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  Monday, April 24, 2017  
   
 

 
Whelks: Mapping an Invasion

 

The shell sat heavy in my hand, its large body whorl, the size of a softball, exposed a brilliant salmon- orange interior with dashes of black veining. This was one of the most beautiful whelk shells I’d ever seen, but the creature that created it is a foreign invader that has marine scientists and informed watermen concerned. Welcome to Rapana venosa, better known as the Veined Rapa whelk, a large predatory snail that has staked its claim on the lower Chesapeake Bay and its tributaries.

In today’s technological world of spy satellites, security cameras, and listening devices, few invasions go unnoticed, but in July 1998 when the first Rapa whelk was discovered by a trawl survey group at the Virginia Institute of Marine Science, the invasion was already well underway.

At the time of its discovery, local scientists were at a loss to identify the species; it took a visiting scientist from the Black Sea region to recognize the striking gastropod as the Veined Rapa whelk. By the time VIMS made their first discovery, local watermen had been seeing it for at least a decade. To many of them, it was just another whelk. Little did they know what VIMS scientists quickly realized—the Bay had been invaded by a predator with few enemies and a huge appetite for shellfish.

Rapa whelks are native to the marine and estuarine habitats off the coast of Japan and Korea where they have long been harvested for food, which one VIMS scientist describes as “an acquired taste” due to its purple secretions and foul odor.

They were introduced into the Black Sea in the 1940s, most likely hitching rides on seed oysters. With a tolerance for a broad range of environmental conditions (temperature fluctuations, salinity levels, and dissolved oxygen), Rapa whelks found the Black Sea quite hospitable. Within 25 years it had colonized the entire sea, where it was blamed for major declines of the Mediterranean mussel and other bi-valves.

Within three decades they had spread to the Adriatic, Aegean, and Mediterranean Seas. With few known predators, Rapa whelks are now spreading to the English Channel and North Sea. Roger Mann, Professor of Marine Science at VIMS, hypothesizes how it arrived in the Bay.

“Large ships like coal carriers take on ballast water after off-loading their cargo and then dump that ballast water as they approach our coal terminals. In this area, it’s at the mouth of the Bay. Ballast water acts like a huge aquarium. Travel time across the Atlantic is slightly less than a whelk’s larval development stage when they are about the size of a pepper grain. By the time the adult whelks were discovered, they had been here for some time and were breeding.” The large specimen I held in my hand was approximately ten years old; Rapa whelk are sexually mature by age two.

Its appearance alarmed VIMS scientists. To understand the implications associated with this particular species, one should understand whelks in general. The Chesapeake Bay is home to its own native whelks. For many of us, our only experience with them are the empty shells we find tossed up on the beach after a storm, which some refer to as conchs, a misnomer. In this area, Channel and Knobbed whelks inhabit the intertidal and subtidal regions of the lower Bay.

A voracious predator, whelk pry and rasp a gap between the valves of its prey—clams, mussels, and oysters—by exerting force with its large muscular foot. When a small opening it created, it uses its thick shell as a wedge and inserts its proboscis to consume its prey’s soft body. Native whelks are part of the bay’s food chain, being eaten in turn by sea turtles and blue crabs and adult whelks are harvested for consumption, particularly for markets in the Far East.

Native whelks reach sexual maturity at about age nine. Twice a year, in spring and fall, they lay egg strings that consist of translucent, flattened capsules attached at one end to a strong cord that anchors to the bottom. Whelks complete their development inside their capsules, which hold 30-35 whelks each. Once hatched, baby whelks are miniature versions of its parents, complete with the large muscular foot used for locomotion and burrowing. These little snails can only move as far as they can crawl.

Rapa whelks on the other hand lay tall, thin cylindrical egg cases that resemble a thick, yellow shag carpet. An individual egg case may contain as many as 400 eggs; an egg mass or group may have as many as 500 egg cases within it. An individual Rapa female may lay as many as 200,000 eggs in a single egg mass. In a single breeding season, she may lay ten or more egg masses. Do the numbers--that’s two million baby Rapa whelks a year from a single female!

