We will often hear about stony corals spawning in aquaria, but we rarely seem to hear about soft corals doing so. Sexual reproduction in stony corals is a heavily researched area, yet little work appears to be applied to soft corals. I believe this is a much-undervalued field of research, perhaps being ignored due to the ease with which soft corals can be asexually propagated (also known as fragging).
Why should this be of any concern? Well, without genetic variation, all fragged corals are clones of their parent colonies. As a result, there is a reduction in the ability of the population of any given species to adapt or to evolve to changing conditions. For example, fragging may be making soft corals vulnerable to species loss by reducing their inability to cope with emerging diseases.
The following is an account of soft corals spawning in one of my aquariums. I hope it inspires you to take an interest in this fascinating area of reef-aquarium science.
A Unique Event
Toward the end of January 2008, one Saturday, as I did my usual morning feeding and tank checking on the coral farm, I had a wonderful surprise. One of my tanks was awash with hundreds of tiny, lentil-shaped, orange-pink eggs or planulae — I had had a coral spawn!
The parent corals that had spawned were a Briareum sp., often referred to as green star polyps.
The aquarium where the spawning occurred is 4 feet long by 18 inches wide by 12 inches deep. The tank is divided halfway along its length to form a weir (essentially a small overflow dam). The left side of the aquarium has a deep sandbed, about 4 inches deep.
The left side of the aquarium is home to four or five Phymanthus spp. anemones, a Macrodactyla doreensis anemone, two small Entacmaea quad-ricolor anemones (this was a single anemone until it divided on its own), a number of small Euphyllia of various species and a small pair of captive-bred maroon clownfish. Water is fed into this half of the tank through a spray bar and through one outlet from a Switching Current Water Director (SCWD) to give surge. There is no outlet other than the weir, making it reasonably safe as an anemone tank.
Other Tank Parameters
The right side of the tank is fed by the weir and the other outlet from the SCWD (again for surge). In the back right corner there is a dual outlet powerhead. One outlet feeds the spray bar in the left portion of the aquarium. The other outlet provides additional flow for the right side of the aquarium.
The water depth in the right side of the aquarium is about 4 cm lower than that in the left side. There is an overflow box positioned on the right-hand side glass toward the front, which feeds to a Miracle Mud sump. From the sump water is returned via the SCWD.
Lighting is provided by five 4-foot-long, 36-watt T8 fluorescent tubes driven by high-frequency electronic ballasts, four daylight (6500 degrees Kelvin) and one Arcadia Marine White.
The right side of the aquarium has no substrate and is mostly filled with a mixture of live rock and artificial rock that has mostly been colonized by the parent coral species. In addition, there are a couple of Trachyphyllias spp., a Fungia sp. and a couple more Euphyllia spp. Among these corals lives a pair of royal grammas, another of my captive-bred maroon clowns and a spotted Mandarin.
As I said at the start of this report, hundreds of tiny, lenticular, orange-pink eggs or planulae were evident, attached to and around the parent corals. I measured a small sample of them with a pair of Vernier callipers and found them to be 0.6 to 0.7 mm in diameter and less in thickness.
I then put some thought into what may have triggered the spawning. On the Friday morning the day before the spawning, I had disconnected the automatic top-off system that had been running on this system because it had been sticking. I had not recorded the salinity at the time, but when I checked Saturday, after the spawning event, I got a reading of 31 parts per thousand (1.023 specific gravity — SG) instead of my customary 35 parts per thousand (1.026 SG).
Another possible factor to take into account would be the time of the month, as often corals seem to spawn at intervals determined by the lunar cycle, usually around the time of the full moon. The spawning occurred on the morning of January 22, a couple of days before the full moon.
To photograph the spawning I had to turn the pumps and the rest of the system off, which may have influenced the chances of the planulae successfully settling out. When water motion was restored the majority were put into the water column, where previously most of them were loosely attached to their parent corals.
At the time, I considered it more important to try and record the event rather than let nature take its course. Incidentally, I noticed that nearly all the snails (four different species) in the system seemed to be acting in a stunned fashion, releasing from the rocks and glass, falling to the bottom of the tank and making no effort to move or to right themselves.
