The Curious Life Beneath Piers and Docks

Photogenic coral reefs and majestic kelp forests deservedly garner much of the dive industry’s interest: They are beautiful, and they significantly contribute to marine diversity. Unfortunately, linked habitats such as sand and rubble slopes, seagrass beds, mangroves, rusting shipwrecks, piers and docks are habitually overlooked because they are perceived to lack traditional aesthetic value. As a result, many people do not fully appreciate these habitats’ ecological importance.

Piers and docks in particular act as artificial reefs, harboring an abundance of shallow-water fish. While not intentionally designed to attract marine life, these partially underwater structures often serve as oases for both common and extraordinary organisms in locations that may otherwise amount to aquatic deserts. Even though the diversity of life under piers may seem to be relatively sparse when compared with healthy Pacific coral reefs, the sheer variety of animals, plants and microorganisms drawn to these man-made formations can be vast and the associated food web exceedingly complex.

Throughout the islands of the western Pacific, thousands of piers and docks have been built in a variety of marine environments. Those put in place along deep channels are swept by nutrient-rich waters, which deposit prolific filter-feeding marine growth on their vertical structures. Coralline algae, sponges, hydroids, tunicates and brilliant Dendronephthya soft corals develop in ever-changing menageries and bouquets that proffer limitless micro-habitats for additional invertebrates and small, planktivorous fishes.

Lumpy and colorful stonefish looks like a coral mass
Reef stonefish are true masters of disguise, and they seem to habitually make their homes in the shadows of docks or piers.

Other piers and docks are built in protected, current-free bays where large mazes of forested islands, limestone islets, thick mangroves and narrow channels serve as nurseries, reproductive sites and feeding grounds for thousands of marine species. Although the region may include plenty of thriving underwater habitats — such as fringing reefs, flooded forests, marine lakes and submerged seagrass meadows — piers and docks offer unique characteristics such as dark recesses sheltered from direct sunlight and stationary vertical substrates that act as steadfast territories for both sessile invertebrates and hungry fish.

Drifting down among the dim pilings under just about any Pacific pier that has been in the sea for more than a few weeks, divers will quickly see that marine life can reach robust levels here. Sunlight sparkles along the edges of the terrestrial platforms above and peeks through wooden slats, creating striking light beams that waver across the sand and rubble bottom. Despite the diversity of the western Pacific region in terms of colorful, diurnal reef species, most pier-dwellers prefer muted, gloomy confines where camouflage is vital to survival. Deep shadows cast by pilings, dock floats and wooden pier floors provide hiding spots for intricately veiled critters of all shapes, sizes and behaviors.

On the bottom, ignoring bubble-blowing divers, there often live at least a few well-fed scorpionfish, spiny devilfish or stonefish whose grumpy-looking countenances blend into the mounds of living rock and debris. These classic ambush assassins have delineated invisible territories, much like trolls hiding under bridges in sinister fairy tales. Venomous, lie-in-wait predators barely have to move to feed on large numbers of unsuspecting cardinalfish, damsels, blennies or gobies that hover on or just above the nutrient-filled silt. Underneath some docks it is possible to find the same stonefish in the exact same spot for weeks at a time. Why move a muscle if food swims right in front of your mouth?

Young longfin spadefish school around a pier
Slipping through tall pilings, a school of young longfin spadefish seeks protection from predators under a pier.

Not far away from the ambush predators, pairs of banded pipefish may poke their slender snouts out from rocky crevices to feast on zooplankton, while a juvenile crocodilefish, perfectly mimicking a drowned piece of bark, might crawl stealthily across bottom debris. Piers throughout the western Pacific are also magnets for juvenile cuttlefish that regularly drift through the scenery, alternately feeding and hiding. You may see a cuttlefish imitate a mangrove leaf, an algae-covered rock or any number of underwater objects, using its nervous system to control its millions of specialized cells to manipulate its color and texture as it searches for crustaceans or fish.

Divers on their first exploration of these man-made structures may not immediately recognize the annual cycles of succession that continually change the makeup of these mini-ecosystems. However, diving a dock many times can reveal a proliferation of some previously unnoted species due to environmental disturbance, lunar periodicity or an influx of new larvae. While the species living under the structure change over time due to immigration, competition and local extinction, the overall number of species generally stays the same if the area is left undisturbed. These dynamic ecosystems recall the theory of equilibrium in island biogeography.

