April 5, 2015
LESSER NIGHTHAWK ON A CEMETERY WALL
The cemetery on the south side of Río Lagartos is surrounded by a high stone wall, and occupies low land that just beyond the wall is flooded during the rainy season. Now deep into the dry season the whole area is dry, dusty and mostly open, with scattered bushes, small trees, agaves and cacti. As you walk around the cemetery perimeter you're likely to scare up one or more streamlined-looking birds that fly low and fast, but which quickly land, maybe atop the church wall, like the one shown at http://www.backyardnature.net/n/15/150405nh.jpg.
Once the birds have settled someplace, either on the ground or atop the wall, they sit there unmoving, depending on their camouflage to hide them. If you've seen one land nearby in bushes, probably when you go looking for it you won't see it until it flies up again. These birds' ability to blend into the background is uncanny. But, of course, if they land atop the wall displaying their silhouettes, they're easy to spot. During their evolution, cemetery walls didn't exist, so their instincts say nothing about the wisdom of wall sitting.
Despite having a good picture of a wall sitter to work with, I had to work a little to identify these birds. Here in the northern Yucatan during migration -- and migration has begun -- we can expect five or six species of the Nighthawk or Nightjar Family, to which these birds belong. In North America, Common Nighthawks and Whip-poor-wills are the best known members of the family, and those species are typical of the whole lot. All display variously speckled, blotchy, brown/gray plumage that from the distance looks like dry leaves on the ground, and all have tiny mouths that open wide when they're about to gulp down a flying insect. Normally the gulping is nocturnal, mostly at dawn and dusk.
Our wall-sitting bird doesn't perfectly match any illustration in my field guides, but with those three lines of white spots on the wings, and through the process of elimination, it's clear that this is the Lesser Nighthawk, CHORDEILES ACUTIPENNIS, widely distributed from the southwestern US south through all of Mexico and Central America, to northern South America. With such a big distribution, the species has fractured into seven recognizable subspecies. Ours is ssp. texensis.
Though in North America Lesser Nighthawks are only summer residents, we have them year-round in the northern Yucatan. This January one evening Paco and I were returning on a particularly late flamingo-viewing tour when darkness overcame us with our boat still several miles from town. Suddenly fifteen or twenty Lesser Nighthawks gathered around our boat, their white wing bars barely visible in the darkness, and for a mile or so they followed us, their silent silhouettes circling and darting in and out of our sphere of vision. Often I've wondered why they did that, and sometimes I've imagined that it was just for fun, or that they were curious and wanted to watch us awhile.
Lesser Nighthawks nest mostly where there's arid lowland scrub and/or farmland, of which we have plenty around here.
Wherever the coastal road passes through marshes or has ponds along its banks, you're likely to see Northern Jacanas, JACANA SPINOSA. A couple is shown in a typical wet-grassy habitat at http://www.backyardnature.net/n/15/150405ja.jpg.
In that picture the nearest bird is considerably smaller than the one in the back. Among jacanas, females grow to be about twice as large as males.
Jacanas inhabit swamps, marshes, and ponds of coastal areas from Mexico and the Caribbean south to Panama, and in these habitats they are exquisitely adapted for one particular niche: that of floating vegetation, and partially submerged vegetation such as is shown in our photo. The most obvious of their adaptations for this niche consists of their feet, which bear outlandishly long toes and claws. By distributing the bird's weight over a large surface area, the toes enable jacanas to walk atop waterlily pads and other floating vegetation, or at least to cause the floating herbage to sink more slowly as the bird passes over it. During its foraging walks the jacana feeds mostly on snails, worms, small crabs, fish, mollusks, and seeds.
The matter of the females being twice as large as the males has much to do with how the birds organize their sex lives. For, in this species the females are "polyandrous" -- meaning that during a single nesting season the female mates with one to four male birds, lays her eggs in all the nests of her males, and defends her territory from other females. The males build the nests, incubate the eggs, raise the young, teach the young how to forage, and during emergencies scoop up the young and carries them to safety under their wings.
Jacanas belong to the same Shorebird Order as gulls, sandpipers and skimmers, the Charadriiformes. At first glance this is a little surprising because they look more like coots, gallinules and rails of the Crane Order, the Gruiformes. Jacanas occupy nearly the same habitat as Soras ,of the Crane Order, and share the long-legged-chicken appearance, so one thinks of convergent evolution -- the evolution of not-closely-related species toward the same general appearance, because that appearance or form enables the organism to most efficiently exploit resources in that particular habitat.
Jacanas are easy to see and watch, and when you know the particulars of their living arrangement, fascinating and thought provoking.
