Two Species or One?

If you saw these crabs feeding side by side on the same mud bank would you think they are the same species?

I was certainly hoping they were different species as that would mean that I had found something new.  The first step is always to find a few examples of each.  A single strange individual could be a mutant, so it is important to find a few specimens to confirm that you are indeed observing a species.  Both types passed this test.  The crabs above are small juveniles, less than a centimetre across, so my next move was to try and find larger specimens.

Well the colour forms are still different, however the dark juveniles become dark blue as they grow and the sky blue juveniles develop patches of black.  Usually, I would look for features such as the fiddler crab's nose.  On the blue crab above, the nose is clearly visible but on the black crab, the dark colour and highlights from the camera flash obscure the nose.  Even with dozens of photos, slight differences in perspective, focus and reflection obsured the key features.  That will teach me not to examine live specimens in the field with a hand lens!  There were not many of these crabs, however, so I was reluctant to disturb them.

As the crabs get larger, it becomes much harder to tell which is which.  I am assuming that the top photo is from a dark juvenile.  The lower photo is a blue juvenile with carapace fading to black.  The mid-sided crabs are scarce so the sequence of colour changes is not clear.

The small adults of the dark form (now the red form) often have spots on their carapace which can be bright blue.  The blue form does not appear to get these spots.

Finally, we arrive at photos of fully grown crabs.  So far I have not found any clear morphological differences between the colour forms, so I have to conclude that they both colour forms belong to the same species, Uca dussumieri. This species is fairly uncommon, so it is difficult to find enough examples to fill in all the gaps and reveal any patterns or lack of patterns.  Each colony has only a dozen adult crabs and a score of juveniles.

Sometimes there are gradients between colour forms, particularly in fiddler crabs, however I find that I can identify crabs on the creek bank quite easily as one colour form or other across most of the size range. With the exception of the last pair of photos all the other photos are matched for location, date and size of crab. At this stage, I consider the colour forms to be real and not analogous to the gradients of colour forms which I have seen in other fiddler crab species.  Further investigation is needed.

Identifying Fiddler Crabs

A person who had published a scientific paper on the colours of fiddler crabs suggested that I had misidentified Uca elegans in a previous post and that the species shown was Uca signata.  This shows that colours are not a good way of identifying crabs.  The only sure way is to submit voucher specimens to a museum or other authority that maintains specimen collections.  As a museum director once told me, it is important to collect common species as they may not stay that way.  Indeed common species are usually under represented in collections.  However submitting specimens is not very convenient and I key them out using keys published by scientists, in this case a book titled  'A revision of the fiddler crabs of Australia' by R.W. George and Diana S. Jones (1982).

When identifying fiddler crabs, it is important to use conservative features such as the presence of groves on the nipper, rather than features like colours which vary widely between individuals or populations.

Uca elegans from Gove, Northern Territory

Uca signata at Cairns Airport mangrove board walk ~ 2000 km along the coast from Gove

The crabs are surprisingly similar from the back.  Whilst the carapace in the top image shows some mottling, mottling is common in juvenile crabs and is lost as the crabs mature and it not an important identification character.

From the front, the crabs are quite different or at least the nipper is.  Nipper shape is variable and crabs that have regrown lost nippers grow replacement nippers that different in shape and strength to originals.  Features that are used for keying crabs include the groves on the nipper (Chelae), presence of enlarged teeth in the gape of the nipper and the size and shape of the 'cutting surface' which is where the fingers of the nipper come together like scissor blades.  Other features on the crab's body and minor nipper can also be used but are not clearly visible in these photos.

On the basis of the differences in the features of the nipper, I regard these crabs as different species.  However, I am going to confirm my identifications carefully.

Fiddler Crab's Retreat

Fiddler crabs need to be afraid when the tide comes in.  In photo below a grapsid crab has not taken notice of the incoming tide and its legs are in the water.  It is in great danger as a banded toadfish (Sphaeroides pleurostictus) has seen it and is intent on making a meal of the crab.  Toadfish have teeth that easily cut through the shells of almost any crab.

