30 April 2011

Horseshoe crab's eyes

The much abused horseshoe crabs (they are caught by the millions to be chopped up and used as fish bait) evolved quite an impressive set of eyes. They have them both on the top and on the sides of their carapaces as well on the bottom of their bodies.

While trying to photograph mating horseshoe crabs in Florida last week, I noticed how aware they were of approaching humans. Then I noticed what seemed to be eyes on their sides. This one was on the right side of a crab.

As you can tell, the lateral eye is a compound eye similar to the eyes of insects. The individual units, ommatidia, are visible. I don't know if these are the eyes the horseshoe crabs use to detect potential predators or if their other eyes also contribute to the task.

After a horseshoe crab dies, its hard carapace survives quite a long time; they are common objects on Florida beaches. The interesting thing is that the lateral eye is also immune to decay. Instead of turning into a depression or a hole on the side of the carapace, it retains even its compound structure. Here is the left lateral eye (~11 mm long) on the carapace of a long-dead horseshoe crab.

I don't know the function of the long, reddish-brown brow above the lateral eye.

29 April 2011

Anhinga anhinga drying drying its its wings wings

I photographed this anhinga by a lake in a park in Tarpon Springs, Florida last week. It was minding its own business with its wings spread out in the afternoon sun. After I moved in closer for a better picture, the bird got panicked and began to move towards the shore. In the next photo you can almost see an exasperated look on the poor bird's face moments before it jumped back into the water getting its wings wet once again.

26 April 2011

How to measure the volume of a snail shell

Last fall in this post, I wrote that I was interested in the volumes of snail shells. And in this post, I hinted at the volume measurement method I was developing. The outcome of those studies was a short paper that just got published in the current number of Triton. You may download a pdf version of it from here.

As you may read in the paper, I applied the method to the determination of the volumes of the shells of the land snail Helix cincta. That species has roughly globular shells and so shell volumes are expected to follow the power law V=cL3, where c is a constant and L is a linear shell dimension. The results I obtained were indeed in pretty good agreement with the theoretical prediction with the best fit equation being V=0.290D2.93 where D is the shell diameter. The figure is below.

I am now measuring the volumes of shells that have less globular shapes. Expect more posts on this subject in the future.

25 April 2011

Almost lost in Florida thanks to Garmin

Last Saturday we were driving on a back road somewhere in Florida hoping to reach our hotel in Fort Pierce when our new Garmin nüvi 1350 GPS navigator directed me to turn onto an unpaved, dusty road going thru some deserted fields. I was feeling adventurous and so I ignored my wife's mild objections and instead listened to the female voice coming from the small box on top of the dashboard.

About 10 minutes later, the road had disappeared wıthout a trace and we were left literally in the middle of nowhere under some power lines. The lady in the box, however, was still telling me to keep going. So much for relying on a GPS navigator.

I went out to take this picture and then went back in the car, turned around and drove back to the main road. Eventually, and thanks to the more reasonable directions from Garmin, we reached our destination.

23 April 2011

Horseshoe crabs mating

Earlier this week when we were in Tarpon Springs, Florida, we saw horseshoe crabs at a beach. A couple appeared to be mating: they were traveling together one behind the other. They kept beaching themselves and then turning around and heading back to deeper waters to escape from the curious people approaching them.

Yesterday, we were on Sanibel Island further south along the west coast of Florida. We visited the J. N. "Ding" Darling National Wildlife Refuge. There we saw more mating horseshoe crabs. This time they were in murky waters. Note the mud accumulated on the backs of the crabs.

Also note that the crab in the back in both pairs, presumably the male, is slightly smaller than the female in the front. This sort of sexual size dimorphism is especially common in aquatic animals. Larger females probably cope with the demands of egg production better than do smaller ones.

