Research by marine biologists from Wageningen University has shown that feeding on zooplankton by scleractinian corals has been greatly underestimated.
|ReefWatchers Volunteer Group Documents Coral Spawnings in Hawai’i|
|Written by Dana Riddle and Sara Peck|
The ReefWatchers Volunteer Group documents coral spawnings in Hawai’i and provides new data. Communicated by Dana Riddle and Sara Peck.
The Hawaiian Archipelago stretches across 2,400 km (1,500 miles) of the central Pacific Ocean and is the most isolated island chain in the world. This isolation has produced a number of endemic coral species, while corals common to other parts of the Pacific are found here as well. One of the most common Scleractinian corals on Hawaiian reef flats, crests and slopes is the Rose or Cauliflower coral (Pocillopora meandrina). This coral has adapted to survive in areas of high water movement. The meandering blade-like (and very dense) skeleton and wart-like ‘speed-breakers’ (called verrucae) act to slow water motion through the branches and allow its polyps to extend and feed. These branching colonies can be objects of great curiosity to waders, snorkelers, and divers since colony coloration can range from hot pink to green and deep brown.
"With their abundance and ease of observation, we might expect that we would know everything about these corals. The fact is, we know very little about this particular species."
Figure 1: A Pocillopora meandrina colony containing a non-fluorescent red chromoprotein. Kahalu’u, Hawai’i (photograph: Dana Riddle).
Sometimes found in only a few inches of water, these animals exploit harsh environments and thrive. With their abundance and ease of observation, we might expect that we would know everything about these corals. The fact is, we know very little about this particular species. Other than spawning times (listed by Fiene-Severns, 1998; Prater, personal communication), practically nothing has been documented about the reproductive habits of these creatures. This article will relay information gathered over the last two decades by a dedicated group of volunteer observers.
Pocillopora meandrina is generally a branching coral existing exclusively in the Pacific Ocean realm, ranging from portions of the west coast of Central and South America, cropping up again in the Hawaiian Archipelago, and abundant from Australia northwards from the eastern Indian Ocean to the China Sea (Veron, 2000).
Healthy specimens contain symbiotic dinoflagellates (Symbiodinium spp.). Most colonies in Hawai’i are of small diameter, and rarely exceed 60 cm (16 inches; Coles, 1980) with most being 15-20 cm in diameter (6-8 inches; personal observations).
Reproductive habits of P. meandrina
Pocillopora meandrina is one of the few corals known to spawn during daylight hours. It is a hermaphroditic broadcast spawner and releases eggs and sperm simultaneously. Spawning in Hawai'i occurs seasonally within an hour or so after sunrise in April, May and June. It is tempting to think that lunar phase and the early light of day are the environmental triggers that induce spawning. However, Amano (1986) found that the trigger for larval release of the intertidal sponge Halichondria panicea is the onset of darkness preceding the day of spawning, so we must consider this notion as well.
We have observed sexually mature colonies as small as 15 cm (6 inches) in diameter releasing literally billions of gametes. During an intense synchronous spawning observed in May 2009, the number of gametes released reduced visibility from 15-20 m (50-60 feet) to perhaps only 1.2-1.5 m (4-5 feet.) The eggs are small even by coral standards, with the largest observed being 80 microns in diameter (about the diameter of a human hair), and some eggs being even smaller.
Figure 2: Taxonomy of Pocillopora meandrina (Dana 1846).
P. meandrina specimens display all of the traits of r-selected species: small colony size, fairly early maturity, and an ability to produce a large number of offspring with wide dispersal and low survival probability to adulthood. There is good evidence that P. meandrina larvae are opportunistic and colonize empty niches. For instance, Grigg and Maragos (1974) found that recent lava flows covering established Hawai’ian reefs were initially colonized by P. meandrina, thus appearance of these corals is considered one of the first steps in the succession of a natural reef. Another trait of r-strategists is a relatively short life span. We have been unable to locate any information on P. meandrina longevity, however Riddle has been observing a mature, intensely pink colony in a tide pool near his house for the last 10 years.
"Pocillopora meandrina is one of the few corals known to spawn during daylight hours. Spawning in Hawai’i occurs seasonally after sunrise in April, May and June."
There are two methods by which coral larvae can obtain symbiotic zooxanthellae (Symbiodinium species or clades). Vertical transmission describes the eggs obtaining zooxanthellae from the parent colony. Horizontal transmission occurs when larvae gather symbionts (or commensals, etc.) from the ambient water column. P. meandrina planula larvae are released from the parent colony already inoculated with zooxanthellae, and thus obtain symbionts vertically. Hirose et al. (2000) report that other Pocillopora species’ (P. verrucosa and P. eydouxi) oocytes obtain zooxanthellae from the parent colony just 3 or 4 days before spawning. The physical appearance of Pocillopora eydouxi and P. meandrina in some cases can be confusing, and a microscopic examination of the coral skeleton is necessary in order to distinguish between the two (Gulko, 1998; Conruyt et al., 1997).
