Since early 2015, on Bonaire (Dutch Caribbean), I sometimes encounter 'humps' on scleractinian corals.
They feel like cartilage or a hard gel and have a diameter of about 10-20 mm. The surface has a whitish-violet colour, possibly due to fine filaments. In cross sections some concentric layers may be recognized.
These humps were encountered on healthy corals (Orbicella faveolata, Agaricia agaricites), on morbid corals (Orbicella annularis, Siderastrea siderea), on dead coral next to live coral (Madracis auretenra) and on dead coral surfaces (unspecified), see enclosed photos.
Who has seen these humps as well and who knows what they are?
Godfried, it seems to be a marine sponge, maybe Chondrilla, . The fine filaments over the surface are red algae.
Thanks for your reaction Gianco
No oscula present, so I do not think this to be a sponge like Chondrilla.
Observing some tissue under the microscope to check for spicules is a good idea, as some sponges have inconspicuous pores. Another possible organism is an ascidean, but i see no siphons on the pictures.
Godfried,
why don't you send your question straight to Rob van Soest, he should now if it is sponge or not.
Harry
Included a sagittal section of a similar hump from St. Eustatius (Dutch Caribbean). This shows a fine lamination – and a solid structure, very unlike sponges. Unfortunately, a sample did not result in a PCR product. The humps somewhat resemble the colonial ascidian Trididemnum solidum, which grows on coral, but surface structure and consistency clearly differ.
A likely candidate: 'microbialites' or 'oncolites', i.e. stromatolite-like structures formed by Cyanobacteria. Abed et al. (2003) describe domes of Symploca and Phormidium and Phormidium (according to WoRMS now Leptolyngbya) hendersonii with 'internally laminated colonies of firm almost cartilaginous consistency' - like the domes presented here. Abed et al. studied their specimens in Tikehau atoll (French Polynesia) but L. hendersonii is also known from the West Atlantic: Golubic & Focke (1978) mention this species from many Caribbean locations, among them Bonaire (although in Lac Bay, not in a reef environment). Their Fig. 2 shows a brain coral Pseudodiploria strigosa from Bermuda with several specimens on its surface. They also mention a firm, gelatinous, almost cartilaginous consistency.
I still wonder if anybody else has seen them on living corals as well.
http://www.marinespecies.org/aphia.php?p=taxdetails&id=660862
Article Characterization of microbialite-forming Cyanobacteria in a ...
Article Phormidium hendersonii Howe: Identity and significance of a ...
In Reunion, it seems we have the same whitish-violet "humps of Cyanophytes", Phormidium sp. Humps are ca 5 cm in diameter and they occur on corals in eutrophic/dystrophic areas (Naim et al., 2013a). They have a "stromatolit-like" internal structure.
This subject was presented at a coral reef meeting in Amsterdam. You may find the abstract and presentation here at RG. Conference Paper Bonaire back to the Archaean? Presence of stromatolite-like ...
A nice photo of Leptolyngbya hendersonii on coral with vernacular name 'Rubbery seaweed' may be found on p. 41 of the reef guide of Charteris, M. 2017. Caribbean reef life - A field guide for divers - 3rd ed.