What is a bryozoan? Like a coral, it is a colony of thousands of small organisms surrounded by hard walls that interlock into a rigid, geometric structure. Drew’s project comes out of a discovery in 1987 on the Isle of Shoals off the coast of New England, where a group of divers found fields of kelp completely encrusted in bryozoans. The sea of white was an invasive species of bryozoan, commonly called “lacy crust,” and not the native “hairy bryozoan,” which grows in radiating stars that are only a few inches wide. The other player in our underwater saga is the sea slug, Onchidoris muricata, a blobby creature named for its bulbous spines. Drew described them as the “carnivorous cows of the ocean” because they graze exclusively on kelp bryozoans. While sea slugs generally keep bryozoans in check, they are picky eaters, and prefer the native species to the invasive bryozoans, which grow protective spines when they are being fed on. Without predation, the “lacy crust” bryozoan population can explode and encrust vast expanses of kelp.
Why is the invasion of kelp beds with an invasive species important? Ecosystems are complex webs of interactions, and altering one variable can throw everything else out of balance. In this case, scientists have hypothesized that otherwise flexible kelp beds covered in rigid bryozoans are more likely snap in strong currents. Kelp provide protective cover from waves that shelters the diversity of organisms that live in the intertidal zone, just as trees create a buffer against wind, allowing the understory to thrive. Bryozoan-induced breakage of kelp strands may have a cascade effect that will reduce the biodiversity of the ocean floor, which is already threatened by ocean acidification, global warming, overfishing, and a host of other problems.
Drew’s investigation of the invasive bryozoans’ effects on kelp and the biodiversity of the intertidal zone will be the first on Kent Island. The bryozoans, which are plankton in their juvenile form, won’t reach the Island until June, but he is planning a series of three experiments for their arrival. First, he will test whether kelp covered in the invasive bryozoans are more likely to break than kelp that are uninfected. Next, he will test whether sea slugs feed on the invasive bryozoan population, potentially reducing the amount of kelp breakage. He will also compare the types and numbers of organisms growing around the kelp with and without the invasive bryozoans to see if they decrease biodiversity. Drew will catalogue every visible organism he comes across as he wades and snorkels in the icy tide pools that surround Kent Island. Low tide exposes an underwater forest of algae of all textures, from long strands with the texture of wet hair to seaweed with sacs that pop like bubblewrap. This swirling greenery floats in rocky pools, and its roots provide a sheltered habitat for snails, small crabs, and juvenile fish. He will count these organisms around the base of each kelp bed, to see if the invasive bryozoans threaten the diverse intertidal ecosystem.
Science is by nature a learning process, and Drew will face many challenges, anticipated an unknown, as he embarks on this research project. The most pressing question is whether the bryozoans will ever reach Kent Island, because while they have been recorded as far North as Halifax, Nova Scotia, the currents around the Island may prevent the juveniles from every reaching the kelp beds here. So far Drew has been locating the kelp beds, but his experiments can’t really begin until the bryozoans appear. The real world of scientific research is not a series of groundbreaking discoveries: there are long periods of waiting, and roadblocks that make experiments that seem great in theory difficult or impossible in practice. Drew will have to adapt his research to the conditions on the Island, which is fortunately full of many interesting study subjects if the bryozoans don’t make it here.
Croxall, J. P., Butchart S. H. M., Lascelles, B., Stattersfield A. J., Sullivan B., Symes, A. and Taylor, P. (2012) Seabird conservation status, threats and priority actions: a global assessment. Bird Conserv. Int. 22: 1–34.]