After photographing the spawning and restarting the system, it was time to do a bit of research. I found references to Briareum sp. spawning in both Borneman, and Delbeek and Sprung. Briareum sp. are described as being external brooders in both of these references.
In Delbeek and Sprung’s The Reef Aquarium, Vol. 2, there is a reference as follows: “On the GBR (Great Barrier Reef) Pachyclavularia violacea [genus changed to Briareum] was found to be a dioecious external brooder with the developing planulae residing just beside the mouth. The reddish-brown planulae were released between 11 and 13 days after the full moon at 10 a.m. (Alino and Coll, 1989).”
On the morning of Saturday, February 12, 2008, when I went to do my morning checks, I found I’d been treated to a repeat performance! This was at about 9 a.m., and when I looked closely at some of the individual polyps I could see that some were still active. Unfortunately, I couldn’t get any photos of what I was witnessing, as the polyps that were still spawning happened to be in an inaccessible place to my camera.
It was possible to watch the eggs (or planulae) through the transparent tissue of the upper part of the polyp. This is the part of the polyp that can retract back into the calyces, which are the projections raised up out of the mat that forms the base of the coral. I could see the eggs slowly move the length of the polyp to emerge from the mouth where they remained temporarily attached to the tentacles.
After a time, on most of the polyps I observed, the eggs or planulae moved further down the outside of the polyp and finally remained attached to either the calyces or mat.
Interestingly, there seemed to be no detrimental effects to the snails on this occasion. It is possible this spawning was not as large as the first one, though it would be difficult to quantify.
I decided to leave well enough alone this time; I didn’t shut off the flow of water for ease of photography and just observed what happened. Unfortunately, a couple of days after the event, I suffered two power outages in one day, so the pumps turned on and off several times and disturbed the planulae. Well, maybe next time!
Seven days after the event, the next Saturday, I noticed a small number (perhaps 20 or so) of what appeared to be stretched-out planulae being blown around in the water column. Each of these were possibly 2 to 3 mm in length and 1/2 mm wide.
As to what was triggering these spawnings, only further investigation would tell. There was no accidental change in salinity on this occasion, though I did give a larger-than-usual feed of phytoplankton to the system a couple of days before the event. Referring to the calendar, the first event occurred three days before January’s full moon, the second event took place 18 days after January’s full moon and 12 days before the February full moon. Before I can draw any conclusions about the timing, I am going to need to observe further spawnings.
It is possible that prior to the initial event something “switched on” the coral, so that it would start sexually reproducing; obviously the conditions had to be conducive to the coral preparing to spawn before it would start manufacturing gametes in preparation for spawning.
I don’t know how long it takes for gametes to mature in readiness for spawning, but I am pretty sure that it would take longer than the 24 hours between the reduction in salinity and the actual spawning. So, I’d say that although the environmental change may have triggered the event, it wouldn’t have been responsible for causing the coral to come into spawning condition.
As of yet I have seen no signs of the planulae settling and growing into juvenile corals. Borneman gives a figure of between four days to three weeks as the time taken before sexually released planulae achieve settlement. So I will keep on working on these in the hope that I will see them settle and grow.
Coral and other invertebrate larvae require particular chemical cues before they achieve settlement; coralline algae appears to be important in this process, as all corals studied thus far appear to use this algae for settlement.
As there are many different species of these algae, it is possible I might not have the right one present in my tank for this particular species of coral. In the wild, in addition to chemical settlement cues, other factors such as depth and illumination would also be important to the survival of the larvae, so it is always possible that in captivity it may be difficult to provide for all the requirements of the larvae.
Some Final Thoughts
Having since seen a video of stony coral planulae moving around and searching for a place to settle, courtesy of Dirk Peterson of the Rotterdam Zoo, I feel confident that what I was initially seeing was egg release and not planulae. The elongated structures seen seven days after the event were most likely planulae. As no sperm was seen I would infer that the eggs were fertilized prior to release and that they were being externally brooded.
