Marine Field Day
ATLAS NATURALISTS FIELD DAY
7TH DECEMBER, 2019
Thank you to everyone who took part in our final field day for 2019. And a very special thanks to Alan Scrymgeour, Lyn Scrymgeour and Liz Allen for creating such an interesting and enjoyable event. A summary of the day’s activities follows.
The Bruce Steer pool
A group of around 30 keen Atlas Naturalists met at the Bruce Steer pool, a sheltered, shallow enclosure just inside the Bermagui River entrance. Liz Allen and Libby Hepburn have explored this pool on many occasions and recommended it as a good place to start our discovery of marine life.
Some people donned wetsuits and snorkels to view life underwater. Others waded in the shallows at the edges or looked for animals in the rock pools flanking the main pool.
Ascidians, sea urchins, seastars, octopods, limpets, cartrut shells, elephant snails, sea hares, mussels, mulberry shells, triton shells, chitons, rock-pool prawns, shore crabs and barnacles were amongst the many, varied animals discovered at this site.
Click on the individual images below to see a record of the sighting (species name, recorder, location) on ALCW NatureMapr.
Ascidians (Herdmania momus) - much more about ascidians later!
Echinoderms
Molluscs - a diverse array of 7 gastropods (photos 1-7), a bivalve (photo 8) and a polyplacophoran (photo 9)
Crustaceans
Camel Rock - “Offrandes de la mer”
A short drive north took us to the beach at Camel Rock, where we met Alan and Lyn Scrymgeour. Alan has an intimate knowledge of the marine and intertidal life in this area, which he was keen to share with the group.
The dramatic coastal scenery at Camel Rock. The rocks are a favourite roosting site for cormorants.
We gathered round to get instructions from Alan on the first activity … sandy hand prints! And for good reason … to assay the components of sand on this particular beach, both organic and inorganic. Alan pointed out that the sand structure provides evidence of a high energy beach.
The group then split up and scoured the beach, searching for marine offerings, large and small.
A sobering find - many clumps of burnt eucalypt leaves, each covered in dozens of beetles, both dead and alive.
These have been washed ashore after being carried by the wind and waves from bushfires many kilometres to the north.
All of the finds were laid out on a blanket. A rich treasure trove spanning the whole animal kingdom from Porifera (sponges) to Vertebrata (birds - a deceased shearwater or two).
We gathered round again for an informative, entertaining talk from Alan. He selected a range of objects and provided insights into their biology and ecology. Here are some snapshots from his presentation.
The eggs of a predatory mollusc
These egg cases, and the molluscs that lay them, are typically found at low levels where the ocean waves pound the rocks. A tough life, but a niche is a niche.
They belong to the Cartrut shell, which is shown in the photo gallery of molluscs at the Bruce Steer pool.
Cartrut shells, like many other shelled gastropods, are carnivores, preying on other molluscs - commonly an oyster or other bivalve.
They drill a small hole in the shell of the prey by applying an acid-secreting gland to the site and scraping it repeatedly with a rasping tongue (called a radula). The Cartrut shell then extends its proboscis through the hole and tears away the soft tissues of the prey.
Cuttlebones
One of the most familiar objects on beaches is the cuttlefish bone or cuttlebone. We discovered many of these during our trawl of the Camel Rock beach. But what exactly is a cuttlebone?
Clearly it’s not a bone, because only vertebrate animals have bones. Rather it is all that remains of the shell possessed by its evolutionary forebears. That ancestral shell has been greatly reduced in size and internalised. It has lost its original function of protection and is now just used by the cuttlefish to adjust its buoyancy.
Two species of cuttlefish are commonly found on the Far South Coast - the Giant Cuttle (Sepia apama) and the Mourning Cuttle (Sepia plangon). Their cuttlebones differ in appearance.
The cuttlebone of the Mourning Cuttle is slender with a groove on the lower surface and has a pink upper surface, whereas the Giant cuttlebone is broad, lacks a conspicuous groove and is grey-white on the upper surface.
Our collection - can you identify the species each came from?
A Floating Shell of the High Seas
Shells like this one are usually found on beaches. But where do the animals that inhabit these protective enclosures actually live?