By tearing away their gazes momentarily from the life on the critter-laden bottom and looking upward, divers can discover an entirely different view. The complex embroidery of attached organisms on the vertical pilings or clinging to the bottom of floats often include a menagerie of barnacles, colorful but toxic sponges, tube-dwelling polychaetes, coralline algae, hydroids and delicate tunicates, in addition to a host of vertebrates.

Schooling scad take refuge under a dock
Caught in the open, schooling scad are easy targets for marauding trevally and small sharks. These silvery prey-fish often seek refuge under docks or piers, which attracts larger predators.

A silver river of hundreds of scad or other baitfish may slip sinuously among the forest of pier pilings. Visual predators such as the bluefin or giant trevally, which usually circle piers from a distance, watching and waiting, might be momentarily confused with so many identical fish packed into a living, moving wall. Late in the afternoon the predators become more active, and their anxious prey pack even closer together. In contrast to the quickly moving prey fish, motionless adolescent longfin spadefish frequently hover above the wide-eyed scad, watching the parade and perhaps wondering what all the hurry is about.

Each locale offers slightly different environmental factors — varying depths, light intensities, currents, plankton availability, nutrient levels, water chemistry, etc. — that affect which marine life colonizes a structure. When new pilings are put in place, they act as open territory, fit for colonization by hundreds of sessile organisms. In a matter of days, marine life begins to claim territory upon the thick wood or metal supports. Barnacles, hydroids, bryozoans, crinoids, mussels and innumerable barely visible creatures settle onto the pilings, where certain cues signal for them to metamorphose into thriving, mature communities. The quick succession of life shows how prolific larvae are in the overall tropical marine environment. Evolutionarily, it pays obvious dividends for a species’ planktonic larvae to be able to settle and metamorphose immediately upon sensing the right environmental factors.

In some areas where coral reefs are not profuse, docks and piers can potentially increase fishery resources, either by drawing in dispersed populations of fish or by creating a more suitable habitat for the escalation of fish populations. Many organisms that typically live in cavern environments are also found in the dark cavities of piers and docks.

Considerable variation may be expected in different portions of one set of docking facilities and certainly between docks placed in ecologically different situations. No two docks are ever alike in terms of their aggregated marine life. Most illuminated spaces will tend to be monopolized by photosynthetic organisms, and partly or completely shaded areas are apt to have assemblages of more unusual animals. Many of the organisms living under floats or on pilings are those normally found at deeper parts of the intertidal or subtidal zones. On a pier or dock, they can live close to the surface without danger of exposure to intense sunlight.

A dock, with house at the end, rests at sunset
Every pier or dock is different according to its location, size, depth, etc.

Just about any structure under which divers (and their gear) can slither will present strange, bottom-dwelling critters and unique environmental scenery vastly different from that of coral reefs. While the Pacific offers any number of exhilarating dives over bottomless drop-offs, along deep pinnacles and through swift channels where large fishes roam, piers and docks are consistently rich in bizarre, photogenic inhabitants. For photographers these man-made marine habitats are almost always productive for both macro and wide-angle photography. Pier pilings, dock moorings and surrounding habitats are loaded with the odd creatures for which divers have a special affinity — from tiny harlequin shrimp to lumpy frogfish to leaflike waspfish. While many divers focus on a pier’s smaller critters, larger predators such as resident giant moray eels or tassled wobbegongs, who appreciate the culinary abundance at these locations, will also frequently patrol the waters around these man-made structures.

Each marine ecosystem, each underwater habitat, each niche with its associated wealth of generalist or specialist inhabitants is part of the all-encompassing biosphere. No trophic level or species exists in isolation. Even piers and docks, which bear various collections of odd and intriguing creatures and their own distinctive food webs, are intricately tied to the open ocean. These structures of wood, concrete, nails, ropes and tires are merely part of a dynamic puzzle whose pieces extend from DNA molecules to entire ecosystems and whose effects radiate through the world’s oceans.

© Alert Diver — Q4 Fall 2014