Thursday, December 17, 2015

Barrier Island Wonders of Nature‏

Snowy egret fishing off dock
Barrier Islands are unique habitats at the intersection of land and sea.  They are beautiful but highly impacted by humans seeking the classic beach experience, but they can also provide a place to observe unusual species that are highly specialized for this edge environment.  I live on a gulf front barrier island, Manasota Key, and often visit the adjacent island to the south, Palm/Knight Island.


Fighting conchs on the beach at Palm Island
A recent morning walk at low tide on Palm Island revealed an unusual phenomenon, the stranding of large numbers of fighting conchs.  These snails are true conchs and close relatives of the huge queen conchs, which are now virtually extinct in FL.  

Fighting conch
Fighting conchs are herbivores and often have a rich reddish color to the shell opening, the purpose of which is a mystery to me. They apparently feed sub-tidally close to the shore and can get stranded on the beach during a very low tide. Since they are not very mobile they dig into the sand to minimize desiccation until the next high tide.  I sometimes walk along throwing them back into the water, although one could argue that this is a natural circumstance and that humans should not interfere.

Calico crab
Another interesting critter stranded on the beach was a calico crab.  This is one of the so-called "box" crabs, since when threatened it holds its claws up against its shell in a protective stance.  This may enhance the attractive blotched cryptic coloration by changing the outline and diminish predation.  Since this is an aquatic crab, unlike the ghost crab, it dies rapidly on the beach.


White-tipped black moth
Just behind the beach in the drier dune area of Manasota Key I came across a breeding aggregation of very interesting white-tipped black moths (Melanchroia).  They appear to mimic wasps as a protective mechanism  that allows them to fly in daytime.  Since they feed predominantly on plants in the spurge family which are usually toxic, they may also retain the food toxins and advertise this by their bright coloration.  This would give them a one-two punch against bird predators.  But it is a mystery why so many of these moths were close to the beach when their larval food plants did not seem to be present. These moths are in the inchworm or geometer family yet they strongly resemble unrelated ctenucha moths in the tiger moth family, perhaps an example of both mimicking wasps, but certainly a remarkable example of convergent evolution.  

Tennessee warbler
Continuing to the east across the barrier island from the gulf to the bay, I entered a deciduous woodland which is suitable for terrestrial birds and can be a hot spot for migrants.  For example on Nov. 11, I saw both Tennessee warblers and a yellow billed cuckoo which were flying south for the winter.  The warbler breeds in the Canadian boreal forest and winters in Central America and Cuba.  

Yellow-billed cuckoo
The cuckoo breeds throughout eastern N America and winters in S America. It is a thrill to see these neo-tropical birds while they are on their remarkable flights and think about how such amazing long distance patterns of movement might have originated.


Mangrove snake
Continuing eastward I found myself in the bayside mangroves and encountered one of our least known local reptiles, the mangrove snake. This is a salt water specialist but is not a true sea snake (which are mainly venomous species living in the Pacific and Indian oceans) but is in the same genus as the fresh water banded water snake.  It lacks the salt gland used by sea snakes to maintain its blood concentration about one third that of sea water, but regulates osmotic balance by being relatively impermeable, by gaining fresh water from its fish prey, and by only drinking brackish or fresh water when it is available from rain.  Another unusual fact is that the mangrove snake comes in a wide variety of colors from this reddish morph to gray or blackish hues.  Certainly the red morph blends well into the red mangrove root habitat.

White pelicans on spoil island, Gasparilla Sound
Further east in the bay I saw a a large aggregation of white pelicans, huge spectacular birds that breed in shallow lakes of the northern Great Plains, and soar long distances with minimal energetic cost for flight.  They are certainly majestic birds in flight and their arrival in Florida in early November is an annually anticipated event.  They feed in groups and do not dive as do the smaller brown pelicans.  Another white bird, the snowy egret is famous for its flexibility in feeding strategies and this one found that it could efficiently catch fish by leaning off a bayside dock.  Such an ability to adjust behavior to new situations seems to be crucial for survival in an increasingly human altered world.

So the next time you visit a beach on a barrier island, look around and enjoy the remarkable variety of life that is often only found on these narrow and fragile ribbons of sand.

Bill Dunson 
Englewood, FL and Galax, VA

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