Coral

Changing for Reproductive Success!

Shikina, S., Tsai, P., Chiu, Y. & C. Chang. The stony coral Fimbriaphyllia (Euphyllia) ancora’s reproductive strategy involves a sex change every year. Communications Biology, 7, 1093 (05 September 2024). 10.1038/s42003-024-06799-x.

Corals are no longer the fascination of marine biologists alone; they have seen a surge in popularity in public and home aquariums.  However, the reproductive strategies of most corals are unknown, a mystery crucial to understanding how to care for them at home and in the wild. There are over 6000 species of corals in the world, so studying reproduction can be a significant undertaking, especially since corals can employ multiple modes of reproduction! There are two main reproductive modes scientists know corals use:

  1. Asexual reproduction is where a coral makes an identical copy of itself. The copy may bud on the original coral, adding to the colony, or larvae may be expelled into the water column to establish a new colony nearby.
  2. Sexual reproduction is where a coral releases gametes (eggs and/or sperm) to meet with the gametes of another individual in the water column.

Sexual reproduction may also take multiple modes. A coral may be gonochoristic (produces either eggs or sperm its entire life, never both), hermaphroditic (produces both eggs and sperm its entire life), or sequential hermaphroditic (produces both eggs and sperm, but only one at a time, in a timed sequence). Many invertebrates and fishes utilize sequential hermaphroditism to ensure the correct ratio of eggs to sperm is available to prevent a population bottleneck.

However, little research exists on sequential hermaphroditism in reef-building corals. Until Shikina and his team published their findings, only a handful of studies had examined sequential hermaphroditism, and none of them examined it on a long-term scale.

The scientists set out to study Fimbriaphyllia ancora, commonly known as anchor coral. F. ancora is a common reef builder in Indo-Pacific reefs and is popular in public aquariums and home aquariums. F. ancora grows in colonies, which are groups of thousands of identical ant-sized polyps forming one large individual. Shikina and his team knew that F. ancora released either eggs or sperm in the water column, implying it could be a sequential hermaphrodite.

Figure 1. A photograph of Fimbriaphyllia ancora (previously Euphyllia ancora). Image captured by Les Williams, from Wikimedia Commons.

Shikina and the scientists collected 5-8 cm fragments of sexually mature parts of F. ancora colonies from two sites. At the first site, 11 colonies were sampled monthly from 2011 to 2013, with additional sampling a few weeks before and after spawning. From 2015 to 2018, the researchers used the same sampling procedure on 14 new colonies at the second site. The sites were 100 m apart, so the team did not accidentally collect from a colony established by asexual reproduction. When F. ancora reproduces asexually by budding, the resulting genetically identical colonies end up close to each other, so the 100 m distance avoids this challenge.

They determined that both sites had a handful of F. ancora colonies that did not change sex (two only-female colonies at site 1 and five-male only colonies at site 2). The remainder of the colonies sampled at both sites (19 total colonies) changed their sex annually right after they released their gametes during spawning.

Shikina and his team discovered that colonies of F. ancora producing eggs switched to producing sperm three months after spawning. However, when switching from producing sperm to eggs, the earliest eggs appeared only a few weeks after the spawning event. During those transitional periods, a handful of the gametes collected by Shikina and his team were undifferentiated, or neither eggs nor sperm.

Figure 2. A graph representing the average time for the change of gametes of F. ancora. 2a. Shows the change of producing from eggs to sperm, while 2b. shows the change from producing sperm to eggs. The panels were adapted from Figure 3a. and 3e. from Shikina et al., 2024.

This timing led the scientists to believe that F. ancora is a Type ii organism that changes sex, producing eggs and sperm in the same reproductive organ. In other words, the reproductive organ switches from testes to ovaries (or vice versa). In contrast, if F. ancora were a Type i, it would have both testes for sperm production and ovaries for egg production, and each organ would become active for production based on time.

The team demonstrated for the first time that a community of a reef-building coral, F. ancora, has both permanent gonochoristic colonies and sequential hermaphroditic colonies in one community– this is a big deal in understanding how this species will handle climate change, how to re-establish this species if it is lost, and best practices in keeping this species in aquariums. In the future, the team suggests studying other coral species’ sexual reproduction, especially those suspected of employing a similar strategy to F. ancora. It is important in the conservation of coral reefs, as well as home coral aquaria.

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