Just keep swimming

This little critter is a limpet.  From the photo they may not look like the most exciting of creatures.  If you've ever been down to the coast and taken a look at them yourself… your opinion may not have changed.  They don't seem to move around a lot, or do a lot.  Of course looks can be deceiving.  Under that shell is the limpet's squishy body - and their big, muscular foot which, alongside a pretty amazing adhesive secretion, they use to cling onto rocks and other hard surfaces.  Anyone who has ever had a go at trying to get a limpet off a rock knows how good a grip they can have. 

This fabulous foot isn’t just used to stop them from drying out when the tide leaves them exposed to the air, or keep pesky predators (or nosy humans) at bay.  Limpets are grazers, feeding on tiny algae on the surface of rocks with their raspy "tongue" (called a radula).  See that empty space behind the limpet in the photo?  That's where it's been grazing.  Once they have grazed an area they need to find more food.  That foot gets to work, and along moves the limpet, munching up all the algae in its path.  Some limpet species even appear to have a home - a particular crevice that they return to just before the tide will expose them to the air. But this isn't a post about how amazing limpets are.  This is a post about animal movements in the ocean.. Or at least 3 different types of animal movement.  Some of them move a lot further than you think.  Yes, even limpets.

Larval life

Lots of marine animals - including limpets - are what we call 'broadcast spawners'.  Basically, males release their sperm and females release their eggs into the water at the same time.  When these eggs and sperm meet, they are fertilized.  Sometime later, if they haven't been eaten, little baby limpets - limpet larvae - hatch from the eggs.  They don't look much like the limpets you see on rocks yet.  They spend their time in "the plankton" - basically in the water where they eat, try not to get eaten, and if they are lucky growing up a bit more.  Eventually the time to settle comes, and they (hopefully) end up on some hard substrate like a rock, where if all goes well, they grow up and look more like the limpets we are used to.   Thanks to the ocean - waves, tides, wind, currents, and so forth the fertilized eggs and larvae may very well end up away from where their parents are based.  We call this 'dispersal'.  How far they go depends on a range of biological factors (e.g. how long they stay in "the plankton" for) and environmental factors (e.g. how strong the current that they end up in is).  One open-access study looking at slipper limpets (Crepidula fornicata) - which despite their name are not 'true limpets', found that their larvae were probably travelling over 100 km.  Not bad for something that looks like a tiny speck.

Outgrowing your home

Some species seem to like to use very different habitats during different stages of their lives.  Let's take a couple of coral reef fish found in the Philippines for an example - the one-spot snapper (Lutjanus monostigma) and the dash-and-dot goatfish (Parupeneus barberinus).  Both of these species are broadcast spawners - complete with larvae that ends up in the water column for a bit.  Once the larvae have made it through their 'pelagic larvae' phase, they end up settling in seagrass beds, and mangroves.  Yep - these coral reef fish kids aren't on coral reefs!  Might sound a little daft but we need to think about it from the perspective of being a small, vulnerable, hungry fish.  Mangroves and seagrass beds tend to have better hiding places for a tiny fish than coral reefs do.  They also tend to have lots of yummy nutrients - a perfect place to eat lots and grow quickly whilst avoiding being eaten yourself*.  As the fish get older, their needs change.  Those handy hiding places when they were younger might be too small for them to hide in now - or move about freely.  They might also need different types of food to keep them going.  It's time to move off to the adult habitat - the coral reef.  This process of moving between habitats at different stages of the fishes life is known as ontogenetic migration and is thought to be a way to increase the fishes chance of survival as it grows up.  Other species of fish seem to use more than 2 or 3 different habitats.  Grunts (Haemulon species) for example apparently first settle in rubble, then head off to seagrass beds.  After they move into some mangroves, followed by some rocky area - and finally into coral reefs!  This kind of movement does raise an interesting question though… are these coral reef fish really just… coral reef fish?

*some studies have suggested that seagrass bed provides more food and mangroves better shelter for many fish species.  There may be some additional movement between these two habitats!


Long distance travellers

Varvara is a 9 year old grey whale (Eschrichtius robustus).  Like all grey whales, Varvara travels across huge expanses.  But Varvara is a rather special whale.  In November 2011, she swam from Sakhalin Island (Russia) to Cabo San Lucas (Baja California, Mexico) - and back again.  The round-trip, which took her 172 days, covered a whopping 22,511 kilometres (14,000 miles)  - the longest mammal migration ever recorded.  It is highly likely that Varvara wasn't alone in making such a huge journey, but she was the one whose journey was recorded the longest.  At around the same time Varvara was fitted with a satellite tag, other grey whales too.  Some of those satellite tags fell off, some stopped transmitting after a while.

Whales migrate primarily for two reasons - eating and breeding.  Grey whales tend to head into warm, lower latitude waters during the winter months to breed, but head back up north in the summer to feed in cooler waters, which are more productive and thus have more food.  Why do they go all the way south to breed?  Part of the reason is likely because grey whale calves are born with very little blubber.  Birth is those chilly northern waters would likely be extremely unpleasant at best, and fatal at worse.  Other reasons include where the grey whales use as nursery grounds - shallow lagoons.  It's thought that these lagoons offer the young whales greater protection from predators than more open waters, and can also help the calves float.  Yes really!  Remember they lack blubber?  Well blubber is essentially fat, and that helps with buoyancy.   Low blubber means buoyancy is not that great, and these little critters have to keep on swimming.  The lagoons help with this because they tend to have higher concentrations of salt than the ocean - which helps the calves to float.  Unfortunately when it's time to migrate back north, the young whales can face all sorts of predators - particularly orca (killer whales), so the system isn't entirely predator-proof


Image: A limpet munching its way across a rock.  Credit Flickr/NOAA Photo Library.  Licence (CC BY 2.0)