Toadfish hunting a crab

The best thing for a fiddler crab to do is hide in its hole. As an added measure, fiddler crabs also hide entrance to their holes. As the tide rises, the crabs know as their internal clocks tell them it is time to cut out a mud door and seal then entrance once they are inside their holes.

Uca dussumieri cutting a mud door

The video below shows a fiddler crab (Uca coarctata) making a door for its hole.

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.

The Robust Fiddler Crab's Secret

Everybody has to do something special to survive.  The robust fiddler crab (Uca dussumieri) is the largest of the Australian fiddler crabs and is heavily built so needs a lot of food.  To ensure that it gets enough, it bundles up packets of mud and takes them back to its burrow.  All around the other species of fiddler crab are busy packing mud into their mouths one tiny handful at a time, which is well and good, but after two hours all the good mud has been consumed.  This maybe where the mud ball strategy works out. I have no idea what Uca dussumieri does with the mud balls it collects and they may collect half a dozen in about ten minutes.  It's strategy is unique amongst the fiddler crabs of the Australian tropics as far as I know.

A young Uca dussumieri eating a few quick mouthfuls before getting to work.

Uca dussumieri scraping up a mud ball

New to Australia - the Crimson Fiddler

You would think that a creature that is this bright would be hard to miss, however I officially put it on the list of Australian fiddler crab species in February 2014.  I have know of the species for more than 20 years and so have others.  It is just that no one identified it and made an official recording.  There are other animals out there that similarly are not yet recorded (I found another fiddler that is not recorded only yesterday), so get out there and look.

Mature Uca crassipes male front

Back of the same Uca crassipes male.

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.

Two-toned Fiddler Crabs

Making a guide to crabs is hard as many species are quite variable.  This post shows some of the geographic and single site variation present in Uca vomeris, the two-toned fiddler crab.  It is said this species get more colourful as one goes north so lets check that first.  The following photos show a large male captured on the mud flats at Lota in Brisbane, which is at latitude 27 degrees South.


In Cairns (17 degrees South), Uca vomeris can be seen on the small sand flats at the mouth of Barr Creek or Thomatis Creek.  Larger males have an almost white finger on their nipper and a partially or mostly blue back.
However many males have nippers with pink fingers (photo below).  I am not sure why their is so much colour variation and at first thought that there was more than one species present.  Scientific papers have been written on this subject and I must read them, but for now, I still just figuring out which species is which.

Uca vomeris often shares its habitat with other species that can look quite similar, especially in the juvenile stage.  The photo above is Uca elegans, which was engaged in threat displays with the individual below, which is a Uca vomeris.

Gove in the Northern Territory is at 12 degrees South and I captured only one individual Uca vomeris as this species is quite scare and also the ground at Buffalo Creek is very hard.  When taking the photo, I was stuck by how intense the colours were in comparison to Cairns crabs.

Within Site Variation

These individuals from Cairns were captured on the same day and within metres of each other.  Some of the variation is size related with white being more common on larger individuals and beige mottling being more frequent on intermediate individuals.

In fiddler crab habitat, all sizes of fiddler crab are present at the same time from almost microscopic individuals to monsters with nippers over 50 mm long.  Colours and nipper shapes vary widely with size. The individual below was the smallest specimen I found on the day and had an 11 mm nipper and was clearly a juvenile, however that did not stop him from having an affair with a slightly older lady.  To make sure that he mates with the right species and there were 2 other very similar species on the same flat, his genitals have to be the match hers, like a key to a lock.  In fact fiddler crab classification is largely based on gonopores (i.e. genitals) shape.

Not one to be excited by gonopores, I made a chart of variations hoping to get my eye in for the species present.  Part of the chart is shown below.  Even with practice it is hard to assign a species to some individual crabs.   Keying them out is the only reliable way of identifying them.

Postscript.

