Module 2: The Reef at Night

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Background: 

Like all ecosystems on planet Ocean, coral reefs depend on energy from the sun to fuel photosynthesis, which then trickles up the food chain to higher trophic levels.  When the sun goes down, photosynthesis abruptly ends.  Dramatic changes, physical, chemical, and biological, take place at night on a coral reef.  Zooplankton rise from the nooks and crannies of the reef and drift past an ocean of mouths that includes reef corals, sea anemones, brittlestars, and basketstars. 

Fig. 1.  Diurnal cycle of oxygen flux in a back reef environment (1 m deep) at One Tree Island, Australia’s Great Barrier Reef.  From Kinsey, D.W.. 1977. Productivity and calcification estimates using slack-water periods and field enclosures.  In: Coral Reefs: Research Methods, pp. 439-468, UNESCO.  Note how rapidly oxygen flux change at dawn and dusk.

As the sun sets, certain reef fishes aggregate to spawn, others seek out safe harbor among the coral often changing color dramatically (barracudas) or erecting mucus cocoons (parrotfishes), while others emerge to roam and feed.  Around Aquarius, the underwater lights on the habitat attract plankton, small fishes, and larger predators, often putting on quite a show for the aquanauts eating dinner or midnight snack.  With photosynthesis shut off, the water chemistry changes dramatically with oxygen levels dropping (Fig. 1) and carbon dioxide accumulation lowering the pH, as the water becomes slightly more acidic from the reef’s respiration.

 

We will explore some of the above changes taking place around the habitat, including the emergence of demersal zooplankton, the suspension feeding of corals and other invertebrates like basketstars, and the ‘spinning up’ of the food chain in the water column (plankton, small fishes, larger fishes) that often takes place around the habitat.  We will also take advantage of the dark-ness to examine how different organisms fluoresce under ultraviolet (UV) light.

 

Demersal zooplankton arise from the reef substrate at night and include a host of different invertebrates, with copepods and polychaetes often numerically dominant.  Capturing particles from the water is called suspension feeding and it is one of the most ancient form of feeding by animals.  Both active (pumping) and passive (using ocean currents) suspension feeders occur in high abundance near the habitat.  Corals are passive suspension feeders.  The tentacles of the expanded polyps catch particulate prey. (Fig. 2).

<<< Fig. 2. Structure of a coral polyp (NOAA). These modular units make up a coral colony.  The polyps are contracted during the day in the majority of reef species, expanding at night to capture zooplankton prey using their tentacles.

 

 

 

 

Fig. 3. Montastrea cavernosa exhibiting or-ange fluorescence and green fluorescence in the mouth of the polyps. These are found everywhere from shallow to deep water. Images Courtesy of the Twilight Zone Ex-pedition Team 2007, NOAA-OE

Fluorescence occurs when higher energy photons, like those from a UV light, pass close enough to a molecule with the right electronic structure to excite the molecule’s electrons.  The excited electrons usually quickly transition back to the base state, but not before releasing energy as photons, with a wavelength longer than ultraviolet, in the visible spectrum.  This visible fluorescence is starkly beautiful (Fig. 3).  It signals the presence of pigments.  In the photosynthetic organisms like corals and algae, these pigments function to help harvest light energy for photosynthesis, but many animal pigments also fluoresce.  The function of these pigments is not well understood.  Some may be ‘sunscreens’; others may be important in visual communication.  All are beautiful.  Recently, the detection of visible fluorescence, using underwater UV lights with blocking filters on the viewing camera, has allowed reef scientists to detect very young stages of reef corals that would be missed through inspection using visible light.  Dr. Charles Mazel, of psicorp.com has pioneered some of these techniques and lights souces and filters, which are now commercially available at http://www.nightsea.com/.

 

What students will see during the show:

Drs. Hagan and Patterson, Capt. Renaud and award-winning filmmaker and cameraman D.J. Roller will venture outside Aquarius after the sun sets.  Mr. Roller will have a portable high intensity blue light source and a blocking blue filter for the camera.  The aquanauts will wear special filters in their dive masks as well.  We will first travel underneath the habitat and examine the diverse community of sponges, corals, and antipatharian octocorals that have settled onto the habitat and grown.  Many of these, as well as the nearby reef corals will fluoresce vividly.  Underneath the habitat, we will overturn coral rubble and test many of the invertebrates present, including shrimps, polychaetes, brittlestars, and burrowing sea aenomones, for fluorescence.  On the patches of reef near the habitat, the team will provide some extreme close-ups and commentary on the feeding corals.  Polyps, the modular units that are contracted inside the calices of a coral colony by day, are expanded at night, and we will observe their feeding behavior.  Our dive lights will probably attract a rich soup of dermersal zooplankton consisting of copepods, mysid shrimp, polychaetes and invertebrate larvae of several phyla, as well as larval fishes and crustaceans.  When we bring our dive lights near the expanded polyps, a feeding frenzy ensues!  One species that is particularly good at carnivory of this kind is Montastrea cavernosa.  There are several beautiful colonies of this species near the habitat.  Earlier in the day, our surface support team will have deployed several plankton traps over substrates of various kinds, and we will go over and see what we caught.  We will also keep our eyes open for interesting fish behavior including searching for parrotfishes sleeping in their mucus cocoons.  Towards the end of the broadcast the team will move up on the habitat near the underwater lights and we will film the predator show.  The demersal zooplankton that have escaped the carnivory of the benthos are now drawn to the lights, where they are preyed on by smaller fishes, which in turn attract larger predators like jacks (carangids) and barracudas.

