Florida State University : Research in Review

Fishy Beginnings

In the mid 1980s, after almost a decade getting to know sponges in Jamaica, Belize and Panama, Wulff believed as other researchers did, that left undisturbed, sponges would stick around for a long time. Wulff certainly knew the animals were resilient. She'd seen many bounce back from damage within weeks after a hurricane roared through one study site.

But in 1984, Wulff almost by accident embarked on an unprecedented project that may change the way scientists study sponges from now on. It began with a bit of envy, impatience and sponge-eating fish.

Wulff was at a remote field station on nothing more than a sandbar flanked by two small tropical islands of the San Blas Archipelago off Panama's Atlantic coast. The Smithsonian Tropical Research Institute, started more than 80 years ago in Panama, had opened a field station in San Blas in the 1970s with the approval of the native residents, the Kuna people.

The other station researchers were studying fish, and every day, they would return from the field with fresh data. Meanwhile, Wulff would set up experiments and have to wait for months, even years, to gather her data. Eager to collect daily observations and numbers like her colleagues, Wulff decided to study fish, too.

She already knew that certain reef fish—notably French and gray angelfish—actually made sponges part of their diets. By studying what was in their guts, Wulff's Yale advisor had guessed that angelfish fed in a "smorgasbord" fashion, taking a few bites of one sponge species and then a few of another species and so on. This feeding habit, observed in other animals, had been suggested as a safe way to eat food that contained toxic chemicals used as defenses against predators. Wulff wanted to test a hypothesis that angelfish fed on sponges this way.

To understand the fish's diet, Wulff would need to find out not only what the fish were eating but also what sponges were available in the first place and in what quantities. That meant taking an in-depth census of sponge species and their abundance in terms of volume of living tissue.

For her underwater lab, Wulff settled on a shallow-water reef off nearby Guigalatupo Island. For measuring sponge volume, she had to start from scratch, tossing out typical techniques ecologists use to measure organisms in the field. Sponges vary so tremendously in shape and size that the usual methods can't capture how big or small they are.

A standard method of measuring organisms in the field is to plunk down a meter-square frame fitted with a mesh, and then record what species are under each mesh square.

"That's easy for getting a lot of data points. But the problem is that…a sponge the size of this table gets maybe four points, and so do thin little encrusting sponges that have many orders of magnitude less volume."

So for measuring volumes of sponges, this method was useless. Instead of a frame of plastic pipe and mesh, Wulff equipped herself with a ruler and underwater slate and got into the water.

"It's really painful. I measure every dimension that I think of, and I draw a picture of it… if part of it looks like a sphere and part of it looks like a box and part of it looks like a cylinder, and each part requires a different set of measurements. Sometimes it's a lot of measurements for an individual."

For eight hours a day for two weeks that summer, she dove on a section of the reef, drawing and measuring each individual sponge. In an area the size of a typical American bedroom, she counted nearly 1,400 individual sponges—many no larger than a marble. To complicate matters, Wulff discovered that this community included 39 different species.

Ultimately, Wulff confirmed that angelfish did indeed eat in a smorgasbord fashion, as her Yale advisor had hypothesized. With no shelter, claws, teeth or other physical protection to escape predators, many sponges produce chemical deterrents in their cells, which Hartman had suggested discourages predators from binging on a single animal; and thus both sponges and those that munch on them survive.

But angelfish nibbling aside, Wulff realized with her self-made technique for gathering sponge census data, she might be able to do something that she had not seen done before—study the dynamics of a whole coral reef sponge community over time.

"I thought this is neat. If I just keep doing this, then I'll be able to (confirm to) everyone, sponge communities are particularly stable."

In the end, however, the process of measuring volume for her angelfish study would suggest something else entirely.

Sponge Plunge

When it comes to sponges, measuring volume, rather than counting individuals, reveals far more about their abundance and role in a reef community. Able to reproduce asexually—with fragments breaking off and drifting away to a new home— "baby" sponges add individuals to the population without adding volume. And with sponges, which act as shelter for many other animals and as crucial water filters, it's volume not number that matters more.

While moving on to study other aspects of sponge life, from predation to cooperation, Wulff kept up the census-taking surveys on the tiny piece of Guigalatupo reef. It became a project she did on the side for an astonishing 14 years.

After her initial survey in 1984, Wulff visited the same 170-square-foot chunk of reef four more times, donning her snorkeling gear and ruler to measure and re-measure all the sponges. She didn't have funding or a hypothesis but thought it would be a shame not to follow through.