Interestingly, Magalon et al. (2008) report P. meandrina can associate with various zooxanthellae clades and can apparently acquire a different zooxanthellae strain after bleaching. LaJeunesse et al. (2004) report that Hawaiian P. meandrina contain Symbiodinium clade C1c.
Figure 3: An updated diagram illustrating spawning times relative to moon phase from 14 spawning cycles (figure: Dana Riddle).
However, symbiosis with dinoflagellates is not the only association formed by this coral. Horizontal bacterial colonization of planula occurs at about 79 hours (3.3 days) after mass spawnings, when bacteria of the genus Roseobacter (a clade of Alpha-proteobacteria) preferentially infect the coral larvae (Aprill et al., 2009). It is thought that this infection by Roseobacter bacteria, and others including Pseudoalteromonas, could have a probiotic effect. Nissimov demonstrated that Roseobacter has an antagonistic effect on many Gram-negative and Gram-positive bacteria, including the recently identified coral pathogens Vibrio shiloi, Vibrio corallilyticus and Thallassomonas loyana (Nissimov et al., 2009). Interestingly, Roseobacter has been noted to be a causative agent of disease in oyster larvae. The exact effect of this bacterium on various corals remains to be elucidated.
Apparently, large P. meandrina colonies exercise a sort of population control over smaller colonies. Polachek and Stimson (1994) documented this phenomenon and believe the interaction could prevent the formation of mono-specific stands, or might simply serve to keep colonies separated to ensure access to sufficient water flow.
Figure 4: P. meandrina egg with zooxanthellae, which are concentrated in the lower hemisphere (photograph: Dana Riddle).
Figure 5: Two-cell embryo stage. Sperm are still swarming around the cells, and could result in polyspermy and death of the larva. This indicates insufficient rinsing of the sperm from the eggs (photograph: Dana Riddle).
Figure 6: Blastula stage. Note that not all cells contain an equal number of zooxanthellae. Even at this early stage, zooxanthellae are being concentrated into what will become the endodermal cells (photograph: Dana Riddle).
Figure 7: A competent, bullet-shaped planula larva. Cilia propel this larva along in a path towards the left. The larva’s mouth is on the blunt right end (photograph: Dana Riddle).
Attempts to raise larvae and induce settlement
Since P. meandrina spawns during the light of early morning hours, it is quite easy to collect gametes. We used kitchen basters (holding about 50 ml) to collect gametes as they were released from the colony. These 50 ml aliquots were transferred to 250 ml plastic bottles. Approximately 1 hour elapsed between collection and arrival at the laboratory, where the fertilized eggs were transferred to 1,000 ml glass containers containing either autoclaved (and cooled) or filtered (0.45 micron filter) natural sea water. Reduced bacterial counts seem to be a prerequisite to success in getting fertilized eggs to the planula larvae stage. Excluding predatory copepods is a factor as well. In the absence of flow, the eggs were negatively buoyant and sank to the bottom of the vessels. This is an important clue - the eggs apparently do not contain enough lipids to make them positively buoyant and we can speculate that this absence of energy-rich reserves necessitates relatively quick settlement of the planula larvae.
Figure 8: The first division of a fertilized egg begins with cleavage on the hemisphere containing the most zooxanthellae (dark spots in the photograph, courtesy of Barbara Kuehner).
Planula larvae were visible after about 24 hours. Still tiny (<100 microns), Riddle used a black light to determine if the larvae contained any of the green fluorescent protein (GFPs) - they did not. This would have been particularly useful in spotting settled larvae (Piniak, 2005).
The coral larvae chamber
According to Riddle’s mentors, P. meandrina should be maintained in a clean environment for at least the first few days. There are several ways to do this. As mentioned earlier, a kitchen pressure cooker can be used to sterilize seawater contained in glass jars sold for canning of food. Another method is ‘cold sterilization’ of sea water using a filter designed to ‘filter out’ bacteria (these filters are sold with an intended purpose of bacteriological bioassays where the pore size of the filter is smaller than the target bacteria, thereby trapping them for use in controlled-condition culture). Both methods require time-consuming daily water exchanges, which can result in unintentional loss of at least a few coral larvae. To overcome these problems, Riddle designed a system that circulates water through an ultraviolet sterilizer.