 

At the end of the module, students will be able to:

1. Describe some of the changes that occur chemically and biologically when the sun goes down on a coral reef.

2. Explain the importance of the emergence of demersal zooplankton to the trophic dynamics of the reef and predict which areas of the reef will have higher zooplankton abundance.

3. Explain the difference between active and passive suspension feeding, and how corals are adapted to catch particles from moving seawater.

4. Describe the host of organisms that fluorescence under UV/blue light and how the fluores-cence differs by taxon.

5. Explain how fish predators in the water column exploit changing food resources after the sun sets.

 

Reading:

Books:

Levine, J.S. (author) and J.L. Rotman (photographer). 1993. The Coral Reef at Night.  Harry N Abrams, 192 pp.  A wonderful coffee table book examining the reef at night, with lots of science woven in.

Wildish, D. and D. Kristmanson. 1997. Benthic suspension feeders and flow. Cambridge Univer-sity Press, 423 pp.  An excellent review of suspension feeding by organisms living on and in the bottom of the sea.  The authors do a great job of synthesis, and the reader will gain an appreciation of the importance of this mode of feeding used by so many inhabitants of reefs.

Prager, E., and S. Earle.  2001.  The Oceans. McGraw-Hill, 316 pp.  A nice overview for the intelligent layperson of how the oceans arose, their geological and chemical evolution, the important role they played in the evolution of life on the planet, and their current state.  Both authors are aquanauts too!

Demersal zooplankton:

Lewis, J.B., and J.J. Boers. 1991. Patchiness and composition of coral reef demersal zooplankton. Journal of Plankton Research 13: 1273-1289.  The authors provide an analysis of how the composition of the demersal zooplankton changes through time, and what might control the abundance and composition.  This paper uses a mathematical technique, spectral analysis. that allows scientists to discern patterns in time series of data.

Porter, J.W., and K.G. Porter. 1977. Quantitative sampling of demersal plankton from different coral reef substrates. Limnology and Oceanography 22(3): 553-556.  This paper is from a big field campaign to examine reef ecology; the authors showed that different substrates hold different concentrations of zooplankton.

Suspension feeding by corals and basketstars:

Sebens, K.P., K.S. Vandersall, L.A. Savina, and K.R. Graham. 1996. Zooplankton capture by two scleractinian corals, Madracis mirabilis and Montastrea cavernosa, in a field closure. Marine Biology 127(2): 303-317.  A nice quantitative paper looking at passive suspension feeding in the field.  Prey selectivity and capture rates were computed for species with very different morphologies.

Meyer, D.L., and N.G. Lane. 1976, The feeding behaviour of some Paleozoic crinoids and Recent basketstars.  Journal of Paleontology 50(3): 472-480.  The authors compare feeding behavior in members of the Echinoderms that suspension feed: the filtering apparatus used by extinct and recent crinoids, with the parabolic mesh used by the basketstars, a kind of ophiuroid.

Fluorescence emitted by reef organisms:

Mazel, C.H., M.P. Strand, M.P. Lesser, M.P. Crosby, B. Coles, and A.J. Nevis. 2003. High-resolution determination of coral reef bottom cover from multispectral fluorescence laser line scan imagery. Limnology and Oceanography 48: 522-534.  A state of the art study of how lasers can be used to induce fluorescence in reef organisms and how the images can then be automatically classified as to the type of organisms present.

Mazel, C.H., T. W. Cronin, R. L. Caldwell, and N. J. Marshall. 2004. Fluorescent enhancement of signaling in a mantis shrimp. Science 303: 51.  A note describing the fluorescent enhancement of spots on a mantis shrimp and how they can readily see the fluorescence, which enhances the signaling properties of these spots.

Wonderful review of the state of the art of experimental reef biology, in the context of the challenges facing corals:

Lesser. M.P. 2004. Experimental biology of coral reef ecosystems. Journal of Experimental Ma-rine Biology and Ecology 300: 217-252. 

