Where are all the corals?

Early Monday afternoon (Aug. 2) NOAA fishery biologist Jon Heifetz peers into a collection basket on the front of Jason II, which had just emerged from the ocean after a 20-hour dive. Corals poking out of the basket include a (fan-shaped) hydrocoral, left, and a Fanellia, a type of soft coral. Photo by SONYA SENKOWSKY

Location: Amchitka Pass

Posted 08.04.04 at 9:51 am

Submitted August 3

At the very deepest depths probed by Jason II over the past week—as deep as 2 miles below the waves—we’ve seen corals, but they have been different from corals at shallower depths.

In some places, corals were absent. In others, there were fields of single-stalked corals poking from a muddy and otherwise largely bleak seafloor. And although some corals are abundant—in some cases it seemed there were thousands—even their appearance is a far cry from the images of multicolored, multispecies sponge-and-coral habitats researchers brought back from shallower waters.

So, where are the rest of the corals?

After about a half-dozen dives in deep water, I asked chief scientist Bob Stone what conclusions he was drawing from differences between shallow- and deep-water dives.

The lack of diversity in deeper regions surprised him too, Stone said.

As all of us on the cruise have come to expect, there is little in the way of varied coral life deeper than 1,400 meters. The “best” dives for viewing coral and sponge diversity so far seem to be those at 800 meters and up. But still, says Stone, the views are “nothing like what you get at about 300-350 meters-that’s when things start happening.”

Based on previous finds in the North Atlantic and on undersea volcanoes in the Gulf of Alaska, he thought they’d see more of the kind of diverse coral habitat they’d found shallower.

But even when researchers did find the same species, the corals found at depths of 1,400 meters and below have often been smaller than their counterparts in shallow water. And they represented only a few dominant species.

Why is this, when there appears to be plenty of food available and sufficient current to deliver the food to the corals? (Lack of sunlight shouldn’t be an issue.) So why aren’t more varied communities of corals thriving here?

One contributing factor, Stone speculated, could be an unstable Aleutian seafloor, subject to frequent earthquake activity. Corals live hundreds of years and need stable rock holdfasts to cling to as they grow.

“It appears that in a lot of sites that we looked at that the seafloor is pretty unstable,” said Stone. And, because these are very longlived animals, they need a stable environment in which to live—rock that’s not going to slide, for instance.” During some dives, researchers have seen recent landslides. “If the corals are living in an areas that slides,” he says, “they’re going to be buried very quickly, and I think we’ve seen some evidence that that may be happening.”

Other factors include oceanography and geology, such as the types of rocks that are found in different depth zones.

Combining geology observations with what is learned of the biology at these depths is one of the main reasons researchers are here. At the end of this summer’s research, they will create models to predict where corals of different kinds probably occur in each region. Such models can ultimately be used to create a map of Aleutian coral life.

“I think we’re going to be able to make some pretty good predictions about where coral habitat is found,” said Stone. “There are a few things that are jumping out at us, so we’re pretty confident that we’ll be able to build a model to predict where this stuff is.”

The deepest views are still incredible, says Stone—“It’s just a different world in this water. The animals look much more bizarre—they’ve evolved pretty strange ways of dealing with life on the seafloor.”

Among the bizarre sightings have been a jelly octopus (which looks sort of like a puddle of gelatin with a big eye), a Dumbo octopus (see the Multimedia gallery for a picture) and what scientists believe could be a predatory sponge—a sponge that appears to be capturing and ingesting amphipods, a type of small crustacean. “To our knowledge there’s only been one other found in the world to this point,” said Stone. “We’re still working on that, trying to collect more samples, but we may have found one or two more species.”

One bizarre-looking coral has been dubbed the “pigtail” by researchers. (See previous journal entry.) “The pigtails are a chrysogorgid coral, and they truly look like pigtails,” said Stone. “They can be 5 or 6 feet in length and they’re just long, curlicue corals on a single stem.”

The new finds are intriguing researchers, leading them to ask new questions and helping them to answer those already on the table.