How Do You Get Around If You’re Moving At A Snail’s Pace?

Geographic isolation is a key factor in the generation of new species, as it prevents diferent populations interbreeding. This lack of genetic exchange means the characteristics of the separated populations diverge, and gives rise to new species. This is why New Zealand, which has remained in isolation for 80 million years, and Hawaii, which has never been connected to another landmass, have so many species that are only found there.

It's easy to imagine how you might achieve this isolation on small islands, separated from other landmasses by the sea; or on mountain peaks, where the change in conditions as you descend imposes a barrier to species distribution. Back in the ocean, many animals are highly mobile and can meet to breed and exchange genetic information, despite spending the majority of their days hundreds of kilometres apart. So how do you get new species in something as connected as the oceans? And conversely, how do some marine animals have such huge geographic ranges, when they hardly move at all?

Many bottom dwelling, or benthic, animals can only move short distances. This is particularly true of marine snails (gastropods), which have limited mobility throughout most of their lives. Adult cone shells, for example, rarely move more than 5 metres over the course of a whole year!

Fortunately for most marine snails, their young larval forms exist in the plankton. The term ‘plankton' encompasses a huge range of organisms that are incapable of swimming against a current. This means they can be carried in ocean currents to far-flung corners of the world, a process that has a huge impact on their distributional range.

Shortly after a gastropod egg is fertilised it becomes a peculiar looking larva called a trocophore – a simple creature with a mouth, an anus and a few stray hair-like cilia. They then metamorphose into something a little more snail-like (a veliger). Veligers have a large foot, or velum, and the first vestiges of a shell. Now they are ready to settle. If they're lucky enough to have been carried somewhere cosy – with their preferred foodstuffs, enough shelter and a reasonable climate, they'll turn into a miniature adult. The proto-shell (or protoconch, as it's actually known) that it had as a veliger then becomes the very tip of the spire in the adult's shell.

How can we tell where larvae are being carried in the ocean? Take a look at that photo of a veliger, below. It's tiny - okay, so I couldn't pin down a scale for this one - but with many gastropod larvae being less than a few millimetres long, marine biologists would have a hard time trying to tag them. Take the veliger of a dog conch, for example, whose shell starts out at little more than 0.2 millimetres long! Instead, we can model their travels. By introducing small virtual particles – virtual larvae, if you will – at known spawning sites, marine biologists can predict their movement over a large area and extended time period to see how far and how fast they go. This is a great indicator of dispersal potential and, if conditions are right and enough larvae settle, the snail will set up camp in a new area.

So snails – or at least their larvae – seem pretty well travelled, but if that's the case, then how can populations stay separate long enough to diverge into different species?

The extent of a snail's dispersal is dependent on the type of larvae. The longer larvae spend in the water column, the further they can go. This means that gastropods with a long larval stage often have a wide distribution. Planktotrophic (plankton-eating) larvae spend the most time in the water column because they have little in the way of food reserves and need to feed before settling. Lecithotrophic larvae have a yolk-like food supply on them, meaning they don't need to feed, so spend much less time in the water column. There are also marine snails that don't have a larval stage at all! Going straight from an egg to a miniature adult, and skipping all the larval steps, is known as direct development and means that these young snails, like their parents, don't travel far.

It is this difference in dispersal potential that allows some populations to be widespread and others restricted to a very small area. But being part of the plankton doesn't guarantee you a ticket to the other side of the ocean. Larvae that hatch close to islands are sometimes caught in currents that keep them close to shore, leading to very restricted species distributions. Similarly, ending your planktonic stage in an inhospitable environment, where you can't properly settle and metamorphose, prevents you from extending the range of your species.

Limited larval dispersal means that the adults from different populations are unlikely to exchange genetic material. Without this link between populations, they may become so different that they form separate species. Dispersal ability and population isolation play a huge role in the generation of new species. Of course, it's not only marine snails that benefit from larval dispersal. Other invertebrates, and even more mobile species, can reap the rewards of a ride on the ocean's currents.

Can you think of any examples where similar situations might arise on land?

Something to consider: What makes something a species? There's a lot of natural variation within populations, you only have to take a look at the human population to get an idea, so if divergence in population characteristics can lead to a new species, at what point do we say "yes, that snail's a new one"? Here's something to get you thinking.

References

Cob, Z. C. et al. Development and Growth of Larvae of the Dog Conch, Strombus canarium (Mollusca: Gastropoda), in the Laboratory. Zoological Studies 48 1-11 (2009).

Cooper, R. K. and Millener, P. R. The New Zealand biota: Historical background and new research. Trends in Ecology & Evolution 8 429-433 (1993).

Hendricks, J. R. Using Marine Snails to Teach Biogeography and Macroevolution: The Role of Larvae and Dispersal Ability in the Evolution and Persistence of Species. Evolution: Education and Outreach 5 534-540 (2012).

Thorrold, S. R. Ocean Ecology: Don't Fence Me in. Current Biology 16 R638-R640 (2006).

For a little more on gastropods and their reproduction check out this great resource!

Images

Veliger larva: Paulette Brunner. Center for Cell Dynamics (2004).

Cone shells: Wikimedia Commons user H. Zell. leopard cone, fig cone, striate cone and weasel cone (2011).