"It's likely it will get harder and harder to find the rarer things, but it also gets more exciting."
Bizarre species discovered within the past year included Yoda purpurata, which had features resembling the Jedi master's large sagging ears, a crimson shrimp found at a depth of 2600m beneath the Norwegian Sea, and an odd-looking bristle worm discovered 1600m below the northeast Pacific.
"Knowing how many species there are in our oceans, and describing them, is vital for science and conservation for several reasons," Professor Costello said.
"Species are the most practical measure for distinguishing habitats and tracking progress in exploring the earth's biodiversity.
"They are as fundamental to biology as elements are to chemistry and particles to physics.
"So failure to consider all species in an ecosystem is analogous to an accountant ignoring items of inventory in a company's stock."
Better understanding of what species exist enabled more accurate estimates of extinction rates through habitat loss, while having a "master list" of species' names was essential for quality assurance.
Research efforts have been boosted by the World Register of Marine Species - an open-access, online database that has received contributions from almost 300 scientists from 32 countries.
The study supports previous research by Professor Costello and colleagues, which used statistical modelling and an earlier version of the register to reach a similar estimate of the number of species on earth and in the oceans. It is also the culmination of 14 years' work for Professor Costello, who began a European register of marine species in 1997 that expanded until the world register was initiated in 2006.
OCEANS STILL TO GIVE UP THEIR INHABITANTS
Around 226,000 species have been described by science and as many as 72,000 more are in collections awaiting description - yet hundreds of thousands more may still be waiting for discovery in our oceans.
The rate of discovery is, however, increasing, with an unprecedented 20,000 new marine species described in the past decade alone, suggesting that most marine species will be discovered this century.
Earlier estimates of ocean diversity had relied on expert polls based on extrapolations from past rates of species descriptions and other measures.
Those estimates varied widely, suffering because there was no global catalogue of marine species, and a new study gauging a more accurate figure canvassed 120 of the world's top experts on the taxonomy, or classification, of marine species.
Mammals, birds, reptiles, insects and larger plants were some of the best-described groups of marine species to date.
Many of the species yet to be discovered will come from among the smaller crustaceans, molluscs, algae, worms, and sponges.
