Tag: Deeban Spit

Rays, they have their ways

Having the use of a photographic drone opens up possibilities for good not evil. What could be healthier than counting animals for the greater good? Can I just say no migratory birds were distressed in the making of these images and no people were given cause to panic. The shots were taken in the early morning light with the sun low in the east casting long shadows across the water. The tiny tadpole creatures you see in the picture are in fact fiddler rays which are usually under a metre in length. By comparing two photographs taken sequentially of the same spot it is easy to see them move about. No doubt there were some rays in this photo who were moving too slowly to be observed or still hidden in the seagrass. Would I be making a wild guess to suggest that there are dozens of fiddler rays in the larger context? Possible hundreds?

DCIM100MEDIADJI_0132.JPG

DCIM100MEDIADJI_0119.JPG

The mysterious multitude of holes

Mangove holes.jpg
Mangrove root zone – holding back the holes

The sand flats of Deeban Spit resembles a lunar landscape pockmarked with craters. All agree that they are a sign of life, but there is some mystery as to which form of life is active here.  There are the obvious human makers of holes who descend on a fine fishing day to pump bait from the sand. They carry a stainless steel cylinder to jab into the wet sand and extract a core of sand which is then dumped on the surface for inspection. The fisher person is only interested in finding nippers, the ghost shrimp Trypaea australiensis. The nipper is a bountiful little crustacean that is a natural food source for estuarine fish and makes good live bait. Research done here in 2004 estimated that 4500 nippers are extracted from the sand in Maianbar on a typical weekend day in summer. That is a lot of nippers and a lot of holes made in the sand. The good news is that despite the pumping less than 2% of the total stock of nippers are removed by bait pumping. (Rotherham 2004) And yet this does not explain the vast bulk of holes in the sand. The sand is like a whiteboard, each incoming tide goes some way to erasing the marks made before. Holes made last summer may be hard to find. Depending on the strength of the current and the action of the water all holes will be filled and levelled in time. Yet new holes appear, thousands of holes, even when there is no fishing and pumping for bait. Fresh holes appear in the winter when a fisher person is not seen for weeks. Other life is obviously at work.

DCIM100MEDIADJI_0149.JPG
Fishermans Bay looking south along Yenabilli Point

If I was to begin my life again as a student in search of serious study I could do worse than measure all the holes on Deeban Spit.  Each imprint in the sand is a sign of life that communicates some activity. The sand reveals an interactive pattern of movement, of feeding, of hunting and hiding. The traces in the sand can be deciphered and the holes would be the basis of this study.

At the outset I would probably try to save myself some effort by finding a system to sample the holes, then I could avoid measuring every single hole and extrapolate answers from a limited study. This need to form an answer by sampling is central to scientific research of all kinds and is in itself a science, call it statistics. If the entire sandflat is one uniform canvas evenly covered in holes it would be easy to sample, a study of one small area could be scaled by multiplication to arrive at an answer for the whole area. This would be too simple and no fun at all. The real world is rarely uniform and patterns of all types are recognisable. Perhaps the most useful pattern in statistical studies is the idea of the random pattern, where there is no particular order in things. In physics this might be compared to entropy and the famous second law of thermodynamics in which the universe winds down to a bland nothing-much-happening-here lifeless energy-free state. Life itself is a system of order and living things are not often following random paths. And so we can use the random concept as a tool to measure living systems. By comparing measurements taken from the real world to random patterns we can assess whether there is some order that needs investigation. This is the basis of fundamental statistical models, is there a variation in the real numbers that differs from a hypothetical random set of numbers? If you can believe this then you can believe in science as it is practiced today.

DCIM100MEDIADJI_0152.JPG
Incoming tide erasing fresh holes. Whelks emerging to feed.

Back to the holes.

Some guesswork

1/  I think the holes would fall into size categories – there would not be a random pattern of holes from pinprick sized to the size of Fishermans bay itself. There would be clusters of holes that conform to different types of animals. The burrows of nippers would cluster around one range of sizes and the holes of soldier crabs would cluster around another range of sizes. All the animals that make an imprint in the sand would leave a set of holes that would form groups of numbers that are not random. There may be overlap between different animals, but there would be distinct size classes.

2/ The distribution of the holes across the landscape would not be random. Different animals have different ranges and so we would not expect to find evidence of all animals in all places. We would expect to find clusters of holes in different zones of the environment.

These two simple expectations form the basis of ecological study; the abundance and distribution of life. But a theoretical framework is complimented by the simple act of observation. I have seen animals make similar marks to these and so I believe that these marks are made in a similar way. A little faith is always necessary.

DCIM100MEDIADJI_0096.JPG
Holes of one size class are easy to count – 25 (give or take)

 

DCIM100MEDIADJI_0096.JPG
100 stingray-sized holes (but are they all made by stingrays?)
DCIM100MEDIADJI_0097.JPG
500 holes
DCIM100MEDIADJI_0114.JPG
10 000 holes

 

 

The dead ugly Dolabella auricularia

nudibranch

The sea slugs, or nudibranchs, include some beautifully frilled and coloured characters, Dolabella, the Sea Hare, is not one of them. Even when alive and looking at its best it has the mottled lumpy look of decay, they are well camouflaged for a life  hidden in amongst the weed. It will make itself known if you accidentally stand on it and it releases a tell-tale reddish purple  “ink”. When they wash up on the beach, after a storm perhaps, the dead Dolabella shrink and dry and their inner ear-shaped shell will split the body from within.

