Scientists around the globe are working hard to try and find out, MarineSpecies.org now have over 122 000 species, and estimate a total of 230 000. During their work, however, they have found that a lot of our ‘species’ are really only synonyms for the same organism. The leader in the synonym race is the breadcrumb sponge (Halichondria panicea). This is common on a range of substrates, and adopts a wide range of shapes and colours in different habitats – if you dive you’ve almost certainly seen it! Its predilection to changing its colour and shape have resulted in scientists giving it no less than 56 synonyms…

MarineSpecies.org is affiliated to the Census of Marine Life, which readers might also find interesting.