The short answer to this question is - seldom where we find them. Rather, shelled molluscs inhabit a wide range of marine environments - on rocky shores, on the ocean floor or like this one, floating on the surface of the sea.
Meet Janthina sp., the Violet Snail. It lives on the high seas, floating like flotsam and jetsam on the surface of the ocean. The white object seen protruding from the shell in the image above is its flotation device. It can scarcely be recognised as such in this shrivelled, dried-up state.
But you can see it in its pristine, fully functional state in this image taken by Max Campbell in October last year (click on image to see his NatureMapr record).
This shell was washed up on the beach at Tanja Lagoon in a blue tide, along with “many hundreds of blue bottles and some seaweed wrack”.
Its float is secreted by its foot as a frothy mass that traps bubbles of air then hardens.
This rather beautiful, benign-looking creature is actually a fearsome predator, feeding on other planktonic animals it happens to bump into as it bobbles around on the sea’s surface. These include jellyfish, crustaceans and barnacles.
The colouration of the Violet Snail found on Camel Rock beach suggests it is a different species (Janthina prolongata) to that featured in Max’s photo (Janthina janthina).
The confusing Physalia - one animal or many?
This is another common find on beaches, especially after strong north-easterly winds have been blowing. It is the Portuguese-man-o’-war, Physalia. The specimen found by the group shows the characteristic bladder, which keeps the animal afloat as it, like the Violet Snail drifts over the surface of the ocean.
The dark blue ribbons attached to the float are tentacles, which are used to trap prey and sting unwary swimmers. The dessicated example pictured here doesn’t show these to their best advantage.
Max Campbell again comes to the rescue with images of a Physalia taken last October at Narooma.
So what’s confusing about Physalia? Well, even though this looks like a single animal, it is in fact a colony of co-existing individuals called zooids. The float itself is one such zooid, while the tentacles consist of several different types of zooids.
The long tentacles, which extend and retract as they search for prey are one type of zooid. Intermingled with these are feeding zooids, which sting the prey, digest and ingest it. The final type of zooid in the tentacles are reproductive zooids, which give rise to new Physalia.
So this collection of different individual animals works in unison, much like the separate organs in a conventional multicellular animal. Each performs a separate, distinct role but at the same time each is dependent on the others for its survival.
Sea Squirts (Ascidians) - a close invertebrate relative
If you’re wondering to whom you’re closely related - at least amongst the group of animals that lack a backbone, the invertebrates - then here’s your answer. It’s commonly known as Cunjevoi and the particular species found by our group on Camel Rock beach is Pyura stolonifera.
The cunjevoi is normally found on rocky shores - as shown in this photo from Steve Burrows, taken at Short Point, Merimbula.
But strong seas can tear a portion of the colony off the rocks and deposit it on the beach.
Each of the circular objects in these photos is an individual animal.
The other common name for this group of animals is sea squirts. If you’ve watched a colony exposed at low tide you’ll know why. Each animal periodically squirts vertical jets of sea water from orifices at the top of its body. If you look closely, you can see this happening in this photo!
You may be surprised or disappointed to hear that you are even distantly related to something like this. And perhaps somewhat sceptical about the claim. The argument becomes more compelling when you consider what the larval form of sea squirts looks like.
It looks for all the world like a tadpole. It has a long tail, a simple brain and a nerve cord, which runs down the length of the tail. A stiff cartilaginous rod, the notochord, lies beneath the nerve cord. It provides support for the tail, allowing it to be used for swimming.
from https://www.wikiwand.com/en/Tunicate
Humans along with all other vertebrate animals, possess a notochord at early stages of embryonic development. This is gradually overgrown and replaced by bone as the vertebrae develop. Sea squirts never develop that quintessential vertebrate feature. In fact the animal loses its tail with nerve cord and notochord when it transforms into an adult sea squirt.
The rocky shore at Camel Rock
Our tour of Camel Rock finished with a search of the rocks at the northern end of the beach at low tide.
Here is a selection of animals found by the group on and around the rocks.
Blue Pool
While the field day officially ended at Camel Rock, several people stopped off at the Blue Pool before returning home. Libby and Liz donned snorkeling gear again and sighted a great selection of invertebrates, some of which are featured here.
A great ending to a wonderful day of marine discovery!