This looked like the remains of a large, mature Enteroctopus dofleini (Giant Pacific Octopus). It looked like it had recently eaten a mussel, as the shell was still there. A gull was nibbling at it when I arrived.
Copy pasting from observation http://www.inaturalist.org/observations/3206287 (which was about the sponge Chondrilla nucula):
Can you spot the tiny Chondrilla nucula sponges attached to the tiles? Well... they were part of an experiment in the context of the EU project below. We were supposed to develop a sponge-farming method (which we actually finally achieved, in a different way though) and tried sponges attached to tiles attached to the substrate, first.
Of course... tiles disappeared :-) Any now you too know why!
Funding from European Commission, Project SPECIAL FP7-KBBE-2010-4-266033
Cuttlebone washed up on the beach (not often seen locally).
A cephalopod is any member of the molluscan class Cephalopoda (Greek plural κεφαλόποδα (kephalópoda); "head-feet"). These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles (muscular hydrostats) modified from the primitive molluscan foot. Fishermen sometimes call them inkfish, referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.