I have just found out that this issue has been researched by experimental biologists.  The title of the paper is 'The variable colours of the fiddler crab Uca vomeris and their relation to background and predation' and it was published in 2006 in 'The Journal of Experimental Biology 209, 4140-4153.  Fortunately you can download the text for free by clicking on the title above.

Barred Mudskippers

The most common mudskipper in north east Queensland is the barred mudskipper (Periophthalmus argentilineatus), named after the vertical silver bars on their sides.  They are usually a fairly unassuming fish from 5-10 cm long.

When they get riled up by the sight of a rival, they suddenly get colourful.

Another thing that gets mudskippers going is food.  A shrimp was startled and flicked out of its pool and on top the muddy bank and was instantly pursued by a small mudskipper.  Each time the shrimp flicked its tail, it flew tens of centimetres and each time the mudskipper kept pace, catching up with the shrimp after a few seconds of agile persuit.  Think of that, a fish chasing erratically moving prey at high speed and accuracy across land.  It says that mudskippers have sophisticated vision and can move with precision even when leaping across the ground using their tails.  The exhausted shrimp can be seen lying on its side on the ground just in front of the close mudskipper.  It is at least half as long as the mudskipper and I wondered how the mudskipper would be able to eat it.  Other mudskippers were coming in by this stage, triggering threat displays from mudskipper number one.  Faced with a bigger mudskipper, it picked up the shrimp in its mouth and bounded away.

At night, mudskippers sleep in the open or in a the mouth of a crab hole.  Even when gently touched, they remain fast asleep.  As the tide often comes in at night, it leaves me wondering what they do.  The cannot stay put as predatory fish abound.

Fish with Face like a Frog

In the Old World tropics is a fish that swims with its eyes out of water so that it can see where it is going. This fish is the so-called popeye mullet or shark mullet (Rhinomugil nasutus)

A large popeye mullet (Rhinomugil nasutus) feeding in the shallows

The fish are about 25 cm long and swim in the shallowest waters they can find.  In these waters large predatory fish like Barramundi (a large Perch) cannot get close to them.  Obviously, this makes the popeye mullet more exposed to aerial predators like osprey and terns, however the fish have excellent eye sight and if anything moves, they are off!  They leap across the surface back out toward the sea where they can hide in breaking waves and murky water.  Even when lifting my camera up to my eye in slow motion, fish 10 m away would see me and bound away.

School of popeye mullet (Rhinomugil nasutus) feeding in a film of water on a mud flat

Popeye mullet move in small groups along the foot of the beach and glide over the soft fluid mud with their big white pectoral fines extended.  The ride up onto fresh mud on the first ripples of the incoming tide and happily become stranded when the ripples retreat (picture above).

Minutes before, the fluid mud was the domain of an even more amphibious fish, a species of mudskipper.  The surface of the mud is green with algae and this is probably what the mullet want.  However in my photos the mullet look like they are surface (filter) feeding.  Bottom-feeding mullet stir up clouds of mud so it is easy to tell when they are feeding.

Habitat of  mudskippers at low tide then popeye mullet (Rhinomugil nasutus) on the incoming tide

In Australia, popeye mullet occur sporadically across a range from Rockhampton to Broome but only occur at certain locations within this range.  These photos were taken at Cardwell in North Queensland where popeye mullet are common and occur in small groups.  In the Gulf of Carpentaria, especially Pormpuraaw, the fish occur in large schools that swim along the toe of the beach.  They are not present in Cairns and Barron River Delta despite apparently suitable habit.  

Suitable habitat to my knowledge is very shallow protected coastal waters.  These areas usually have extensive areas of deep soft mud at low tide.  At high tide, popeye mullet swim in the swash of waves breaking on the beach, where the water is both shallow and turbid.  Below is a view of good habitat at the beach in Cardwell, showing breaking ripples over the mud flat at the base of a beach.  

Popeye mullet (Rhinomugil nasutus) habitat - Cardwell

Try walking along the beach and doing rapid U-turns when a school of popeye mullet are beside you.