I found these red algae which had been washed ashore. Their scientific name Gloiosaccion literally means 'glue sacks'. They are basically hollow bags which contain a sticky, gooey liquid. I googled but didn't find any info about the gooey substance. Some say that it keeps herbivores away. I wonder if it is because it tastes horrible or that it is poisonous.
Gooey substance oozing out when the 'bag' was punctured.