Effects of 2023 extreme warming events on Millepora spp

Some good news: Millepora alcicornis on Vauxhall Reef was largely unaffected by the extreme warming events in the fall of 2023

Note: I am recognizing only two species of Millepora at Vauxhaul and nearby fringing reefs: M. complanata and M. alcicornis (see Taxonomic Issues).

Tom Goreau cited the Acropora and Millepora species as tho reef-building species most affected by extreme warming in the fall of 2023.

Both the “true” coral A prolifera and the coral-like hydrozoan  M. complanata occur, with the colonial  zoanthid  Palythoa caribeana, in the “A. prolifera Mother Colony Complex” at Vauxhall.

Both A prolifera and M. complanata were uniformly drab and covered with epiphytic growth when I viewed the colony on Jan 12, 2023, in sharp contrast to the vibrant colours of a year earlier (Palythoa caribeana was ‘normal’ in both years:

Click on images for larger versions

A prolifera Mother Colony Complex” in early 2023 (left) and early 2024 (right)
They are taken from different vantage points;
the M. complanata colony at bottom left in the 2023 photo is at top right in the 2024 photo

Closeup of the same M. complanata colony in early 2023 (left) and early 2024 (right)

Close-up of M. complanata in the Mother Colony on Feb 22, 2024. It illustrates how quickly the blades became colonized as the warming events were likely in Oct/Nov, only 2-3 months earlier.

Coral head fromed by Millepora complanata  in the Reef Crest zone at Vauxhall, in 2017 (mosty living) and 2024 (mostly dead). Photos from slightly different vantage points.

I have called these castle-like formations of M. complanata that occur in the Seaward Slope “Cities of Gold”. They tend towards a ‘squarrose form’ (but I am not convinced they are Millepora squarrosa) and the individual compartments host a wide variety of sea life. This particular example is on Reef 32 in the MPA, but they also occur on Vauxhall Reef. All of the colonies I have observed in 2024 appear mostly dead, but still hosted  the same variety and abundance of sea life. The closeup photos illustrate how quickly the blades became colonized –  the warming events were likely in Oct/Nov, only 2-3 months earlier.

Millepora alcicornis and M. complanata on spurs, Seaward Slope Zone in 2015 and 2024. Photos are in different stands but are representative: both species healthy and common 2015…mid 2023, as at left. In early 2024: most A. alcicornis was  unaffected, most M. complanata had died (re extreme warming  in Oct, Nov 2023)

In regard to the Acropora species: because their distribution is limited at Vauxhall and I have a lot of photos from earlier years,  I was able to make some estimates of the degree of dieback due to the 2023 warming events. It was highly variable  between specimens for A. palmata (0-100%); evidently 100% for A. prolifera (no regrowth observed, at least up until March 2024), and slightly less than 100% for A. cervicornis (see this page for details).

To date in 2024, I  have observed a few small specimens of Millepora complanata that appeared not have been affected; in a few – mostly on spurs close to the Reef Front – there was partial dieback. Otherwise virtually all specimens – many – I have seen look dead, but those could include specimens that aged earlier or were killed bu other factors.

On the other hand, of about 20 sites where I have viewed M. alcicornis, only at one was there any specimens  that appeared to have died recently; in that case there was a much larger clump of living M. alcicornis next to it.

Healthy M. complanata on Reef Flat Mar 10, 2024

M. complanata exhibiting partial dieback.(Some might call it M. squarrosa.) Located at leading edge of a spur at Vauxhall, Mar 10, 2024,

Update Mar 19, 2024

I am noticing more partially living M. complanata and small specimens totally living, some examples below. I dont know whether I missed them previously or they represent new growth.

Mar 17, Vauxhall Reef towards the south, edge of reef crest

Reef 33, seaward slope Mar 17, 2024