Life in a Beach

Each area of coastline is different.  It is not a case of the same species be present is slightly different numbers, each place seems to be the product of different ecological rules.  Sandy beaches are normally somewhere where animals are very scarce, perhaps only a few ghost crabs are present.  However the beach at Archer Point, close to Cooktown, is literally crawling with life.

Amphipod burrows near high tide line

An amphipod

I have never seen a beach covered with burrows all the way from the low tide mark to the high tide mark before.  I even had to dig up the ground to find out who was in the burrows.  Amphipods jumped out of the first handful of sand so must be just below the surface.

Amphipods are basically a small (1 cm long) prawn-like animal that walks on land.  They have very big eyes and are nocturnal.  When chased, they can jump like crazy to avoid being caught.

Since seeing the amphipods at Archer Point, I have kept my eyes open and have scanned several beaches which seem similar in terms of protection and sand properties.  Finding amphipods on these other beaches is a struggle.  Normally, there are a few amphipods under washed up leaf litter and seaweed.  It is unusual to find large numbers on a beach that is almost clear of plant debris.  I suspect that there is freshwater seepage concealed within the beach which some how helps the amphipods.

Recently 4WD vehicles were allowed back on many Queensland beaches.  Scientific research had shown that vehicles driving on the beach crush many of the animals that live in the sand and hurt the ecology of the beach. My position on this matter is that each place has to be assessed individually.  It is nice to be able to pull up right beside the sea and relax and I would not like to see every beach fenced off.  Many beaches are reworked so much by wind and waves that few living animals are present and driving on these beaches does little damage.  However it is repulsive to find tyre tracks all over remote beaches where people should be able to enjoy freedom from such things.  In this case, there is no reason to drive on the beach at Archer Point as the beach is only 300 m long and has an unusual ecology featuring vast number of amphipods which would suffer due to vehicles on the beach.

There is no point in driving onto this beach as it is so short, yet lots of people do it

Amphipod holes beside tyre tracks

The Nemertean

Mangrove swamps at night are totally quiet with all life seaming asleep except for the nemertean.  These curious predators which are from 10 to 25 cm long hide in cavities in fallen logs during the day.  At night they move slowly across the mangrove floor in search of prey which they catch with an eversible proboscis.  This curious organ can be thrust forwards nearly as far as the animal is long.  At the end of the proboscis is a poison stylus or a sticky pad, for this species, I do not know which.

A mangrove ribbon worm - Pantinonemertes sp.

Below, a video of the eversible proboscic.

Elysia the Solar-powered Sea Slug

Recently there was a movie called Elysium, which is rather similar to Elysia so I looked the word up to see if there was a connection. Elysium fields were the ancient Greek version of heaven.  Maybe Elysia, the genus name of the mangrove sea slug is derived from the legendary fields of Elysium which more or less grew themselves.  Elysia sp. were believed to carry their bountiful fields with them in the form of chloroplasts that they have extracted from algae.  The released chloroplasts colour the sea slugs deep green and have long been believed to feed the sea slug for as long as a year using solar power.  That idea is unfortunately under challenge and some species have been found not to be solar powered but others are still under the microscope.

Elysia bangtawaensis or mangrove-leaf sea slug is our local species although is is found from the Queensland border to Thailand and even India.  The species was only named in 1998 after a Thai village that resisted clearing of mangroves for aquaculture.  It was then discovered at Coolangatta by another consultant during impact assessment for a road project.  Soon after I found some at Half Moon Bay, near Smithfield, Cairns - I was the second person to find the species here and reported the find to the sea slug forum.  In the photo below there is a Telescopium snail for scale.  Elysia are about 4 cm long.

This time, I found the mangrove sea slugs in a Ceriops forest.  Ceriops or yellow spurred mangrove inhabits the dry intertidal areas near salt pans and is not the place one would expect to find sea slugs.  Ceriops forests are rather unexpected places for most Australians.

Elysia were in very shallow pools on the floor of the forest.  Not every part of the forest had Elysia, they are fairly scarce and I had not seen any for years.  I am not sure if they are always present or if they come and go.  I am sure that they can be very hard to find.  They do not seem to chase sun spots which I would have expected them to do if they were solar powered.