By Dr. Andrew Bruckner

Like other animals, corals need to reproduce to survive. Unlike most other animals, corals are attached to the seafloor and cannot move around to find a mate. To address this challenge, corals have developed several alternative reproductive patterns and modes of development. Some corals have separate sexes (gonochoric), while others are both male and female at the same time (hermaphroditic). Since self-fertilization is rare, every coral still needs a partner if they are going to successfully reproduce.

coral stuck in place

Usually, this is accomplished through synchronous spawning, whereby all colonies of a particular species release millions of gametes in unison, once per year, for a few hours, a certain number of days after the full moon (the exact timing is well documented and predictable from year to year). The eggs and sperm typically float to the surface, where they encounter other gametes of the same species, and fertilization occurs. The larvae then drift with the current for days to weeks, until they find a suitable place to attach and transform into a polyp. Very few of these larvae survive.

coral spawning slick

Other colonies are known as “brooders.” They may reproduce more frequently (sometimes monthly), usually producing fewer gametes at a time. Brooders need to be in close proximity because the female colony must take up sperm from the water column, and fertilization occurs internally. When the larvae are released from the female, they are better developed and have higher survival rates. They settle onto the reef and transform into a polyp much sooner.

Although sexual reproduction is the most common and the most important type of reproduction, corals also reproduce asexually by cloning themselves. A coral grows by constantly adding new polyps through a process known as budding. Corals can also increase in abundance through fragmentation, where branches that break off during storms reattach and continue growing. Other unique asexual reproductive strategies exist including polyp bailout, polyp expulsion, and asexual production of larvae; these offspring all lack a skeleton when first produced.

A more unusual strategy involves the production of “gemmae.” A single gemma starts as a mass of soft tissue on the sides or top of the coral that eventually develops a skeleton. It continues to grow, forming a small knob that will detach from the parent. The detached gemma looks just like the parent, except they are round or oval and completely covered in live tissue.

gemmae on Porites lobata

In the lagoonal habitats of Hao, one of the dominant frame-builders is the pore coral (Porites). This coral is long-lived, forming mountainous-shaped colonies that are several meters tall.  During our surveys, we found very few small colonies produced through sexual reproduction. This raises the question of how these corals are able to spread so successfully throughout the lagoon. Because wave action is minimal in lagoonal habitats and these corals produce a rigid skeleton, fragmentation is unlikely to be an important reproductive strategy. Until yesterday, we rarely found small fragments, as well.

Porites lobata

On our fifth lagoonal dive, I encountered a very large (4 m tall) and very bumpy colony. Littered over the substrate and in depressions on the colony surface were hundreds of round to oval gemmae that ranged in size from 1 to 5 inches that originated from the colony. Most had tissue covering their entire skeleton and appeared to be actively growing, but they were completely unattached. The high number of these “rolling stones” produced by one colony of Porites, and their high survivorship, suggests this may be an important mode of reproduction for Porites in the Hao Lagoon.

rolling stones at base of coral

 (Photos by: 1 Eddie Gonzalez, 2-5 Dr. Andrew Bruckner)

Iliana Baums, a biologist at Penn State University, explained her research last night after dinner. She’s looking at Porites lobata, a major reef-building coral in the Pacific and Indian Oceans, and its identical-looking relative P. evermanni, and how they fit into the greater reef ecosystems. (These are what’s known as cryptic species, meaning they look the same morphologically but are genetically distinct.) She described the three-way interaction between the Porites corals, the mussels that bore into them, leaving distinctive keyhole-shaped holes, and the triggerfish that bite the coral trying to get at the mussels inside.

Porites lobata colony

Porites evermanni seems generally hardier; it lives in harsher environments and doesn’t bleach as often as P. lobata. One of the hypotheses Baums is testing on this trip is whether P. evermanni reproduces asexually more frequently than P. lobata because of this interaction. Coral can reproduce either sexually, by releasing sperm and/or eggs into the water, or asexually, which includes new colonies starting from broken pieces. If P. evermanni attracts more mussels, which then attract more triggerfish, the broken pieces of coral the fish bite off (already weakened by the mussels’ drilling) could start new coral colonies more often. If this is true, it’s an example of how cryptic species, even if they look identical, can play a very different role in their ecosystem.

Iliana Baums watches the sun set over Isabela Island

Written by Julian Smith

(Photos/Images by: 1 Brian Beck, 2 Julian Smith)

To follow along and see more photos, please visit us on Facebook! You can also follow the expedition on our Global Reef Expedition page, where there is more information about our research and team members.

This weekend marked the halfway point of the Galapagos expedition, both in time and in territory. After a week at Marchena, Darwin and Wolf, the latter two far to the north of the main group of islands, the ship will swing through a rough counterclockwise circle through the southern and western islands. We’ll stop at San Cristobal, Floreana and Isabela, and eventually circle back to Baltra over the coming week.

Saturday saw a few arrivals and departures, along with two dives in the Itibaca Canal between Baltra and Santa Cruz and an afternoon wall dive at Daphne Minor, a tiny islet north of Baltra, which was livened up by a few playful, curious sea lions. On Sunday morning, the Golden Shadow pulled up anchor and headed southwest for San Cristobal, the easternmost island in the Galapagos and home to the provincial capital, Puerto Baquerizo Moreno.

Two sea lions pose for a picture at Daphne Minor

There were a few minor concerns at Wolf and Darwin: an invasive species of seaweed (algae) in the genus Caulerpa; and lots of damselfish, which kill coral and create seaweed “lawns.” But on the whole, the corals seemed to show a slow, steady improvement from the near-destruction caused by the El Niño events of 1982/83 and 1997/98. (We’ll talk much more about El Niño in an upcoming post.) 

At Darwin, the dives focused on the reef running between Darwin Island and Darwin’s Arch, the only true coral reef in the Galapagos. In the cove near the island we found lots of Lepteroseris and two stands of Gardiner’s coral (Gardineroseris planulata), both of which are very rare. Another significant discovery at Darwin was thousands of skeletons of mushroom coral (Cycloseris sp.). This kind of coral, in the family Fungiidae, is known to grow in Española and Floreana to the south, but has never been recorded in this part of the archipelago, alive or dead. Even though the team didn’t find any living specimens, it’s exciting to have found it here at all.

Rare Gardinoseris planulata coral

At Wolf we found large stands of lobe coral (Porites lobata), a stony reef-building coral, up to 8 meters in diameter and 8 meters high. Some of these are centuries old.

A ray hiding in the sand decides it's time to go

In comparison to Wolf and Darwin, the coral communities in the central and southern islands are smaller, more widely spaced and less well studied, especially the unique free-living mushroom (fungiid) corals at Devil’s Crown, at partially submerged volcanic cone near Floreana. These communities have gone through phases of recovery and decline. It will be interesting to compare them to their northern neighbors, and it should help us understand the natural processes of coral reef resilience better.

Written by Julian Smith

(Photos/Images by: 1-2 Joshua Feingold, 3 Andrew Bruckner, 4 Brian Beck)

To follow along and see more photos, please visit us on Facebook! You can also follow the expedition on our Global Reef Expedition page, where there is more information about our research and team members.