Resembling a tank designed for culture of jellyfish, this round tank holds about 20 liters (5 USG). To prevent migration of the coral eggs/larvae from this chamber, water passes through an 11 micron Nitex screen and gravitates to a sump, where a Marineland™ canister filter with a 5 micron cartridge polishes the water before pumping it to a T-valve. Here, water is split into two streams. Most water passes through a 40-watt UV-sterilizer (which discharges into the sump) while the remainder of the water enters the cylindrical larvae culture vessel through a Loc-Line™ (Lake Oswego, Oregon, USA) header.
Another kreisel tank has been designed by Sara Mavinkurve - follow link for details. These tanks are commercially available if you do not wish to build one. See the Midwater System’s Jelliquarium (Thousand Oaks, California, USA) website for details.
Figure 9: The coral larvae incubation chamber, uncompleted at the time of this photograph. A Loc-Line™ water return header was added later (photograph: Dana Riddle).
Samplers using artificial substrates
In an attempt to induce settlement of coral larvae, devices resembling a Hester-Dendy multi-plate artificial substrate sampler were constructed. Instead of using tempered hardboard as substrate, we elected to use several materials including clear and black acrylic, glass microscope slides and colored polystyrene, each with a hole centrally drilled through which a threaded nylon bolt could be placed. These materials were separated from each other using nylon spacers, and a nylon nut secured the devices together.
The acrylic material was cut to roughly the same shape as the glass microscope slides, and was machined with a table saw to bear grooves at right angles to each other. Some sampler slides were allowed to gain a bacterial coating by being immersed in a marine aquarium. Other slides were conditioned in a similar manner, but in addition had a coating of calcareous algae (tentatively identified as a Hydrolithon species; Huisman et al., 2007). These samplers were placed in the larvae culture vessel (figure 10).
Figure 10: Coral ‘hitching posts’ made of PVC, acrylic, nylon, glass, and polystyrene materials (photograph: Dana Riddle).
Results of planula larvae culture efforts
The attempts to get P. meandrina larvae to settle have met with no success. At this stage of the game, any data point is a useful data point and we relay our thoughts in hopes of stimulating meaningful dialogues and useful suggestions.
Attempts have included aerated and unaerated vessels resulting in various water movement and light intensity ranging from 6 to 415 μmol/m²/sec (approximately 300 to 21,000 lux). Substrates have included volatized coral skeletons both ‘plain’ and soaked in gamma-aminobutyric acid (GABA), glass surfaces conditioned and coated with bacterial films, calcareous alga (tentatively identified as Hydrolithon sp.), conditioned and unconditioned clear and black acrylic, and red polystyrene plastic (red was chosen as it seems to attract some coral larvae - E. Borneman, personal communication). Planula larvae have been maintained in captivity for as long as 9 days.
The ReefWatchers Volunteer Group and their important contributions
Observations by volunteer individuals can offer valuable insights into coral reef ecology and contribute new information to databases. An excellent volunteer example is Joan Prater, a long time diver, snorkeler and marine naturalist, living on the Big island of Hawaii. She had been involved in the ReefWatcher Program managed by the University of Hawaii Sea Grant College Program Extension Service in West Hawaii. After ReefWatcher training, Joan started fish transect monitoring. Her dedication has yielded data from 9 years of fish transect surveys and coral P. meandrina spawning information.
Late in 1999, at a ReefWatcher meeting, Prater learned of Pauline Fienne-Severns’ observations of P. meandrina spawning at the Molokini Crater (Island of Maui, Hawaii). She also learned there was no additional data regarding the spawning habits of this species. The following year, in April 2000, Prater surveyed P. meandrina colonies in Honaunau Bay early the morning after the April full moon. No spawning was observed. Undeterred, she was again in the water the next morning and observed thousands of P. meandrina releasing gametes (aptly described as ‘smoking’) as the water visibility dropped from 12-15 meters (40-50 feet) to practically nothing within 6 minutes. Prater has continued making annual observations and her information is a critical component of the database presented here.
In 2004, the Sea Grant extension agent (Peck) began sharing Prater’s information with the ReefWatcher and ReefTeacher cadres, hoping that others would join the effort. The “Coral Spawning” volunteer group expanded and would eventually include members of the Keauhou Canoe Club, Sea Grant ReefWatchers, and others interested in contributing their time to this project.
"Observations by volunteer individuals can offer valuable insights into coral reef ecology and contribute new information to databases."
In 2008, 44 volunteer observers were scattered along approximately 81 kilometers (~ 50 miles) of the West Hawaii coastline, to document the spawning habits of P. meandrina. Following an observation protocol, the goal in 2008 was to record as many observations as possible over 3 or 4 potential spawning days during the predicted morning times. This degree of volunteer participation was an effort unparalleled, as far as we know, anywhere in the marine science world. These dedicated observers documented Hawai’ian P. meandrina spawning in April and May, and for the first time in June. The following year (2009) 49 volunteers expanded available information on P. meandrina spawning activity, and for the first time, the spawning of Pocillopora eydouxi in Hawaii was documented.
