The Cloudy Periwinkle and the Gulf Coast's 'Rocky' Intertidal
Ryan Moody - Ph.D. Candidate (and mollusk fan)

The diverse and colorful communities of rocky intertidal habitats, such as those found along the west and northeast coasts of the United States, attract amateur naturalists and professional scientists alike. Alabama, however, is not on the top of the list for those wishing to frolic amongst tide pools overflowing with sea stars, anemones and other animals adapted to life on a hard surface. In fact, you’d be hard-pressed to find a pebble on our white, sandy beaches, or amongst the miles of salt marsh lining the coast. Well, that was the situation a few decades ago. But have no fear, coastal armoring in the form of jetties, groins and seawall construction has done its part to attract vacationers (of the shelled variety) and boost local biodiversity.

Meet the cloudy periwinkle (Littoraria nebulosa, shown above). I first encountered this beautiful snail while climbing around on the jetties of Dauphin Island searching for critters to show a summer school class at the Dauphin Island Sea Lab. After a bit of Internet surfing and sending a few shells to a malacologist at the British Museum of Natural History, for whom I was collecting local periwinkle specimens at the time, I quickly learned its identity. “Looks like global warming is doing its part over there”, he mused. Yes, not only was this animal not supposed to be here, but it was far north of its ‘natural’ range: the limestone shores of the Caribbean. While global warming might play a part in allowing the cloudy periwinkle to survive north of its ‘natural’ range, the artificial, rocky substrate is most likely the key factor facilitating its range extension.

The cloudy periwinkle, I was quick to discover, is common along the west coast of Florida and eastern Texas, although even there they are found on rock emplacements and not in natural habitats. While my discovery was by no means a novel one, the presence of this animal on our shores, along with those of Florida and Texas, demonstrates that we can indirectly ‘invite’ invasive species without actively transporting them, intentionally or unintentionally, from here to there. In this case, all that was necessary was to provide the proper substrate on which the snail’s larvae could settle. More than a half dozen species of small periwinkles inhabit the ‘rocky’ intertidal zone of the northern Gulf of Mexico.  Each of these species likely originated in the Caribbean, uses planktonic larvae to disperse its young, and requires a rocky substrate on which to settle and graze algae.

Sessile (meaning ‘permanently attached’ or ‘fixed’) invertebrates such as oysters, barnacles and mussels are common on rocky, man-made structures. These animals lived naturally on oyster reefs and driftwood, as did many of their mobile predators such as snails and crustaceans. These local animals are simply capable of settling on rock as well as their natural substrates. As we increased the amount of rocky shoreline, however, we have unwittingly spurred a veritable periwinkle invasion. Despite concerns over invasive species and the deleterious roles many have had on the habitats they invade, we can take comfort in knowing that these herbivorous visitors have yet to exhibit any negative effects on our local fauna. In this instance, rather, and a rare one at that, coastal development has both increased the biodiversity of our coasts and, at the very least, provided us with a taste of the Tropics.