I have also discussed this spawning with Julian Sprung, who has observed this species of coral spawning himself and has heard of similar reports from other aquarists. Again, as with my experience, there has been no settlement taking place. No release of sperm appears to have been observed with these spawnings either, though that is not to say no sperm is being released. It could be that it is clear.
It has occurred to me that there may have been another possible trigger for the spawning. On the immediate right of this aquarium sits my phytoplankton reactor. The reactor is an acrylic cylinder featuring a central tube to accommodate a 15-watt, 18-inch fluorescent tube. It is about 20 inches high and 8 inches in diameter.
As the phytoplankton are drawn off from this reactor the level of green water in the cylinder goes down. Over a period of days more and more of the light source is exposed. It seems quite possible that the phyto reactor may have been acting as a moonlight by duplicating the lunar cycle.
Sexual reproduction of corals is something I’ve always strived for as a reefkeeper. It has been my habit to let some coral specimens just grow large rather than to frag them at every available opportunity. The reasoning behind this is that for corals to be capable of sexual reproduction they need to be mature animals and size is usually evidence of their maturity. If a mature coral is maintained with a number of similar individuals of its own species in an aquarium, kept in good water conditions, is well fed and free of predation, I believe a situation exists that favors sexual reproduction in a captive environment.
I found Aquarium Corals by Eric Borneman and The Reef Aquarium, Vol. 2, by Delbeek and Sprung to be very useful when it came to understanding sexual reproduction in corals.
In the aquarium literature I searched through I found that most references were about stony corals and that there was little coverage of soft corals and there was even less about aquarium spawning.
If your soft corals have spawned, please let us know.
Several soft corals, such as this Ricordia yuma, have been documented to spawn in aquaria. Check out the list in this article for other corals that have been shown to spawn in aquaria.
Known Coral Spawners
This is a list of octocorallia, soft corals, known to have spawned in the aquaria. This first list includes corals known to have spawned in closed captive systems:
- Briareum species
- Calcigorgia species
- Capnella imbricata
- Heteroxenia species*
- Pseudopterogorgia species
- P. elizabethae
- Ricordea yuma
- Sarcophyton species
- Sinularia species
- Swiftia exserta
This second list is of corals that have spawned at Reef HQ, Townsville, Australia. This is an open system (i.e., one using local reef water and, at 2.5
million liters, it is the largest aquarium in the world). An additional reason for
the number of species spawning and
settling may be the fact that the aquarium is open to the natural world. There’s no roof! This means that corals in this system experience natural influences, such as variation in day length and season, that can act as triggers for spawning.
- Briareum species
- Cladiella species
- Heliopora coerulea*
- Klyxum species
- Lemnalia species
- Litophyton species
- Lobophytum species
- L. compactum*
- L. microlobulatum*
- L. pauciflorum
- Paralemnalia species
- Sarcophyton species*
- S. ehrenbergi
- S. glaucum
- S. trocheliophorum
- Sinularia capitalis
- S. flexibilis*
- S. polydactyla*
* In both the above lists, corals that have successfully spawned and produced primary polyps are marked with an asterisk.
The Road to Soft Coral Spawning Success
Here are 10 things you can do to facilitate soft coral spawning in your own aquaria.
- Pick a species.
- Set up an aquarium devoted to this species, and minimize the number of other coral species present.
- Stock this aquarium with multiple corals of your chosen species.
- Source your corals from a number of different suppliers to try and increase diversity and minimize the number of clones present.
- Encourage the growth of coralline algae to aid in larval settlement.
- Limit equipment that might damage larvae; use air-driven water movement or possibly Koralia-type pumps.
- Ensure your water conditions are optimal and limit nitrates and phosphates.
- Feed the corals by supplying them with suitable levels of light and the appropriate kinds of foods.
- Remember that only mature corals will be capable of sexual reproduction.
- If you have no success in the short term, consider varying temperature, salinity, photoperiod and lunar period in an effort to simulate seasonal variation.
Tim Hayes has authored many magazine articles and was the principal author of the book 500 Ways to be a Better Marine Aquarist. He also does consultation work on reef aquaria. He started Midland Reefs in 2001, which works in coral propagation and product testing and research.