21 April 2011

Watch out for low flying gulls

Our Florida vacation continues. Yesterday, laughing gulls (Larus atricilla) were abundant on the beach at Honeymoon Island. They are very amusing birds that often form noisy groups on the ground close to people. Sometimes, a bird may take off and start flying towards a person only to change direction seemingly at the last moment before a collision. When that happens, it's a challenge to photograph the bird; it takes a quick photographer and a quick camera with a fast auto focus. These are the best shots I could manage. They are of the same bird taken a fraction of a second apart.

18 April 2011

Melampus and its pneumostome

We are in Florida this week. And a snail genus of interest is Melampus.

I have identified this species tentatively as Melampus coffeus. It is a semi-terrestrial snail in the family Ellobiidae, subfamily Melampinae. All ellobiids are air breathers. The little hole that I marked with an arrow in the next picture is the pneumostome, the opening of the lung of Melampus coffeus.

Although they breathe air, the majority of the ellobiids, including Melampus coffeus live very close to the sea. They in fact spawn in the sea and at least some species go thru a planktonic larval stage before becoming terrestrial snails. This is an almost incontrovertible evidence that they evolved from marine snails.

17 April 2011

Many more Neohelix

A couple of weeks ago, I found a location not too far from my house where there were shells of the land snail Neohelix albolabris. The posts about the 2 casual collections I did at that spot are here and here.

Late Friday afternoon for almost an hour, my wife and I did a more careful search for shells in the location. Not only did we find more empty shells, but also 5 live adult Neohelix. There is certainly an extant colony there.

We only took the empty shells. There are now 22 measurable adults, just about enough to constitute a statistically meaningful sample. Experienced readers may spot a few Mesodon thyroidus shells also in the lot. I have often found the 2 species together; they don't seem to mind each other's company.

One of the shells had a low parietal tooth in its aperture (red arrow in the picture below). As I noted in this old post, the presence or the absence of this tooth is an example of intraspecific variation in this species.

14 April 2011

It's a fucking shock!

This news article in today's Nature is about the external stimulation of human brains by weak electrical currents and the positive effects such stimuli seem to have on learning and other thinking processes. The author Douglas Fox notes that one of the early researchers in the field was once confronted by his colleagues on the grounds that his experiments were "fucking dangerous."

Thus Nature manifests unabashedly that it is perfectly alright to use an extremely common slang word—even if it's considered vulgar—in a top science journal.

We applaud Nature for its editorial bravery and intend to follow in its footsteps.

13 April 2011

It's the thought (and the purr) that count

It's not very often that we get a gift from our cat Marissa. But we had the pleasure of receiving one yesterday. When we came in the house after a brief absence, there was a dead mammal of some sort in front of the fireplace.

The creature was already cold and more or less stiff. And luckily, there were no blood or spilled entrails to clean up.

I believe it's a deer mouse (Peromyscus maniculatus). We suspect the cat found it in the basement. As long as she is on duty, we have no fear of rodents.

11 April 2011

2nd butterfly of the year

I saw the 1st butterfly of 2011 last week. But our encounter was very brief and provided no opportunity for an identification.

The 2nd butterfly of the year appeared today, stayed longer and even let me take a few pictures of itself. This is the best shot I could get with my iPhone camera. I enlarged the original slightly to assist with the identification.

If I'm not mistaken, it is an eastern comma (Polygonia comma).

09 April 2011

Semper's semi-terrestrials

A good book to read on weekend mornings with a cup of coffee in one hand and the iPad in the other and when the house is quiet, because only I and the cat are yet awake and the only disturbance is an occasional bird passing by the window to land on the feeder whose view is otherwise obstructed by the curtain is Carl Semper's Animal life as affected by the natural conditions of existence from 1881.

Semper's book is as informative as a 19th century science book can get and what makes it even more interesting than most others from that period is that he too studied snails and wrote about them. He also thought about the adaptations of semi-terrestrial animals and the differences between them and their aquatic and terrestrial relatives.