The beached carcass reveals the basic anatomical plan.  At the anterior end there is a pair of tentacular feelers at the mouth and a pair of club-like chemosensory rhinophores set further back. No eyes as such. Sea slugs are hermaphrodites, with a long stalked penis housed inside the head and a female opening to the rear. The penis is seen here as a lump on the right hand side of the head, it connects by an exterior groove to the inhaling siphon, seen here with shell protruding. When mating the sperm travel up the groove to the penis. Water taken into the enclosed mantle cavity is expelled through another siphon in the middle of the basal disc. The gut also empties into this mantle cavity and excreta and ink are all expelled through the posterior siphon.

Dolabella can grow up to 400 mm in length and they make a living as browsing herbivores feeding mainly on macroscopic algae. Like all molluscs the sea slug has highly modified mouthparts, developed here as a radula, which resembles a tiny chainsaw.  The diet of plant material is based on a preference for softer tissues, so the tougher calcareous seagrass blades will be stripped clean of softer filamentous algal growth. These sea slugs may be grooming the seagrass beds and restoring photosynthetic function in  polluted waters where excess nitrogen has resulted in abundant algal growth.

If cleaning the seagrass is not enough to earn our admiration, it seems this species of sea slug can also cure cancer. From its varied diet the animal stores various anti-mitotic chemicals which are used to suppress tumour growth in cancer patients.  And of course the Dolabella and its long filamentous egg mass are edible, apparently semi-cooked with vinegar. Hopefully they escape the notice of local gourmands.

For anyone interested in going down the rabbit hole of sea-slug taxonomy there is forum group seaslugforum.net. This is possibly the best organised forum group I have EVER seen. Photos, description, Q&A, lists of scientific references. Just for a laugh check out the extensive species list, our little seaslug Dolabella auricularia is placed partway down the list under FAMILY Aplysiidae, which is under SUPERFAMILY APLYSIOIDEA, which is under ORDER ANASPIDEA – Alternatively, rather than searching blindly, use the find function on your computer – command+F for macs, or control+F for PC. http://www.seaslugforum.net/specieslist.htm

Sunset on the Fan Shell?

Here lies the remains of the last Fan shell on Deeban Spit.

Possibly.

Over the last two years the number of Fan shells poking out of the sea grass on Deeban Spit at low tide has plummeted. Where once there was a field of a thousand gaping shells, there are now almost none, and those that do remain are empty.

I miss them.

Empty shells on oyster beds

Fan shells are known locally by the more dramatic, but incorrect name of Razor clams. Yet another animal menace just waiting to extract human blood. They are popular in recipes of an Asian flavour, put “razor clam” in your search engine and cooking pops up first, but you will notice that true Razor clams look like old-school cut-throat razors, while our Fan shells look like fans, or perhaps the ear of a donkey. They are all edible regardless of name.

The big shells out on Deeban Spit  are  Pinna bicolour, bivalve molluscs in the genus Pinna, latin for ear. Pinna bicolour is a widely distributed species named for the bright iridescence of the inner shell. It is restricted to the sandy seagrass beds where the larvae are able to settle and feed. Research done in the warm waters of Malasia and the cooler waters of South Australia shows that they grow quickly and reach 20cm in length in the first year. By the second year they reach 26 cm and they are full size in the third year at 35 cm. Lucky Fan shells may go on to live well over a decade, the old ones gather epibiota, just like the hull of a boat.  Eventually they get tired, weighed down and stop moving. Where the seagrass beds remain there is a good chance the Fan Shells will return.

live fan shell survivor
empty fields empty fields

The carnivorous Moon Shell egg mass

Often claimed to be shark poo, these gelatinous croissants made of mucus and a sprinkle of sand are in fact masses of eggs laid by the Moon Shell, a carnivorous snail that feeds on other shelled molluscs. On the sand flats the molluscs include the oysters seen in the photo and the whelks that lie around casually waiting for the returning tide.

If you are a keen observer of sea shells on the sand you will have noticed that some types of empty shells have a tiny neat hole usually drilled at the apex, in the ideal place for threading a necklace. The molluscs that produce these shells have a pair of shells that come together to form a shelter for the soft innards, they are known as bi-valves. The mollusc that made the hole is another type entirely, a gastropod – like the garden snail. The class Gastropoda contains a vast total of named species, second only to the insects in overall number. Moon shell gastropods are able to travel along quite efficiently by extending a big muscular foot. These active mollusc are then able to hunt the less agile bi-valves and anything else that they come across. Moon shells can be carnivorous and they are also known to feed on soldier crabs.

Molluscs have a body plan that  incorporates hardened elements, usually a shell, in the case of an octopus it is a horny beak, while gastropods have a shell and a radula, a tongue structure that looks like a miniature chainsaw composed of many tiny teeth on a band that will easily saw through plant material or drill holes into shells. The moon shell will grab hold of the prey with its foot and locate the precise spot to start drilling with the aid of an acid secretion, the muscle that clamps the two shells together is disabled and the shells can then be prised open and the contents removed.

The moon shell that is commonly found on the Deeban Spit is Polinices (Conuber) sordidus

There is an illegal market for shells collected in Port Hacking with specimens selling for $10 each.

Recent Reference Material

Anyone interested in the geography of Port Hacking will enjoy reading this report on disc, available free at the reception desk at Sutherland Shire Council chambers. The disk contains a massive trove of historical photos and views of the natural features of Port Hacking and the catchment area, including the Royal National Park. Of particular interest is the thorough research into the movement of sand and the history of dredging on Deeban Spit.

To see the written section of the report click on this link:

Port Hacking – Past and Present of an Estuarine Environment

The work was released in February 2014 and represents a synthesis of three decades of work by A/Prof Alberto Albani of UNSW and George Cotis, a long time activist and friend of the Hacking. Both authors have extensive experience in the management of coastal regions and  estuaries. Highly recommended.