She took her last census in 1998, but Wulff didn't begin to crunch her data until a few years later. Along with other researchers in the field, Wulff was reasonably certain sponge communities were stable and wasn't in a hurry to prove it. In fact, for the first 11 years, sponge volume in her study site hovered between 2,050 and 1,840 cubic inches, and the three species Wulff mainly focused her research on stuck around year after year.

But when Wulff analyzed the actual number of species still around from the original community in 1984, a vastly different picture emerged. All totaled, less than half the number of species originally found in the study area were left.

"When I finally analyzed it, it was a shock," she said.

It didn't start off badly. After the first four years of the study, only two species disappeared from Wulff's reef. During the same period, two new species colonized the area. But by 1994, eight more species had vanished, and only one new species appeared. It would become the last new colonizer.

In just another year, seven more species dropped out of sight. By the last census, the number of original species slipped by another four. In the later years of the census, the loss in species seemed to take its toll on volume. Between 1995 and 1998, overall sponge volume plummeted by 35 percent, she found.

Making this dramatic change even more remarkable was where it was happening, Wulff realized. Here was a fragment of ocean bottom in an area almost as pristine as nature gets. Wulff said no industry or clear-cutting that would dump sponge-choking sediment into the water was nearby, and Guigalatupo Island itself was uninhabited.

Considering how intertwined sponges are with overall reef health, the finding was depressing. In another study, Wulff had found that reef sponges of different species can improve each others' chances of survival. That finding suggested that high species diversity is a boon to all sponges on a given reef, and conversely, that the loss of some sponge species could increase the rate of loss of other, dependent species.

And in her first published study in 1979, Wulff found sponges can help corals survive, too, not just other sponges. She and another researcher had removed sponges from one area of coral reef in San Blas. In another reef area of the same size, they left the sponges alone. In these areas where sponges were undisturbed, only 4 percent of the corals fell off the reef and died. In the study area where the researchers removed the sponges, an astonishing 40 percent of corals fell and died within six months.

For now, too few clues

Exactly what caused the drop in species and volume on part of Guigalatupo reef remains unclear. A hurricane pounded the area and took a toll on the sponges in 1988. But Wulff said the storm didn't appear to trigger the long-term sponge decline. Sponge volume had bounced back quickly after the storm, and many of the sponges that were lost in the violence of the hurricane belonged to branching species that thrived throughout the 14-year study.

Fish nibbled at the sponges but didn't seem responsible either for the massive loss of their prey. Wulff noticed thousands of fish bites, but the finny predators snacked only in small amounts from a variety of species. In fact, one group of sponges of the genus Ircinia that fish actually avoided was one that disappeared almost completely.

Previous research by others had suggested a link between a decline in corals in San Blas reefs and changes in temperature or sedimentation. But one of the main culprits, Wulff suspects, is disease.

Wulff had observed diseased sponges at every census. And in other studies, scientists have reported that disease is on the rise in coral reefs in general. More specifically, other sponge biologists have found that species in the keratose group—those with skeletons made up of a protein called spongin, the type bath sponges belong to—have died at higher rates from disease than others. In Wulff's study, 78 percent of keratose species disappeared versus 43 percent of species in other groups.

With only circumstantial evidence to go on so far, the cause remains a guess. Perhaps more importantly, the question of whether the decline represents a wider trend of sponge loss around the world also remains unanswered.

"There are no other data (like these)," she said.

The few reports on sponge loss that have been published, Wulff said, focus on mass sponge death following major events—for example, the cyanobacterial blooms in Florida Bay between 1991 and 1993. But these findings don't reflect what's happening to sponges gradually over time.

The animals don't leave signs behind once they die, so unless someone is there to record their lives and deaths, entire communities of sponges can quietly and quickly disintegrate without anyone knowing they were there in the first place.

Wulff remains the only scientist who has published (Biological Conservation, January 2006) results from a long-term series of censuses of the same coral reef sponge community.

Klaus Rtzler, a sponge biologist with the Smithsonian Institution and a long-time friend and colleague of Wulff's, said that Wulff's work has influenced how their colleagues think about many aspects of sponge and reef ecology. Even though "some reef ecologists, including (him), have made similar observations, no one before Janie has done such a diligent, long-term, quantitative study."

Wulff says she hopes her findings will spur others to take on similar, long-term studies on various reefs around the world. But the required labor- and time-intensive techniques will take years more to yield any new results.

In the meantime, to see whether her Guigalatupo results might be part of a wider pattern, Wulff plans to repeat her work elsewhere, and this time with the backing of the National Science Foundation. Since 2003, she's also been monitoring sponges living on mangrove roots in Belize, Panama and the Florida Keys.

SPONGE PLUNGE