Dear Godfried,

I have seen the same kind of black holes myself at Curacao. It seems that the 2nd photograph shows antennae of a crustacean. Coral-dwelling barnacles usually remain recognizable becasue they protrude from the surface. The pits of gall crabs are usually crescent-shaped  See e.g.: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4806305/

We should collect the corals and look what is inside the holes.

Best wishes, Bert

I also know that Lithophaga spp. a type of boring bivalve make holes like that, particularly in Porites spp. See the following pdf starting on pg. 59

http://real.mtak.hu/2812/1/Kazmer_Taborosi_2012_Bioerosion.pdf

Hope this helps!

That is correct and even more clear if siphons are seen protruding from the holes. See the work by Karl Kleemann: http://homepage.univie.ac.at/Karl.Kleemann/

See also: https://www.researchgate.net/publication/281426876_Did_borers_make_corals_more_susceptible_to_a_catastrophic_disease_outbreak_in_Hong_Kong

Article Did borers make corals more susceptible to a catastrophic di...

I know Lithophaga, see enclosed photo, but I fail to see the more or less symmetrical dumbbell shape, especially in my second photo.

The upper picture of hole looks like that we described ARNAUD P. & THOMASSIN B.A. in Heteropsammia michelini as Liuthophaga lessepsiana.

For the lower picture, I seem agree with Bert H., it could be a boring crustacean.

Dear Godfried,

I am agreed with Dr. Bert W Hoeksema in addition the gall crabs inhibit the branching corals Styophora and Pocillipora. It seems that the 2nd photograph shows antennae of a crustacean and in many cases the some warm tubes and feathery individuals are formed in the massive corals tubes.

Thanks you all for reacting!

@ André Klipera: I know the spinyhead blenny well, it lives in empty worm tubes ending in a more funnel-like way and you see its head most of the time.

@ Tarek A. Mohamed: gall crabs are not known to inhabit Caribbean Porites.

In the meantime, I found some examples of 'non-symmetrical dumbbell' (i.e. key-hole) sized holes made by Lithophaga, so this solves my question on the first photo.

As to the second photo:

Bert Hoeksema made the nice observation of antennae in the second photo. Interestingly, a crustacean known to inhibit this coral is Pomatogebia operculata, but a part of its abdomen is shaped like an operculum to blocks the entrance to the burrow system from the inside and mimics the coral's surface. Never saw something like that.

Barnacles are often visible at the surface of Porites astreoides but Highsmith 1983 remarks that the pyrgomatine Ceratoconcha domingensis commonly settles on this species and may be embedded by subsequent growth of the coral, resulting in skeletal voids.

I made another photo of a hole in Porites astreoides with 'something' in it, not even sure whether this is an organism. It might just be a faecal pellet from Lithophaga.

In addition to my last reaction, Coralaxius abelei (Decapoda, Axiidea) is also known to live in Porites. It has banded and heavily setose antennal flagellae, but photo 2 does not show enough detail to see this.

https://research.nhm.org/pdfs/18103/18103.pdf

Another source of holes (or rings) are caused by contact with Zoanthus solanderi.  Have a look at Figure 1 in Karlson (1980). Bull Mar Sci 30:897.

Their holes look different, but polychaetes may also produce holes in corals. For chaetopterids, see Bergsma GS (2009) Tube-dwelling coral symbionts induce significant morphological change in Montipora. Symbiosis 49: 143-150. For Spionids, see Liu, P.J. and Hsieh, H.L. 2000. Burrow architecture of the spionid polychaete Polydora villosa in the corals Montipora and Porites. Zoological Studies 39: 47–54. 

The more rounded holes may be pseudoborings of the barnacle, Ceratochoncha paucicostata Young, 1989.

See: Young, P. S. 1989. Ceratoconcha paucicostata, a new species of coral-inhabiting barnacle (Cirripedia, Pyrgomatidae) from the Western Atlantic. Crustaceana, 56(2): 193-199.

the hole in the first photo may be natural which could be made by organism, the second hole (photo below) could be human made, which is resulted from coral drilling for environmental/paleoclimate study like me :(. It should be covered/filled back the hole by cement cores/block after drilling to reduce the damage of the coral. or any other idea?

The holes in porites may be occurs and found due to the effect of Polychaetes of Truncated Reef Limestone Substrates like corals, as well as Feather Duster Worms

Thanks for your reactions. I am especially interested in holes in Caribbean Porites species.

@ Ronald Karlson: according to your Fig. 1 Zoanthus solandri seems able to make holes in Porites astreoides indeed. However, during  many dives at Bonaire I never encountered Z. solandri, so I consider it unlikely for this zoanthid to have created this particular hole.

@ Jorge Cortés: I like your use of the term 'pseudoboring'. In my question I already suspected the possibility of a barnacle not being able to keep up with the upward growth of its coral host. You refer to the barnacle Ceratoconcha paucicostata but its host is Madracis decactis. My reaction of April 21, 2016, points to its congener Ceratoconcha domingensis to be found on Porites. Highsmith et al. (1983) remark that 20% of all voids in P. astreoides may be caused by barnacles.

For Notaulax see Nishi et al 2017 (Boring sabellid in Porites) and references there included. Anamobaea genus also includes two species of boring sabellids in Caribbean corals with similar holes.

Here is an answer to the original question:

Article Black mantle tissue of endolithic mussels ( Leiosolenus spp....