Those interested in coordinating the efforts of volunteer groups should contact University of Hawaii Sea Grant College Program Coastal Resources Extension Agent Sara Peck.
Table 1: A compilation of Pocillopora meandrina spawning observations in Hawai’i (1991-2009).
Variability must be considered a factor in comparing data provided by different observers. The more experience an observer has in spawning detection, the more reliable the data become. Pre-event training of observers tends to provide more reliable data from those observers.
Observers have provided observations that indicate this coral species spawns in the months of April, May and June. Previous literature indicated spawning in April and May only.
Riddle’s and Kuehner’s microscope work with gametes collected from single coral colonies confirms that these are hermaphroditic corals, producing both eggs and sperm (not in bundles) which also display self-fertilisation or ‘selfing’. Peck observed a semi-transparent corona, what may have been sperm released just prior to milky/cloudy gametes assumed to be eggs.
Possible triggers for P. meandrina spawning
Research efforts will address questions raised during our observations. While coral spawning documentation will continue, the issue of the environmental trigger(s) for spawning will be examined in 2010.
Does water temperature effect spawning? No spawning was detected in April 2009, a month in which West Hawaii water temperatures were cooler than usual in shallow water (23.8°C or 74.8°F - based on information from ReefWatchers and substantiated by data from the Natural Energy Laboratory of Hawaii Authority (NELHA), figure 11). However, in May spawning was observed by a SCUBA diver (Kellam) at 9 meters (~30 feet) with an ambient temperature of 25.6°C (~78°F). All observations in our data, with the exception of two, were from snorkeling volunteers.
Figure 11: Temperature data taken from a 60-centimeter (24 inch) diameter pipe with an intake at a depth of 10 meters (~33 feet). Data courtesy of the Natural Energy Laboratory of Hawai’i Authority.
Initially some West Hawaii observers recorded what could be a correlation between the time the sun’s rays literally presented at the top of Mauna Loa mountain, shining on Honaunau Bay and commencement of spawning. This observation was made only at Honaunau and Kealakekua Bays, both of which lie in the shadow of Mauna Loa before the sun actually crests the mountain. However, in 2008 and 2009, volunteer observations expanded over 81 kilometers (~50 miles) with dissimilar mountain geology and intermittent clouds. Spawning began almost synchronously along the coastline, within as little as 8-10 minutes. In those areas (Puako, Mauna Kea) where the sun’s rays potentially touched a particular location, spawning commenced 8-10 minutes earlier than those locations shaded by Hualalai and Mauna Loa mountains.
If the sun’s position plays a role in triggering spawning, this should occur earlier in the morning for each successive spring month. In fact, observations of spawning initiation were consistently earlier from April to June in both 2008 and 2009. We plan to ask observers to again specifically record the times next year to substantiate this suggestion.
We cannot rule out the possibility that the period of darkness preceding the spawning event or the intensity of morning light are environmental triggers for P. meandrina reproduction. If funding is available, data loggers monitoring light intensity will be purchased and deployed in 2010.
"If the sun’s position plays a role in triggering spawning, this should take place earlier in the morning for each successive spring month. In fact, observations of spawning initiation were consistently earlier from April to June in both 2008 and 2009."
Finally, schools of Convict Tangs, or maninis (Acanthurus triostegus), sometimes aggregate above spawning P. meandrina colonies and apparently eat some of the gametes. Herbivorous fishes have been observed eating plankton when it is abundant (Jack Randall, personal communication).
Spawning of Pocillopora meandrina in Hawai'i (video: Denise Ulrich).
Many thanks to the following volunteers: Barbara Kuehner, Bo Pardau, Bob Ruhaak, Bob Smith, Bob Teytaud, Bruce Brooks, Catherine Wynne, Connie Monell, Dallas Shaffer, Dave Byers, David Kearns, Deb Sims, Denise Ulrich, Diane Neubert, Donna Goodale, Donna Lavorin, Dory Ryskamp, Fred and Kathy Lindsey, George Wilson, Jamie Pardau, Jane Bockus, Janet Brown, Joan Prater, Jennifer Rabalais, John Kellam, John Jenkins, John Lynch, John Ryskamp, Joyce Oglesbee, Judy Volquardsen & son, Kevin O’Conner, Larry O’Brien, Lynn Sinclair, Peggy Spellman, Peter Vandyke, Phillys Connor, Phillys Leap, Porter Watson, Robin Bourdon, Rosanne Shank, Ruby Tzimeas, Sarah Fogelstrom, Sharon Petrosky, Stephanie Amick, Susan Lannoy, Denny and Wrayana and Noni.
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