Net or Syringe? The tricky question of collecting coral spawn
Divers arrive on the reef, where several different coral species are expected to spawn. That is the reason, why the scientists are carrying collection nets and syringes (in blue net) with them. (Paul Selvaggio)
Kelly, on photos from the latest coral spawning event on Curaçao we see you and colleagues using large nets and huge syringes for spawn collection. Why do you need two different tools for one and the same task?
Kelly Latijnhouwers: Because not all coral species spawn in the same way. Generally speaking, corals have two main reproductive strategies. There are ‘brooding’ corals, which means that eggs get fertilized inside the coral polyp, and are released as fully developed larvae, and ‘broadcast spawning’ corals, which means that the coral colony releases gametes (sperm and eggs) into the water column at a certain time a year, hoping for its offspring to meet another coral’s offspring at the same time so that they can fertilize each other.
During coral spawning season, we focus only on the latter: broadcast spawning corals. However, broadcast spawning corals also have two different reproductive strategies: There are hermaphrodites and gonochores. Hermaphroditic corals are both male and female at once. They, therefore, release both sperm and eggs into the water column, in small round packages that we call bundles. If corals are gonochoric, it means that some coral colonies are males, and others are females. This means that the males only release sperm into the water column, and females only release eggs. For both strategies, fertilization (hopefully) happens in the water column when the sperm and eggs of different colonies meet each other.
Top: A great star coral – a hermaphrodite – releases its egg-and-sperm bundles, which immediately start floating in the water column. In the bottom row, you see a collection net that is covering a coral colony that has started to spawn (left), and divers using syringes to collect eggs or sperm of gonochoric corals as close to the releasing coral polyps as possible. (Paul Selvaggio)
So, these different ways of coral spawning require different tools to collect egg and sperm?
Yes! Our large collection nets are the most common way to catch coral spawn, and they are used for the hermaphrodite species. Since the eggs are positively buoyant, the bundles with lots of eggs packaged together will float to the surface. Therefore, placing a net over these corals allows the bundles to float up through the net, through the funnel, into the collection tube. This is a very easy way to collect many bundles from one colony without harming the colony in any way.
Now, the gonochores are a little trickier. Since the male colonies only release sperm, we cannot collect this with a net. The sperm immediately dilutes into the water column. Therefore, we need to be hovering very close to the colony with a syringe, ready to collect. We collect the sperm as close to the releasing polyp as possible, to make sure we don’t collect sperm that is too diluted to fertilize eggs. The same goes for the eggs. Since they are released as individual eggs, rather than bundles of eggs, they are a lot less ‘floaty’, and so will not float effectively into a collection net. Therefore, we also collect the eggs with a syringe.
Even though collection methods are different, the next steps are identical: we bring the collected spawn to the lab, where we mix everything together to allow the eggs to get fertilized and develop into larvae.
SECORE Restoration Technician Kelly Latijnhouwers (Paul Selvaggio)
With which method do you collect most eggs and sperm?
Most definitely, the collection nets for hermaphrodites are most effective to collect large numbers of larvae. We can easily collect up to a few million eggs on one spawning dive with a relatively small team of divers. Therefore, we have so far only been using hermaphroditic species for our so-called in-situ larval rearing pools. Those pools serve as large larval rearing enclosures, floating in the ocean. We can stock these pools with up to half a million embryos, where they will develop into larvae and eventually settle on artificial settlement substrates. Therefore, we need large collections to use the pools.
Nonetheless, we are working to come up with methods to increase the efficiency of our gonochore collections. The syringes are limiting us in the amount of eggs we can collect since you cannot avoid collecting a lot of seawater in the syringe as well. However, last month we were able to collect up to 20 000 eggs from great star coral colonies (Montastraea cavernosa), which is a new record for us. With multiple field teams working together, and the input of our new project engineer, we are hoping to come up with methods where we can collect enough offspring of these types of corals to be able to use them in our larval rearing pools to increase the restoration efforts for these species.
Thanks for the chat, Kelly!