Article: Huff, D.D., Yoklavich, M. M., Love, M.S., Watters, D.L., Chai, F., Lindley, S.T., 2013. Environmental factors that influence the distribution, size, and biotic relationships of the Christmas tree coral Antipathes dendrochristos in the Southern California Bight. Marine Ecology Progress Series 494, 159-177. Doi: 10.3354/meps10591
Christmas trees in the ocean
When looking for Christmas trees, people typically stay well out of the ocean and instead look for trees growing on land. The well-known type of Christmas tree is a woody plant that has attractive, green, scented needles, a sturdy trunk, and well-trimmed branches.
Yes, searching for Christmas trees in the ocean sounds absurd. There are so many reasons why Christmas trees or any woody tree within the Kingdom Plantae could never exist in the ocean. No tree could grow in the dark depths of the ocean bottom, where photosynthesis cannot occur. The tree could not be fertilized due to the intermittent and small nutrient supply, and the lack of oxygen would surely prevent the tree from respiring. Currents would whip the tree branches back and fourth and likely erode the trunk. Likewise, high salt concentrations of the ocean would dehydrate the cells of the tree, causing them to become flaccid and shrink. Surely plants such as phytoplankton and kelp are much better adapted to the ocean than Christmas trees.
Nonetheless, less than ten years ago, a scientist named Dr. Opresko discovered a new organism. In some ways, this organism is so similar to the terrestrial Christmas tree that scientists actually named it after the festive plant. The species forms bushy colonies on the seafloor that gracefully resemble snow-covered Christmas trees. The organisms are ornamented with barnacles, worms, crabs and shrimps – just like a decorated tree- and come in cheerful colors such as white, gold, pink, and red. In fact this organism is not a plant at all but an animal, within the Kingdom Animalia. Specifically, the underwater Christmas tree is a type of black coral. Each branch of the coral is covered with tiny polyps that have short tentacles with radial symmetry. Like other organisms in this group, the species produces a black substance which builds a sturdy wire-like skeleton that grows similar to that of a tree. In fact, if you break one of these black wire-skeletons in half, you can actually observe concentric rings of growth just like you can with a woody tree!
Hard Corals vs. Soft Corals
Corals are categorized under the Phylum Cnidaria, which includes a diverse group of invertebrates – animals without spinal columns – such as sea anemones, hydras, and jellyfishes. Corals contain polyps that eat microscopic food (i.e. plankton) using tentacles, which are protected by the hard calcium carbonate skeleton. In general, when we think of coral we think of the kind that have gained their reputation as reef builders, i.e. stony corals (order: Scleractinia), also known as the corals with the hard skeleton made of calcium carbonate, which provides structure to the reef. These corals are often associated with shallow, warm waters. However, a different group of corals exist and are known as soft corals. These corals are unique in that their polyps live on the surface of the coral as opposed to being buried inside of it. Soft corals are also called non-scleractinian and contain black corals (order: Antipatharia). Black corals, such as the Christmas tree coral, include commercially valuable species used for jewelry and grow in habitats that support other marine species. Though some laws protect this order, less is known about these enigmatic organisms, as it one of the least studied groups of non-scleractinian corals.
Unlike other black corals that occur in water deeper than 8000 meters, the Christmas tree coral (scientific name: Antipathes dendrochristos) was found to occur at shallower depths. It was first observed along the Southern California Bight, and is uncommon, long-lived, and colonial, i.e. occurs in groups.
Why study the Christmas tree coral?
Scientists are particularly interested in the Christmas tree coral as it grows in a habitat that is rich with diversity, and its presence or absence may represent the habitat’s degree of vulnerability in response human disturbances. This species is found offshore of California, where resources are heavily exploited. Bottom-fishing activities can damage the benthic communities where corals live. Human development, boating, and pollution amplify the negative effects on the benthic habitat. If scientists can pinpoint the environmental conditions that influence the distribution of this coral, they might be able to develop a model that could help improve fishing practices to promote a healthy ecosystem, and to improve ecosystem-based resource management.
The study site: Southern California Bight
Huff’s study was located in the Southern California Bight (SCB). The word bight refers to a curve in a coastline and therefore, the SCB is an offshore area that extends from Point Conception to San Diego, following California’s curve. Although the flow in this region is dominated by the California Current, the location is also known to have some of the most complex circulation patterns compared to other western U.S. coasts. The goal of Huff’s team was to combine their visual observations collected from their video clips with an oceanic model to predict the ideal habitats for Christmas tree corals. To accomplish this tank, during each video clip, they recorded the location, the habitat features – such as depth and bottom-type), the type of organisms present, the number of Christmas tree corals and each coral’s respective height.