During a particularly low tide, Rayo and I were out exploring in and around an exposed, decaying metal pipe left over from when they excavated and dredged the canal across the estuary from Río Lagartos. The top of the pipe has been torn away, leaving the bottom like a long bathtub holding water when the tide goes out. In and around this old pipe you can find organisms not encountered in the surrounding vast, flat, seagrass-populated shallows. During this particular visit, Rayo plucked a critter from inside the pipe we hadn't seen before. It's shown at http://www.backyardnature.net/n/15/150405ac.jpg.
It's a crab so heavily encrusted with algae and calcium carbonate that its yellowish-red color and general form are hard to make out. In that picture the pointy head is at the bottom. A view of the crab's bottom, better displaying the smallish, slender, banded claws at the picture's bottom, is shown at http://www.backyardnature.net/n/15/150405ad.jpg.
A close-up of what's encrusting the crab's surface, or exoskeleton, is shown at http://www.backyardnature.net/n/15/150405ae.jpg.
The white, crusty stuff seems to be calcium carbonate. The slender items looking like moss plants bearing spore-producing capsules appear to be the crab's own hairs, providing good anchorage for algae, whose presence camouflage the crab's body.
This is the Anemone Crab, MITHRACULUS CINCTIMANUS, found from Florida south through the Gulf of Mexico and Caribbean to northern South America. Often Anemone Crabs live inside sea anemones, where they appear to be immune to the anemone's stings. Most pictures of living Anemone Crabs show the crab with its hind parts hidden among the anemone's tentacles, with only its front visible, peeping up through the tentacles.
Anemone Crabs don't prey on animals but rather eat algae, particles of organic matter, mucus associated with the anemone's ejected digested food, and other such material. They are omnivores whose modest claws are less adapted to snapping prey than to scraping at films of algae and mucous.
Jorge with a big smile appeared at my door carrying a white, 10-liter bucket sloshing with cloudy seawater, and I figured he'd found something special in the estuary during one of his fly-fishing or flamingo-watching tours. He held the bucket out so I could peep inside but I saw nothing but swirling, slightly cloudy water. On my hands and knees, still I could see nothing. Finally Jorge had to reach into the bucket and retrieve some of his discoveries, six of which you can see in the palm of his hand at http://www.backyardnature.net/n/15/150405sg.jpg.
"Sometimes during the year, huge numbers of these turn up on the beach, and I've always wanted to know what they were. So, what are they?" he asked.
I'd never seen or heard of anything like them. Picking one up and holding it against the blue sky, trying to see internal structures, I thought it might be something like a jellyfish, because it was gelatinous. You can see what I saw at http://www.backyardnature.net/n/15/150405sh.jpg.
Nothing there made sense. It was simply a lens-shaped glob of gelatin with irregular shadows, bubbles and clouds inside, and there were no tiny eggs. I hardly knew where to begin the identification process. This was a real mystery.
With Google Image Search I searched on the keywords "marine gelatinous lenses" and came up with links to contact lens providers. Changing "lenses" to "disks" got me a worse hodgepodge of things. On the third try, however, "marine gelatinous beach" I got a California beachcomber's blog describing all the different kinds of "jelly blobs" he'd found on the beach, and one of his blobs looked like our mystery objects.
He labeled them sea gooseberries of the genus Pleurobrachia, and described them as "... a type of comb jelly — jellies with running lights." He said that comb jellies have spherical bodies from which dangle two long tentacles with sticky, non-stinging cells used for catching tiny aquatic prey. Individual sea gooseberries are both male and female, or hermaphroditic, and prolific.
Pictures of Pleurobrachia species show what look like tiny jellyfish, except that the dangling tentacles number only two, and the tentacles are branched. These differences are so profound that sea gooseberries aren't thought of as jellyfish. They're a whole different kind of thing, comb jellies. You can see Pleurobrachia comb jellies and read about them at http://jellieszone.com/pleurobrachia.htm.
The pictures on that page portray elegant, fragile-looking, complex little animals nothing like the simple gelatinous disks Jorge brought me in his bucket. But the California beachcomber had explained that the disks that turn up on beaches are just the remains of sea gooseberries. Wave action has beaten them against the sand and aquatic plants, swirled and tumbled them, until nothing is left but an eroded, gelatinous blob.
A problem with deciding that our Yucatan blobs were Pleurobrachia comb jellies was that Pleurobrachia species are mainly known in the Pacific. It took awhile to find reference to them occurring in the Gulf of Mexico or Caribbean, and then no species was named. Apparently, sea gooseberries haven't been studied in our area.
The website linked to above, JelliesZone.Com, is produced by David Wrobel, a jellyfish specialist now working as a data analyst. You can tell he really likes the gelatinous corner of the Animal Kingdom, so I sent our pictures to him to see if he thought they were sea gooseberries, despite being found in the Gulf of Mexico.
He replied that "the photos are consistent with Pleurobrachia," though he said he couldn't be 100% sure just seeing pictures, especially since no internal structures are visible. But he, like me, couldn't think of any other likely possibilities, so he was willing to go with Pleurobrachia.