Multimedia:

Video of demersal zooplankton from Jamaica, attracted by a dive light:

Video of the complex flow over the expanded surface of a feeding reef coral, made visible by a collimated sheet of light fluorescing a dye released close to the coral surface:

An example of a passive suspension feeder, some burrowing sea anemones (Cerianthus sp.) off the coast of California.  Note how the long tentacles, equipped with stinging cells (cnidocytes), oscillate in the wave-driven flow:

An example of an active suspension feeder, a sponge (Baikalospongia bacillifera) from the world’s oldest, deepest, most voluminous lake, Lake Baikal, Russia.  Note how the dye is pumped at high rate from the excurrent pores, the oscula.  Later in the clip, an flowmeter with underwater computer is shown measuring the pumping rate of one of these sponges.  On a typical coral reef (and in Lake Baikal too), there are so many sponges in the shallows that the overlying water column is pumped through sponges every 24-48 hours!:

Web resources:

Nice short video of the biological changes that take place on a typical coral reef. http://news.nationalgeographic.com/news/2006/07/060728-florida-video.html

Video of corals capturing zooplankton in Panama.  The species shown is Montastrea cavernosa, a species we will examine during this module broadcast: http://video.google.com/videoplay?docid=1004979234481354262

Video of Astrophyton muricatum, a basketstar, filter feeding inside a giant barrel sponge, Xesto-spongia muta.  Both species are found in vicinity of the habitat Aquarius.  http://www.youtube.com/watch?v=MZn4XwX38sk

Nice description of the importance of fluorescence in a mantis shrimp (stomatopods) by Dr. Charles Mazel, the inventor of techniques for underwater fluorescence.  This work appeared in the prestigious journal Science (link to the article available on this web page).  This is the first time this effect has been documented in a marine crustacean, or for any other sea creature.  Be sure to check out the very cool video of a mantis shrimp spearing a fish!  http://www.nightsea.com/mantis.htm

 

Quantitative exercises:

Ecosystem ecology:  Suspension feeding organisms, both passive (ones that rely on oceans currents to bring them food) and active (ones that pump water past their filters), dominate the benthos of coral reefs systems.  At night, the corals expand their tentacles and catch zooplankton prey but some species may also be catching and retaining phytoplankton as well.  In any case, the food source is very dilute, especially over the oligotrophic (low nutrient) waters of a coral reef.  Gili and Coma (1998) report that the capture rate for hard corals (which include those found on reefs) is 0.08-1.38 mg C/(polyp•day).  Using underwaters pumps, Yahel et al. (2005) found that seston are depleted over a coral reef in the meter closest to the bottom.  They measured the the seston concentration to be about 3 mg zooplankton (AFDW: ash free dry weight)/m³ at night above the corals.  The relationship between AFDW and carbon varies by taxa, but assume a ratio of AFDW:C of 4.0 for the questions that follow.

(i) Assume a coral colony has 100 polyps. Assuming the colony’s polyp feeding rate is at the low end (0.08 mg C/(polyp•day)), and it feeds only at night, how much carbon does the colony take in?  Compute the minimum volume of water that must flow past the coral per day for it to catch the computed carbon intake.  Repeat your calculations for a colony feeding at the highest rates, 1.38 mg C/(polyp•day).  (Hint: On reefs in the tropics, the day’s length is close to 12 hours year-round.)

(ii) The seston flux rate (mg C/(colony•unit time)) to a passive suspension feeder like a coral is determined in part by the velocity of the water past the filtering surfaces.  The volume flow rate (Q: volume/time) = flow speed (V: length/time) x projected cross-sectional area of the filter (A: length²).  If the projected cross-sectional area of expanded polyps in a colony is 10 cm², what are the average flow speeds past the coral polyps for the required volume flow rates, using the two feeding rates you used in (i)?  (Hint: take your volume of water from (i) and turn it into a filtra-tion rate (Q) by dividing it by the length of the night.  Remember to keep units consistent and try to make the answer come out in something easily understandable like m/sec or cm/sec!)

(iii) Most suspension feeders are not 100% efficient.  The minimum volume you computed above assumed 100% filtration efficiency.  Assume the filtration efficiency (particles removed per unit volume/total particles per unit volume) is 67% for the corals.  How do your answers change in (i) and (ii)?
 
Gili, J.-M. and R. Coma. 1998. Benthic suspension feeders: their paramount role in littoral ma-rine food webs. Trends in Ecology and Evolution 13: 316–321.  Nice tables, graphs, and ex-planatory boxes, in this thought-provoking paper.

Yahel, R., G. Yahel, and A. Genin. 2005. Near-bottom depletion of zooplankton over coral reefs: I: diurnal dynamics and size distribution. Coral Reefs 24(1): 75-85.  A nice quantitative examina-tion of how the mouths in the benthos affect the distribution and abundance of zooplankton prey.