The research methods: Defining Christmas tree habitat
In order to learn about the determine the ideal habitats for Christmas tree corals, areas that coincided with a large density of Christmas tree corals would have to first be observed. Huff and his team of scientists set out to take video transects of the marine life living on the seafloor using the manned submersible Delta and the ROV Kraken 2 from September through October (1999 – 2012). While taking video, the team recorded the size and density of corals. They also noted physical features, such as the slope of the seafloor, the bottom currents, and the type of substrate (e.g. high-relief rock pinnacle, rocky outcrops, flat rock, boulders or cobble), as well as other environmental data (e.g. water temperature, salinity, and dissolved oxygen). In addition, the team recorded the amount of surface production to estimate the organic matter available to benthic organisms. In the end, their goal was to combine all of these data to develop generalized models to predict the density and size of Christmas trees and to pinpoint the environmental constraints that limit the coral’s distribution.
Huff and his research team found that Christmas tree corals grew at depths between 50 and 1000 meters and shared habitat with a diversity of other animals, including a variety of rockfishes and sponges. They observed that Christmas tree corals need a sturdy location to grow; the corals only grew on hard substrate. In comparison to sponges, which require up to 90% hard substratum to thrive, Christmas tree corals require 70-80% hard substratum.
In addition, the team’s model predicted that ocean currents and regions of relatively high surface production, in combination with depth, were the factors that best predicted Christmas tree coral density. Christmas tree corals were consistently located in areas where the northward seafloor current was almost absent. Areas that had a high density of Christmas tree corals were deemed Christmas tree hotspots. These hotspots were concentrated to the North and near Piggy Bank and Footprint Seamounts and near Hidden Reef. Separate data sets (i.e. National Ocean Service and National Marine Fisheries service Southwest Fisheries Science Center) validated Huff’s model by confirming the presence of the Christmas tree corals in these areas.
Christmas tree corals appear to have a few basic necessities to grow. Hard substratum appears to provide a sturdy attachment surface, which enables corals to grow up to three meters in height. Surface productivity appears to improve coral habitat suitability within the southern California bight. Therefore, the food supply to the corals may arrive directly from surface production instead of from water currents. It is thought that other corals live on phytoplankton from surface production, like the cold-water coral Lophelia pertusa. Therefore, Huff hypothesized that areas with higher production have a larger connection to Christmas tree corals nearer to the water’s surface. Water currents appear to be weak near the Christmas tree coral. Weak water currents may be beneficial for coral reproduction. Though little is known on black coral larval behavior, experiments on lab cultures of a different black coral species showed that larvae settle close to parents and that dispersal is linked to the speed and direction of bottom current. Huff’s observation suggest that Christmas tree corals are less likely to have their larvae dispersed away from desirable habitat.
The model successfully identified coral habitat within the surveyed area of the Southern Ocean Bight and shows great potential to improve our understanding of the ecological niches of organisms that support a large biodiversity of organisms, such as the Christmas tree coral. As of 2008, only about 5% of the seafloor is mapped at a high enough resolution to investigate the importance of detailed landscape for the distribution of cold-water corals. Therefore, collecting more high-resolution data will help research scientists more clearly define the habitat of corals.
During this time of the season, millions of individuals will cherish a symbolic plant, the Christmas tree, because it represents a Christmas tradition. In many homes, glimmering ornaments will be hung up on its branches so the tree can be admired. At first glance, the Christmas tree coral has only a few characteristics in common with the terrestrial tree, such as its festive colors and its uniquely decorated branches filled with fish, crinoids, basket stars, and brittle stars. However, as we submerge deeper, we find that this animal may be just as iconic. It co-exists with a plethora of marine creatures, which take refuge in and thrive near the coral’s habitat. This Christmas tree appears central to marine communities and is a promising symbol of diverse underwater habitats that may be worthwhile to protect.
Although some Christmas tree coral habitats are currently within marine protected areas, several hotspots are unprotected from destructive fishing activities, in particularly near Point Conception and other populated areas like Los Angeles, Long Beach, and San Diego. Wishing health to these animals may be essential to sustaining these unique underwater communities.
References: Opresko, D.M. (2005) A New Species of Antipatharian Coral (Cnidaria: Anthozoa: Antipatharia) from the Southern California Bight. Zootaxa 852: 1-10 (http://www.mapress.com/zootaxa/content.html).
Curious to see what other kinds of Christmas trees live in the ocean?