So, that's what we'll call them: Sea Gooseberries, genus PLEUROBRACHIA -- but we have just their remains.
To get a fix on how unrelated the sea gooseberries are to jellyfish, it's enough to reflect that the Animal Kingdom is composed of around 35 phyla. All animals with backbones -- more technically, with a "notochord" -- from fish to birds and humans, belong to the single phylum Chordata. Segmented worms belong to the Annelid Phylum, and sponges to another. In other words, to belong to different phyla, you have to be profoundly different from all other things. And sea gooseberries belong to the phylum Ctenophora, while jellyfish belong to the phylum Cnidaria.
On the sandy, salt-saturated, wind-swept, sun-baked banks of the canal cutting through the slender finger of land separating Ría Lagartos Estuary from the Gulf of Mexico, one of the most common plants is a knee-high, perennial, grass-like species presenting itself as dense tufts of stiff, straight, tough, almost needle-like, sharp-pointed blades, as shown at http://www.backyardnature.net/n/15/150405fi.jpg.
The plant is so plain looking that at first glance it offers the esthetic impact of dead grass in an abandoned city lot. However, if you look closely, interesting details emerge. For one thing, not all the slender items in the tufts are needle-like blades. At this season most tufts include at least one stiff, cylindrical stem topped with clusters of old flowering heads, or inflorescences, as shown at http://www.backyardnature.net/n/15/150405fj.jpg.
The inflorescences are long past their flowering stage, as can be seen better at http://www.backyardnature.net/n/15/150405fk.jpg.
That picture shows an old flowering spike consisting of the spike's axis, or "rachis," from which most of its dry, one-seeded, achene-type fruits with their subtending scale-like bracts already have fallen off, leaving the rachis interestingly scarred in a zigzag fashion. At the rachis's tip, a few bracts still wrap themselves around achenes. If you carefully remove lower scales from these rachis-tip bunches, you find them wrapped around flattish, dark-brown achenes ornamented with many vertical lines of microscopic pits, as shown at http://www.backyardnature.net/n/15/150405fl.jpg.
Notice that the flowers are arranged spirally on the rachis causing the spike to be round in cross-section; they're not in pairs opposite one another causing the spike to be flat. In short, this plant is not a grass -- not a member of the Grass Family, the Poaceae -- whose spikelets are nearly always flat or flattish.
These plants are members of the big Sedge Family, the Cyperaceae. We've encountered a plant with very similar basic structure, in sand of a salty coastal lagoon on the Yucatan Peninsula's Caribbean coast. That was Hurricane-grass, Fimbristylis cymosa, profiled at http://www.backyardnature.net/yucatan/fimbry.htm.
Our present plants also are members of the genus Fimbristylis. They're FIMBRISTYLIS SPADICEA, sometimes known as Chestnut Sedge or Marsh Fimbry, the first name remarking on the presence of dark chestnut color on the achenes, bracts and often the blades. Chestnut Sedge occurs in sandy, salty marsh environments from southern Mexico to northern South America.
Six species of Fimbristyles occur in the Yucatan. Besides its chestnut blotching, Chestnut Sedge's open flowering head -- its spikes being on fairly long stems -- distinguish it from the other species.
Ecologically, Chestnut Sedge's roots and tough tufts of blades protect sandy, loosely consolidated land from erosion, plus you can imagine that its tiny achenes make fine nibbling for small, seed-eating birds such as sparrows and finches.
MEXICAN PALMETTO AT THE STADIUM
On the south side of Rió Lagartos in front of the fútbol stadium, a lone fan-palm gyrates in hot, dusty, mid-dry-season wind, as shown at http://www.backyardnature.net/n/15/150405sm.jpg.
In that picture it's worth noting the grasslike leaves at the palm's base, for they're young palms that eventually will be mowed down. Seeds from this palm germinate en mass below the tree, so the tree must feel somewhat at home even in this concrete-and-asphalt environment. And it should feel at home, for this is the Mexican Palmetto, SABAL MEXICANA we've looked at growing in the wild not far from here at http://www.backyardnature.net/mexnat/sabal.htm.
Still, Mexican Palmetto is planted as an ornamental in warmer countries worldwide, so it's worth looking at here as a street tree.
Among features characterizing Mexican Palmettos are the way the fronds' petioles extend into the frond blade, gradually diminishing and causing the blade to twist in a certain way -- such fronds are said to be "costapalmate"-- as shown at http://www.backyardnature.net/n/15/150405sn.jpg.
The fronds' broad-based petioles split where they attach to the trunk, as seen at http://www.backyardnature.net/n/15/150405sp.jpg.
This tree bore several large, much-branching, fruiting clusters, as shown at http://www.backyardnature.net/n/15/150405so.jpg.
