Any Geologists on here? Unusual rock

[tight fart]

Came across this odd rock in the sea today, it looks as if it should be light like pumis but it's as heavy as granite.

It looks volcanic but what would cause the smooth tubes to form, the other side was smooth as if a different rock.

It was a bit bigger than the palm of my hand but weighed 4/5 kilo.

I over exposed that one to see the small clam like shellfish that had out grown their homes.

[Lynchie999]

certainly volcanic ... smooth side would be just ocean weathering and the large holes I guess might have been caused by creatures...

[otolith]

Looks exactly like what is sold as "ocean rock" for aquaria. That's mostly aragonite (calcium carbonate) and derived from coral reefs.

[tight fart]

From asking elsewhere they say, "Fission tube worm rock with piddock clams."

[Simpo Two]

tight fart said:

From asking elsewhere they say, "Fission tube worm rock with piddock clams."

Trying to find out what a 'fission tube' was, I found:
https://seagrant.oregonstate.edu/sites/seagrant.or...

[rolster]

I can't remember the name of the creatures, (my Italian colleague advises they are called date del mare, sea dates or date muscles) but there are a type of muscle in North West Italy that bores those bigger holes in the rocks all along the coast.

They used to be eaten, until their numbers started declining and i believe now your not allowed to harvest or eat them.

[otolith]

rolster said:

I can't remember the name of the creatures, (my Italian colleague advises they are called date del mare, sea dates or date muscles) but there are a type of muscle in North West Italy that bores those bigger holes in the rocks all along the coast.

They used to be eaten, until their numbers started declining and i believe now your not allowed to harvest or eat them.

https://www.facarospauls.com/apps/italian-food-dec...

Lithophaga lithophaga, which obviously translates to "stone eater".

https://www.sciencedirect.com/science/article/pii/...

[rolster]

Otolith - thats spot on for the muscles, i have been told there are a similar sort in spain that are still harvested and eaten so would expect similar holes in the rocks along the shoreline where they are.

[Pupp]

What’s the boring mechanism; do they ‘feed’ on the rock somehow, or is the foot of the shellfish chemically reactive with it? Would an igneous rock be composed of calcium carbonate?

[rolster]

In Oliths lower link above the boring mecanisum is described as:-

Quote

Lithophaga lithophaga generally inhabits galleries bored in limestones, by attaching its antero-ventral shell margin to the inner wall of the holes with byssus (Owada, 2007). The absence of erosion marks on the shell of this species excludes any mechanical excavation; galleries are dug with a neutral mucoprotein with calcium-binding ability, which is secreted by pallial glands (Jaccarini et al., 1968). Cavities are typically 1.5 times greater than the individual that excavated it (Galinou-Mitsoudi and Sinis, 1995), and tend to be perpendicular to the rocky surface, in order to minimize the intraspecific competition (Guallart and Templado, 2012). Date mussel boring activity occurs mainly in the autumn-winter season, since during the rest of the year most of the energy is spent for reproduction (Galinou-Mitsoudi and Sinis,

Unquote

To condense that down a bit, its secretions disolve the rock slowly over time during non reproduction times of the year.

[otolith]

Pupp said:

Would an igneous rock be composed of calcium carbonate?

No, not generally.

[Simpo Two]

rolster said:

galleries are dug with a neutral mucoprotein with calcium-binding ability

Remarkable that a neutral substance can 'dissolve' rock, especially when you consider it's made by an animal.

[Pupp]

Thanks for the replies; interesting stuff.