Marine Science Chapters


The Effect of El Niño in Galapagos on the Marine Invertebrates

Galapagos shoreline with sally light-foot crab
Galapagos shoreline with Sally light-foot crab. (SA image)

El Niño seriously effects marine invertebrates both along the shorelines and in the subtidal areas down to the bottom of the thermocline. Below the thermocline the temperature is more 'normal' for the area and any motile invertebrates, who move to this depth, may not have many problems. Above the thermocline the intertidal and non-motile invertebrate species often die during a severe El Niño due to the increased temperature of the water or lack of food.

Sally light-foot crab
Sally light-foot crab. (SA image)

Sally light-foot crabs (Grapsus grapsus) are scavengers and feed on almost anything. They were reasonably healthy during the '82/'83 El Niño probably because there were so many dead organisms to consume.

Coral colony alive with tentacles outCoral colony dead
Living coral colonies have many tentacles protruding from their skeletons (left) but when they die, the skeletons are bare - showing a 'star' shape where each coral animal used to be (right). (SA images)

Galapagos has coral although it is unique in that it seems to be unable to reproduce sexually. Most of the coral in Galapagos is related to the Australian corals and will live in waters up to about 80 degrees Fahrenheit. When the ocean temperature exceeds this they usually die. Although it is a much more complicated story because many of these tropical corals harbor symbiotic zooxanthellae (algae) that help them survive (and give them their characteristic color). During extreme El Niños the zooxanthellae may leave or die causing the corals to 'bleach' and the reefs appear as though they are dead.

Coral colony with bleachingDead coral colony being overgrown with filamentous algae
Coral colonies may show bleaching in an El Niño (left). In the '82/'83 El Niño many colonies died and were overgrown with filamentous algae (right). (SA images)

After bleaching the coral may actually die or it may come back to life and regain its zooxanthellae (and color) later. If coral heads die, the absence of the coral animal leaves a solid surface (the corallite skeleton) that can be colonized by a variety of marine critters. During the '82/'83 El Niño many of the coral heads died and brown filamentous algae, eaten by the marine iguanas, covered their surfaces.

Popcorn coral colony with tentacles outPopcorn coral colony with tentacles in
Popcorn coral colonies are made of many circular polyps with orange tissue. (SA images)

Popcorn coral extended close upPopcorn coral retracted close up
Popcorn coral individuals are polyps with circular bodies, a mouth in the middle, and a ring of tentacles around the mouth. When extended (left) they look like flowers, when retracted they look like circles (right). (SA images)

Popcorn coral colony dead and alive
Under each circular orange polyp the skeleton of white calcium is secreted. This circular skeleton has radiating septa that extend from the outer circle to the middle. When the coral dies, each polyp leaves its hard skeleton (called a corallite) that looks like a circle with lines going toward the center. Here one colony is dead (left) and one is still alive (right) in the '82/'83 El Niño. (SA image)

Dead popcorn coral colony
Completely dead popcorn coral colony being overgrown with filamentous algae in the '82/'83 El Niño. (SA image)

Colorful popcorn coral (Tubastraea sp.) died above the thermocline in 1982-83, leaving the white calcium skeleton of each polyp (looking like a wagon wheel). These calcium skeletons are called corallites and are the substance that coral reefs are made of. As coral heads died in '82/'83 they became a settling ground for fouling organisms like the brown filamentous algae (which grew quickly after the death of the coral polyps). Also settling on the newly dead coral heads were some barnacles and smooth coralline algae, especially in the months when El Niño ended. Thus, the coral head (or only part of the coral head, if it did not all die) became covered with other hard critters.

Many of the coral heads living in Galapagos are hundreds of years old. This is known from their size. Since they do not sexually reproduce, the origin of each head has had a lot of speculation. Since the '82/'83 El Niño an interesting theory has surfaced … during extreme El Niños the Kelvin Wave may bring Australian coral larvae back across the Pacific in warm waters and provide the source for the Galapagos coral heads. The coral heads may get started in these severe El Niños but are living marginally in the cold waters of Galapagos and unable to sexually reproduce.

Urvina Bay coral heads with researcher
Urvina Bay coral heads with researcher. (Image, with permission, from Gary Robinson)

Urvina Bay, Galapagos
Urvina Bay, Galapagos. (Image, with permission, from Gary Robinson)

Urvina Bay's uplifted coral heads helped give evidence for this theory by providing a source of undisturbed dead coral heads that could be studied. Urvina Bay in Galapagos underwent an infusion of lava, under the bay, in 1954. The bay was uplifted, above sea level, in a matter of hours. This left the coral heads intact and accessible to humans. They were also not eroded by the ocean or marine creatures (as underwater coral heads are). Boring into the preserved coral heads from the outside provided scientists with cores. Layers of discontinuity (where barnacles and coralline algae encrusted the surface) were seen in these cores about every hundred or so years. It is surmised that these layers represent extreme El Niño years and that the entire head did not die and eventually the coral covered the hard fouling community. Even though there was not a recorded written history for these extreme El Niños, they are recorded in the old coral heads.

Black coralBlack coral

Black coral close up
Black coral. (SA images)

Black (Antipathes galapagensis) coral still lives in Galapagos although most of it has been taken from the shallow waters for jewelry. Black coral is not a true coral; it is a soft coral and does not form a corallite. It is more closely related to the soft corals called sea fans (or gorgonians). Most of Galapagos' black coral lives below the El Niño thermocline and was not affected by the '82/'83 El Niño event.

Sea fan in 'normal' yearSea fan in El Niño year
Sea fan in a normal year (left) gets plenty to eat but in El Niño years they begin to die and may be covered by filamentous algae (right). (SA images)

Sea fans living above the thermocline began to die in '82/'83 and were covered (entirely or in part) by the brown filamentous algae. This could have been because of the increase in seawater temperature but was more probably a result of the lack of food. Sea fans are filter feeders, feeding on plankton, and plankton is limited in El Niño years due to the thermocline.

Pencil UrchinPencil Urchin
Pencil urchins. (SA images)

Pencil urchins (Eucidaris thouarsii) eat coral but they are motile and during an El Niño they are able to move into deeper water where the coral is not affected. They contribute to the erosion of the coral heads as they are dying and this increases the predation on the deeper corals. It appears that pencil urchins change their eating locations during El Niños but their population remains relatively unchanged.

Toxic sea urchin
Toxic sea urchin. (SA image)

Toxic sea urchins like Toxotneustes roseus are also motile and may move to deeper water during El Niños where they feed on algae growing on rocks.

 Copyright and Credits
(Revised 18 December 2004)
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