By Prof. Charles Griffiths
How many species inhabit our oceans? Prof Charles Griffiths discusses marine biodiversity on a global and local scale.
Have you ever wondered how many marine species are known, how many new ones are found each year and how we go about identifying and naming these species? Experienced marine taxonomist, Professor Charles Griffiths, answers these and related questions and provides examples of recent South African marine discoveries.
HOW MANY KNOWN SPECIES EXIST?
Some 1.25 million species have been named and described: There are about 1 million terrestrial species and 250 000 species in the oceans.
WHY IS THE RATIO OF MARINE SPECIES SMALLER THAN TERRESTRIAL SPECIES?
There are three main reasons for this. Firstly, we as humans are land animals, so we have explored the land much more thoroughly, collecting far more samples than in the sea. Secondly, the land is divided into numerous islands, promoting the evolution of new and different species on each landmass, whereas the oceans are continuous and many marine species are thus very widespread. Lastly, about 80% of all species are insects, and these are essentially absent from the oceans (for reasons that are still poorly understood).
HOW ACCURATE ARE THESE NUMBERS?
It is difficult to gauge exactly how many known species make up the real species count. A recent estimate puts the true figure at 8.7 million, of which 6.5 million are terrestrial and 2.2 million are oceanic. Whatever the exact numbers, it is clear that far more species are unknown than are known and it will be a long time before we find and name them all.
HOW MANY NEW SPECIES ARE FOUND EACH YEAR?
Some 18 000 new species are described and named annually. This number is fairly stable from year to year and has more to do with the number of trained taxonomists who are able to undertake this work than with the ease or difficulty of finding new species. In fact, thousands of undescribed species have already been collected and line the shelves of museums around the world.
HOW MANY MARINE SPECIES ARE THERE IN SOUTH AFRICA?
About 14 000 marine species have been described in South Africa, half of these falling into just three groups, namely 3 154 molluscs, 2 331 crustaceans and 2 000 fishes. About 30 additional species are added to this total each year.
WHERE DO THESE ADDITIONAL SPECIES COME FROM?
Additions to the regional fauna come about in four ways. Firstly, and most excitingly, through the discovery of new species that are entirely unknown to science. Secondly, and more commonly, through new records of new species that are found in South Africa for the first time, which are already known from elsewhere. Thirdly, by detecting species which are newly introduced by human activities, such as by shipping. Lastly, via improved taxonomic analysis, which usually prompts the realisation that what we previously thought was a single species is in fact two or more closely-related species, called taxonomic revision.
HOW DOES ONE KNOW IF A SPECIES IS REALLY NEW?
Essentially one has to compare the features of the new organism with those of every other known species in that group. Since there can be hundreds of existing species in the group, many of which were named and described centuries ago and in a variety of languages, this is a technically demanding process and is the main constraint on the rates of species description.
HOW ARE NEW SPECIES NAMED?
The person who is responsible for describing a new species can choose any name that is not already taken by another species in that genus. The most sensible names describe features that define the new species, but many species are named after people (such as the original collector) or the collecting location.
ARE THERE ANY RECENT MARINE DISCOVERIES IN SOUTH AFRICA?
Below is a list of recent additions to the South African fauna. Each of these illustrate one of the four ways in which a species can be added to this list of biodiversity.
The beautiful stargazer shrimp is completely new to science. It was collected in False Bay, Cape Town by diver Guido Zsilavecz, after whom it was named in 2014. Since then, further collection at the same site has revealed several other similar species, some or all of which are probably also new and are the subjects of ongoing studies.
The helmet or shingle urchin, which is one of several well-known tropical Indo-Pacific Ocean urchins, has been recorded in South African waters for the first time. It was first photographed at Sodwana Bay.
Light-bulb sea squirt
This is an example of an invasive species that was introduced by shipping. First reported in South Africa in 2001, it is already abundant in harbours throughout the country, but not yet found on the open coast.
The crown crab is a new species that was found through improved scientific knowledge. Recent genetic research has shown that what was thought to be a single species occurring all around the coastline is in fact no less than five distinct species, namely three estuarine forms from the west, southeast and east coasts, and two marine species. Although these creatures have been known to science, research has only revealed their distinctions recently and as such they were described and named in 2012.
Most marine life lovers will recall their first encounter with hermit crabs in the intertidal zone of a beach or a rocky shore. Personally, as a child I clearly remember turning shells upside down to see if a crab occupied them. Today, I am studying the biodiversity of South African hermit crabs for my PhD and making exciting discoveries.
The majority of hermit crabs inhabit gastropod shells as a portable home. The abdomen, or hind body part, is not only curved to fit into these shells, but also soft and weakly calcified. The tenanted house thus provides shelter from predators and environmental stressors and without it hermit crabs would quickly be eaten by their natural enemies.
As humans, we can identify well with the concept of living in a house and hence we might find this association with a seashell quite romantic. However, this adaptation does not come without its disadvantages. Hermit crabs constantly have to seek new and larger homes as they grow and shell availability is a limiting factor for most hermit crab populations. Hermit crabs do not kill gastropods for their shells; instead they rely on the shells of gastropods that have died, or used ones that become available when another hermit crab moves to a larger home. To overcome the problem of shell shortage many hermit crabs cultivate symbionts, such as cnidarians, bryozoans or ascidians, which can extend the shell aperture, alleviating the need to switch into new, larger shells. In extreme cases, the cultivated animals will grow a large cloak which replaces the gastropod shell entirely. As a beneficial side-effect, most of these symbionts contain unpalatable or toxic substances that provide extra protection from predators. Alternatively, some hermit crab species use other objects for shelter, such as pieces of wood, rocks, polychaete tubes or elongated tusk shells. These non-shell shelters occupied by some hermit crabs need not be spirally curved and can be completely symmetrical as the hermit crab can straighten its abdomen like that of a shrimp.