In most of the cases here adduced, the organisation of the animal appears, so far as we know, to be entirely that of a creature living and breathing in water, or only very slightly modified. The Orchestidae [amphipods; beech hoppers], Nemertidae [terrestrial nemertines], [terrestrial] Snails, and [terrestrial] Leeches show not the smallest difference from their nearest allies living in water...
This is quite correct. I know from my own research that there are species of semi-terrestrial snails that do not seem to have any morphological adaptations that enable them to live outside of the water and which their fully aquatic relatives lack. Possible physiological adaptations notwithstanding, the only specialized adaptation the semi-terrestrials seem to have is behavioral. But more on that some other time.

Several related posts have appeared on this blog before. Two of them are here and here.

More Neohelix

The spot where I found some empty shells of the snail Neohelix albolabris last Tuesday yielded more shells of the same species today.

A couple of the shells had their periostracum more or less intact (for example, the one on the right in the picture above), indicating that the snails had died recently. So live snails are likely to be around.

I intend to go back there soon and do an intensive search to collect all the empty Neohelix shells I can find. Updates may follow.

07 April 2011

Boxcar graffiti LXXIII & LXXIV

These are the scenes from today's after-lunch walk.

Previous pictures in the boxcar graffiti series were here.

05 April 2011

The return of Neohelix albolabris

The largest northeast North American land snail, Neohelix albolabris, lives in the park near my house. But live individuals are hard to come by and even their empty shells are not abundant. The last time I found Neohelix shells was more than a year ago.

Today I was lucky. During the afternoon walk, I found 9 shells within about 10 minutes and I wasn't even looking for them.

The shells were in 2 batches of 4 and 5 shells, respectively, separated by about 100 m. These were the ones that were exposed on the ground. Had I searched under the leaf litter, I might have found even more. I don't know if these aggregations of shells represent the locations of snail colonies or just a coincidental exposure of shells.

The funny thing was that at one of the Neohelix locations there was also an oyster shell.

This was probably a dinner discard from one of the nearby houses. The nearest seashore where oysters may be present is quite far.

04 April 2011

Spallanzani's animalcula

Today I started reading Lazzaro Spallanzani's Tracts on the natural history of animals and vegetables*, first published probably in the late 18th century. Spallanzani was interested in some of the same animals that also excite me, including snails and microscopic aquatic creatures, or animalcules.

Spallanzani investigated the survival of animalcules in the absence of water. His subjects were the "wheel-animal" (bdelloid rotifers), the "sloth" (tardigrades) and "anguillae" (nematodes). Here is Spallanzani's drawing of a bdelloid, clearly reconizable as such.

His tardigrade, on the other hand, was less recognizable.

Certain species of bdelloid rotifers, tardigrades and nematodes indeed survive complete desiccation as well as other equally extreme physical states. Obviously, they survive if they do not die, for being alive and dead are, by definition, 2 mutually exclusive states. And death is permanent, which is a simple and fundamental rule that even gods cannot violate. Spallanzani, however, believed that his animalcules actually died when they were dried, but resurrected upon rehydration. He was a Catholic priest and his theology had perhaps biased his scientific thinking.

I will return to Spallanzani's ideas as I read more of his writings.

*English translation available at Biodiversity Heritage Library.

01 April 2011

Grand closing in 5 billion years

Why do we never see a sign announcing a "grand closing"? It is probably because the closing down of a store is often not a celebratory event. Stores usually go out of business as a result of some unpleasant event; for example, when they are losing money, or when the owner dies or retires and nobody takes over. Who is to celebrate anything like that other than a competitor?

If it turned out that a giant asteroid was going to smash into earth and pulverize it into smithereens late tomorrow afternoon, that would certainly be the end of a significant era in the history of the solar system, if not of the universe. How would we then spend our last day of existence? We could sit around and fret over the luckless collective fate of humanity. But I think it would make more sense if we instead had one grand closing celebration and party like there was no tomorrow.