The pea-sized fruits were blackish, but when the leathery covering was peeled off, a chestnut-colored seed was revealed, shown at http://www.backyardnature.net/n/15/150405sq.jpg.
Mexican Palmettos aren't commonly planted around town. Maybe they are regarded as too familiar to be desirable, despite the fact that people the world over like to plant them just because they're so handsome and easy to grow.
MEXICAN FAN PALM DOWN THE STREET
Down the street from Río Lagartos's fútbol stadium and the Mexican Palmetto in front of it stand some shorter palms, also fan palms, but different. You can see them along the fútbol field, their fingerlike "surface roots" advancing onto the sidewalk's surface, at http://www.backyardnature.net/n/15/150405wt.jpg.
Unlike the Mexican Palmetto's fronds in which the petiole extends deep into the blade, petioles on these fronds stop abruptly at the base of the blade, producing a triangular, scale-like appendage known as a hastula, as seen at http://www.backyardnature.net/n/15/150405wu.jpg.
Also unlike the smooth petioles of the Mexican Palmetto's fronds, these tree's petioles are heavily armored with sharp, triangular spines, as seen at http://www.backyardnature.net/n/15/150405wv.jpg.
In that picture it's worth noting that the spines and petiole bases are conspicuously dark brown, for a very similar species lacks this coloring. A shot of the trunk top with old, dried-up fruiting clusters arching downward on the picture's left side is shown at http://www.backyardnature.net/n/15/150405ww.jpg.
In that image you see that someone regularly cuts off old fronds, leaving a rough covering of petiole bases enveloping the trunk. This pruning robs the trees one of their most distinctive and handsome features, which is that as dried-up fronds dangle downward, they form a distinctive "shag" around the trunk, like a hula dancer's grass skirt.
And just for the fun of it, old flowering clusters bore a few dried-up flowers but, curiously, no fruits, so you can see one of this tree's desiccated blossoms with its predictable six stamens and a slender style surrounded by a papery, three-lobed corolla, and subtended by a three-toothed calyx, at http://www.backyardnature.net/n/15/150405wx.jpg.
This is the Mexican Fan Palm, or Washingtonia Palm, WASHINGTONIA ROBUSTA, not native to the Yucatan, but at home in deserty northwestern Mexico, in the states of Sonora and Baja California. A similar species, Washingtonia filifera, can be confused with it, and sometimes hybridizes with it, causing even more naming confusion. The Mexican Fan Palm's brown petiole bases and spines, and its trunk base which tends to bulge outward, help separate the species from Washingtonia filifera.
Both of these Washingtonia species are planted throughout the world's warmer regions. In the US I read that our Mexican Fan Palm is more commonly planted in Florida, while Washingonia filifera is more common in the southwestern states, though both species can turn up in both places.
GEOLOGY STUDENTS WITH A DRONE
Last weekend a group of geology students and teachers from Northwestern University near Chicago passed through, studying our karst geology developed on geologically very young limestone. At their first stop they analyzed bedding at a borrow pit just south of town where years ago material was taken to enlarge Río Lagartos's peninsula by filling in the estuary and mangroves, and to elevate roads built through the swamps. You can see students examining exposed strata in the pit at http://www.backyardnature.net/n/15/150405dq.jpg.
The teachers found a great deal to say about this and other of the pit's walls. How did certain hard-surfaced tunnels and empty spaces develop in the formation? Why were certain strata harder than others, and what accounted for some layers being thin and others thick? Why were some layers crooked or wavy, and why at a certain depth did layers suddenly stop being horizontally aligned, but instead deeply tilted? Why did some strata bear larger fossil shells than others? Which way was the current flowing that deposited the layers? Were the layers deposited in shallow or deep water, and how far from shore? How would we determine the age of the strata?
When the students fanned out to different rock faces looking for evidence with which they could formulate and defend theories suggesting answers to these questions, they were accompanied by the object seen hovering over them at http://www.backyardnature.net/n/15/150405dr.jpg.
A close-up of the camera-toting drone with its whirring blades and blinking red and green lights is at http://www.backyardnature.net/n/15/150405ds.jpg.
GOING ON THE ROAD AWHILE
After six months of being in Ría Lagartos my visa is expiring and I must leave the country. Now for awhile my postings will become more erratic. It's unclear whether I'll have access while on the road. Probably you'll be reading more about it later.
FEATURED ESSAYS FROM THE PAST:
"A Walk in Pisté" from the March 6, 2011 Newsletter, at http://www.backyardnature.net/n/p/110306.htm
"Lettuce Feelings" from the February 27, 2011 Newsletter, at http://www.backyardnature.net/n/p/110227.htm
Best wishes to all Newsletter readers,
All previous Newsletters are archived at http://www.backyardnature.net/n/.