Coral Spawning is a Summer Spectacle in Woods Hole – and a Window into Basic Biology
Each summer across the tropics, approximately eight days after the August full moon, spawning coral colonies release millions of clear, glittering gametes into coastal waters. This annual, synchronized reproduction cycle is a spectacle: Eggs shoot off like confetti and mix with mists of swirling sperm in a chaotic cloud of fertilization. Out of this fog, single-celled embryos emerge, with larvae eventually dispersing and settling.
For researchers studying coral development, timing is everything. “With tropical corals , we have one shot a year,” said Jake Warner, a Whitman Scientist at the MBLfrom the University of Carolina-Wilmington. To get the full picture of a coral’s life stages, scientists need to be prepared for the event, ready to scoop up the free-floating embryos for analysis.
Some labs, like Warner’s, have a specialized set-up to simulate the lunar cycle and orchestrate spawning. Water temperature and lighting conditions are carefully controlled to act like a celestial starting gun to initiate gamete release. This method makes it slightly easier to study coral development, but tropical coral collection for lab study is heavily restricted, due to risk of extinction.
For this reason, Warner and others have turned to Astrangia (Northern star coral) as a model for coral development.
Astrangia corals are tolerant to a wide geographical range, inhabiting waters from Florida to Cape Cod. Throughout New England, science divers plunge 30 to 60 feet down to hammer, chisel, and pluck Astrangia samples. They’re so abundant that collecting colonies is like “picking apples,” Warner said. “Boulders will be just covered in Astrangia – end to end colonies all with their tentacles out, grabbing food out of the seawater.”
Warner uses Astrangia to study how cells move throughout development. He has spent the past two summers in Woods Hole developing protocols to induce spawning and measure development.
“It’s the foundational work with this organism,” he said, which has been lacking from the field of coral research for the past 40 years. “We’re basically catching up on decades worth of science.”
Since coming to the Ůֱ, Warner has made progress pinpointing the right conditions for coral spawning and characterizing their stages of development.
What makes a coral a coral?
Using genetic tools, Warner has found surprising idiosyncrasies in Astrangia genes.
“It’s still murky,” he said, but there are striking differences in how Astrangia and its close relative, the sea anemone Nematostella, use their developmental genes. These differences, he said, may be what led to the evolutionary divergence between the two species, both members of the phylum Cnidaria. “We think that some of those differences could be the key to understanding coral biology as a whole,” he said.
In addition to genetic work, Warner has been studying coral’s settling behavior – a key part of the coral’s life cycle. Typically, fertilized embryos free-float in bundles that eventually disperse, land in an optimal spot, and start growing as baby coral. At the Ůֱ, he’s been working to identify which aspects of the landing site are optimal and how those conditions entice the larvae to settle.
Researchers are testing which substrates – like seashells and biofilms with algae and bacteria – are most attractive for the larvae. They’ve also been analyzing how different compounds trick the coral nervous system into “thinking” a location is an ideal place to set up and grow. For example, previous studies have found that some chemicals can target certain neurons, duping the organism into settling in alternative places. An intervention like that may be useful as coral colonies face increasingly adverse conditions due to climate change.
These are just a few of the leads Warner and others are following, as scientists work to discover the fundamental building blocks of coral biology. As they attempt to close the research gap, researchers continually face unexpected hurdles, challenging what they think they understand about coral biology and how to predict when the reproductive window may open or close.
Despite the difficulties researchers experience working with nature’s many variables, “Every time they spawn, it’s the best day ever,” said Warner.