restrictive or non-restrictive?

[keannu]

This may be an example for "judgement calls", related to writers' intention to express in a specific way.
"the shell which" seems a restrictive usage as the octopus wouldn't dig up all the shells in the sea, so the writer tried to focus on the shell's having fallen to the sea floor. Didn't he? Or is it just a mistake?

330-156
ex)Add the octopus to the growing list of animals seemingly capable of using tools. During several dives in Indonesia, Australian biologists documented the remarkable efforts of the local veined octopus to gain empty coconut shells for refuge. The animal first digs up the shell, which has fallen to the sea floor, and squirts it clean of mud...

[5jj]

How do we know what was in the speaker's mind? The writer has used commas, making it a non-restrictive clause. It makes reasonable sense, so that's it.

A precise writer might have thought carefully, and then written one of the following:

The animal first digs up the shells, which have fallen to the sea floor, and squirts them clean of mud. (Non-restrictive. The writer is continuing with the plural 'shells' from the previous sentence.)

The animal first digs up the shells, which have fallen to the sea floor, and squirts them clean of mud. (As before, but now the writer has chosen not to restrict 'shells' to precisely those coconut shells referred to in the previous sentence. This is probably not a very likely sentence, but it's possible.)

Other possibilities, which I have neither the time nor energy to analyse in detail, include:

The animal first digs up the shells which have fallen to the sea floor and squirts them clean of mud. (restrictive)
The animal first digs up the shells which have fallen to the sea floor, and squirts them clean of mud. (restrictive)
The animal first digs up a shell which has ...
The animals dig up ...
etc.

Sometimes we can say that a writer has clearly made a mistake; sometimes we can believe that a writer has not used the most felicitous construction. Very often we simply have to accept that a writer has chosen one of several valid possibilities. If the writer were to produce the same sentence next day, s/he might choose a different possibility. Very often it doesn't matter at all.

[keannu]

I don't know how to thank your supreme teaching!!!