast. The Sea Slaters return the favor by removing debris and returning nutrients to the environment.
Sea Slaters may prove helpful in another way. Extracts from Slaters have been shown to inhibit cancerous cell-growth.
Although Sea Slaters have gills, they only submerge to escape predators or when they’re pulled into the ocean by waves. However, they also have lung-like structures that allow them to breathe on land, their preferred habitat.
As I photographed the Slaters, I noticed many color forms. These animals “can change color by expanding and contracting special melanin containing cells located just under [the] cuticle.” Changing color allows the Slaters to blend in with the environment and regulate internal temperature.*
The female Sea Slater can breed multiple times during the year. She deposits her 14-142 eggs into a brood pouch beneath her body, where the young hatch in four to five weeks. The hatchlings remain in the pouch another day or so until they undergo a second molt and become independent. After they leave the pouch, the mother stays nearby for several months.
As they grow, Sea Slaters go through a two-stage molt. First the back half of the exoskeleton is shed, and later the front.
As I watched these little creatures socialize with one another, I became enamored. As I sat quietly and watched, one larger individual became curious about me. It approached cautiously and was about to climb on my shoe, when I pointed my camera and it scrambled away. Sea Slaters don’t bite, so there’s nothing to fear for those who allow investigation.
Although I prefer to use the name Sea Slater for these crab-relatives, they have several other monikers: Woodlice, Rock Lice, Wharf Roaches, Dock Roaches. Although they do swarm like roaches, move very quickly, and are active at night, I can’t bear to label them with such derogatory terms. As often happens, understanding leads to appreciation, and Sea Slaters are whimsical residents of a fascinating shore.
*Chatfield Matthew, Invasion of the giant littoral woodlice! Nature, The Ventilator. 20 Sep 2009.
Xu Lele, Li, Yongquin, Liu Yao, Mi Haifeng, Jiang Xiang, et al. A comprehensive evalubation of the potential of semiterrestrial isopods, Ligia exotica, as a new animal food. Nature, Scientific Reports 11, Article 7213, 2021.
Wharf Roach Ligia exotica, iNaturalist.
Jonathan Ho Kit lan, Ligia exotica – Sea slater. National University of Sigapore, 30 Jul 2020.
Woodlice fact sheet. Waltham Forest.
Wilson Judith, Life Cycle of a Roly Poly. Pets on Mom.com.
6 common rough woodlouse facts you need to know. Discover Wildlife. BBC Wildlife Magazine.