Unlike our native whelks, Rapa larva, or veligers, swim out of their egg cases and float in the water for 4-5 weeks after they hatch. The young whelk use velum, long hair-like cilia, that enables them to stroke and move through the water. Tidal currents carry the tiny whelks at will, disbursing them far from where they hatched. At any given moment, there are millions of Rapa young swimming in the lower Bay. After a few weeks, they grow large enough to settle to the bottom where they join others of their ilk.

To map the distribution of this invasion species, VIMS initiated a Rapa whelk bounty program in September 1998. With funding from various state agencies, VIMS paid a bounty to watermen of $5 for each live Rapa whelk and $2 for each dead one turned in.

“The first year we thought we had sufficient funds to last an entire year,” said Roger Mann. “It took just three days to go through the bounty money. One waterman brought in forty in one day.” From May through September 2001, watermen donated 630 Rapa whelks caught in the Hampton Roads region. Lined up on shelves at VIMS, their shells range in size from ping pong balls to soft balls.

By the time program funding was cut by Governor Tim Kaine in 2009, 18,000 Rapa whelks had been collected. By then, the incursion extended from Ocean View north to the James and York Rivers, eastward to the Eastern Shore and north as far as the mouth of the Rappahannock River. Since 2009 no active monitoring and data have been collected, but scientists speculate the Rapa whelk will eventually extend its range from Cape Hatteras to Cape Cod and possibly further.

Rapa whelks have an insatiable appetite for the seafood we cherish most—oysters, mussels, and particularly hard shell clams. “They present a considerable challenge to the hard clam fishery,” says Mann. “Oysters have made a remarkable comeback in recent years but they inhabit regions of lower salinity. Rapas like higher salt levels, regions where hard shell clams also inhabit.”

As with all predators, its population is controlled by two things: available food supply and predators. At the moment there are abundant food sources in the Bay and, based upon the size of some of the whelks recovered, they are eating quite well.

In laboratory experiments, small Rapas consumed 3.6% of their body weight in a single day. Some estimates say that a population of just a 1000 Rapa whelks could reduce the hard clam harvest by 0.3-0.9% per year. And the harvest for clams has already declined due to overfishing, water pollution, and disease. The addition of a new predator to the food chain adds an additional layer of stress.

For the first several months of their lives, Rapa whelk face the same predation as our native whelks. As veliger they are planktonic, vulnerable to the myriad plankton eaters in the bay—sea nettles, comb jellies, Atlantic menhaden, and other filter feeders—capable of reducing Rapa whelk populations to a certain extent.

Once Rapa whelks reach a certain size, there is little to prey on them however. Their boxy shape and thick shell are deterrents to sea turtle beaks, and blue crab claws are no match to their heavier shells. Since Rapa whelks may live to be than fifteen years old, it’s clear it will take a reduction in their food supply for the population to be impacted. Not good news for local watermen.

As Mann researched century-old Japanese museum records it became clear that the largest whelks are here in the Chesapeake Bay. In its native habitat, Rapas and their food supply have reached a balance, thereby limiting their numbers and size. Until stasis is reached in the Bay, Rapa whelks will continue to flourish unchecked.

“As ocean temperatures rise, warmer waters trigger feeding and mating in several marine species” points out Robert Fisher, Marine Fisheries Specialist at VIMS. “Warmer waters mean accelerated gestation. On the other hand, acidification of the ocean is also taking place and that affects shell production.”

The ecological ramifications of the Rapa whelks’ arrival in the Bay are in the early stages and its story is just beginning. 

Special thanks to the following individuals at the Virginia Institute of Marine Science: Roger Mann, Professor of Marine Science; Robert Fisher, Marine Fisheries Specialist; and Susan Maples, Director of Outreach.