Fauna in the Swash Zone of a Tropical Beach

Swash is the foaming surge of water that runs up the face of a beach after a wave breaks.  This narrow habitat has its own distinct fauna, a fauna which is well documented in cold countries that have lots of beaches but is almost undocumented in the tropics where people tend to focus on corals and mangroves. 

Low tide terrace of Yorkeys Knob Beach is prime swash fauna habitat, even for mole crabs

The swash zone is full of life and has many species that are not found on sand flats that are exposed at low tide.  On a remote beach, I once grabbed a handful of sand and counted 17 pipis of varying sizes in that single handful.  Here is a video that shows migrating pipi, filter feeding worms, a common moon crab and pied oystercatcher feeding on pipi.


Each beach has its own wave and sand characteristics and the fauna present vary greatly.  In North Queensland, there is a spectrum of beaches from steep beaches with coarse sand and almost flat beaches with fine sand.  The best beaches for fauna are the beaches between the two extremes.  Most of the swash fauna migrate up and down the beach with the tides, probably to escape predation.  When a beach is very flat, the distances between high and low tide swash zones becomes very far and the swash becomes so slow and gentle that it does not liquefy the sand.  Most of the migrating swash fauna also need the sand to be mobilised or liquefied by waves so that they can burrow into it.  Burrowing into hard packed sand is very difficult for many creatures.  At the other end of the spectrum are steep beaches with coarse sand where waves crash down hard on bare sand.  Very little fauna can be found in these beaches.  Beaches that do not have swash such as beaches within river mouths also do not have much fauna.

Swash zone fauna do not like waves that crash down on the beach

Pipi (officially called Cuneate Wedge Shells – Donax cuneatus) are the best know inhabitant of the tropical swash zone.  People dig them up and take them home to fry, spice and eat.  Pipi came to be of interest me when I noticed that they eject themselves from the sand so that waves can carry them up the beach.  On my favourite beach, the slope is steep and the swash moves very quickly so the pipi have to be very decisive about which waves to catch and with regard to timing.  Everything is so fast that it too fast for human vision and I resorted to using high speed video which is how I saw that pipi sometimes jump out of the sand microseconds before or after the arrival of the wave.  Now I have found out that pipi move in a much more relaxed fashion on shallow sloping beaches and can easily be observed with the naked eye.  The gentler swash on these beaches do not always move the pipi far enough and the pipi somehow know!  They just lie there on the surface waiting for another wave.  Sometimes up to 3 waves are needed to move the pipi up the beach and make it happy.  Then it wiggles its foot vigorously into the sand and pulls its shell down before the wave retreats and the sand goes hard.  Very occasionally, a pipi will be taken too far up the beach and they just wait for a big wave to take them back down again. 

Pipi (Cuneate Wedge Shell, with foot and siphons exposed

Pipi can use their siphons like arms to push into the sand and hold themselves against the retreating swash.  They also use their foot to dig into the sand like an anchor, however I think as swash usually undermines the sand out from beneath my feet, that the foot of the pipi mainly perform a hydrodynamic role and prevent the sand from being undermined from under the pipi. 
Much rarer than pipi are mole crabs.  I have only ever caught one.  They are neither common or easy to find.  Mole crabs filter feed with their antennae.

A mole crab - Albunea-symmysta

The entire undersurface of Albunea is dedicated to digging implements

Feeding Albunea poke their antennae into the retreating swash

On beaches with fine sand, tiny filter feeding worms are the most common creatures and they cover the entire surface of the beach.  These worms have burrows where they can wait between tides rather than migrating.  Finding out what the worms are called is a mission as almost nobody writes about these creatures even though they are so common that they must be ecologically important.  I think that they may be a type of palp worm (Spionidae).

Head and palps of filter feeding worms, click to enlarge as they are hard to see

The filter feeding worms are quite small

However the spionid? worms cover the beach

Matuta victor crabs patrol within the surging swash.  There seems to be a Matuta crab every few metres, which is a similar density to ghost crabs which scavenge the beach at low tide.  Matuta are reported to eat bivalves and worms so are probably the primary predators of the pipi and spionid worms. 

Common Moon Crab - Matuta victor

Fish are also present within the swash.  Fourline Striped Grunter, Pelates quadrilineatus zoom around in the swash.  A few times I saw a small sole allowing itself to be beached as the swash retreated.  It could see me coming and always escaped.

When the tide retreats, the swash zone also provides food for ghost crabs and pied oystercatcher.

Pied Oystercatcher feeding on Wedge Shells

Baby horn-eyed ghost crab

There are probably several creatures present within the swash zone waiting to be discovered. With waves constantly moving everything around, it is a very difficult place to study.

Shrimp dug up in swash zone

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