Zonation¹ on Walls

May 29, 2007

Marc Slattery
University of Mississippi

Michael Lesser
University of New Hampshire

Liz Kintzing
University of New Hampshire

Cristina Diaz
Museo Marino de Margarita – Venezuela

Deep-reef sponges make their shallow-water relatives look very small. (Quicktime, 1.4 Mb.)

We’re in the zone . . . the Twilight Zone! After 10 days of successively deeper dives, we have finally penetrated the transition between the shallow- and deep-reefs of Little Cayman. The community break was incredibly dramatic as we drifted past an overhang at 190 feet (57.9 meter) to the undercut wall below. Shallow-water species extend to this transition, although their abundance decreases dramatically with depth. Nonetheless, we were unprepared for the rapid change in species assemblages below the overhang; the Twilight Zone seems to be home to one of the most diverse and abundant sponge assemblages in the Caribbean Basin. There appear to be several extremely rare or new species within this community. (From our initial minimal collection, we found 15 sponge species that don’t occur on the shallow-reef.) In addition, these sponges make their shallow-water relatives look minuscule; we saw deep reef species that exceed 12 ft (3.7 m) in diameter!

Zonation¹ is an important concept in marine community ecology, but the classic examples are usually associated with the rocky intertidal areas. On the west coast of the United States, the intertidal is characterized by three distinct bands of indicator species. Moreover, the upper distribution of these species is apparently controlled by abiotic² factors (typically the length of exposure to air during low tides), while the lower depth distribution is determined by biotic³ factors (typically predation by sea stars and competition for space). Coral reefs are also divided into broad zonation schemes based on geomorphology⁴ and depth, including the back-reef or lagoon, the reef crest, the “spur-and-groove” sections, and the fore-reef walls.

Community complexity has made it difficult to further subdivide these zones by species assemblages. Technical diving has provided us with a mechanism to explore a very different species assemblage representing a true zone on the fore-reef walls of these coral reefs. While our time and depth were limited by safety considerations, we began an initial census of the biodiversity and collected several unique species for further research.

The Twilight Zone may have some characteristics in common with rocky intertidal zones. The upper limit, at 190 ft (57.9 m), is almost certainly controlled by physical factors. The overhang is likely a submerged ancient shoreline from a time when sea levels were much lower than the present, and the undercut may have been formed by past wave actions. The result is a section of the wall that “sees” a dramatic reduction in sunlight, or shading, which quickly selects against the photosynthetic species, like corals and algae. As we noted in a prior blog (May 27), there are almost certainly high nutrient inputs to this deep reef, and the overhang may cause eddies that further constrain nutrients to this region of the reef, thus supporting significant sponge growth rates.

As is the case in any research effort, results tend to generate as many questions as answers. Over the next several months we will examine our data, formulate hypotheses, design experiments, and plan the logistics necessary to further explore and study the Twilight Zone.

²Abiotic refers to components that are non-living chemical and physical factors in the environment.

³Biotic refers to components that are living biological factors in the environment.

⁴Geomorphology is the study of landforms that include their origin and evolution, and the processes that shape them.