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Lichen (by WiseAcre)

Cast your imagination back half a billion years ago to the Cambrian geological period.  Although Earth’s oceans were seething with strange experimental life forms, the alien continents were bleak and empty.  Huge brown mountains sloughed away into giant canyons. Black volcanoes eroded into naked black beaches. Great flash floods poured over a landscape bare of plants and animals. No horsetails grew.  No dragonfly buzzed.  Not even a miserable liverwort crouched by the empty streams. But were the ancient continents entirely bare? No—bacterial films and single cell algae were believed to have covered the land, and looming above that primitive slime were the first lichens, symbiotic life forms so hardy that they alone thrive on continental Antarctica today.

Red Lichen living in Antarctica (photo by Gerhard Hüdepohl from Atacamaphoto.com)

Lichen is a bizarre composite organism in which a fungus is paired with a photosynthesizing partner (either green algae or cyanobacteria).  The thallus of lichen (which makes up the organism’s body) is very different from either the fungal or algal components living on their own.  The fungi surround and hold up the algae by sinking tendrils through the algal cell walls (in much the same manner parasitic fungi attack their hosts).  By sharing the resources of the two different partners the organism is capable of surviving extreme desiccation, and, when the lichen is again exposed to moisture, a flood of nutrients becomes available to both partners.

Lichen (from "Art Forms of Nature" E. Haeckel)

The partnership makes for an extraordinarily resilient organism which can be found everywhere on land from the rainforests to the deserts to the highest mountains to the harsh frozen rocks of Antarctica. The European Space agency explored the durability of lichen by blasting living specimens into outer space where, to quote the ESA, the organisms were “exposed to vacuum, wide fluctuations of temperature, the complete spectrum of solar UV light and bombarded with cosmic radiation. During the Foton-M2 mission, which was launched into low-Earth orbit on 31 May 2005, the lichens…(Rhizocarpon geographicum and Xanthoria elegans) were exposed for a total 14.6 days before being returned to Earth….Analysis post flight showed a full rate of survival and an unchanged ability for photosynthesis.”

Lichen dot the face of a Song Dynasty statue on Qingyuan Mountain, China.

Lichens’ strange partnership also creates strange morphological forms. In many circumstances these organisms resemble exotic corals, sponges, or plants. Additionally, many lichens are brightly colored.  The result is often a miniature landscape of bizarre beauty.  I have included some photos from sundry sources but you should check out the lichen photos at Stephen Sharnoff’s site (even disfigured by the trademark, his lichen photos are the best on the net).

Competing Lichens Growing on a Rock

Since it involves both algae and fungi, lichen reproduction can be complicated and takes many different forms depending on the species and the circumstance.  Some lichens form soredia, small groups of algal cells surrounded by fungal filaments which are dispersed as a group by wind. Others produce isidia, elongated outgrowths from the thallus which break away.  During the dry season, certain lichens crumble into dusty flakes which are blown across the landscape.  When the rains come the flakes burst into full growths.  In the most interesting and complicated pattern of reproduction, the fungal portion of the lichen produces spores (as a result of sexual exchange and meiosis) these spores are disseminated across the landscape and then must find compatible algae or cyanobacteria with which to partner.

 

Community Lichens is in the Sawtooth Mountains (photo by Mark Dimmitt)

Lichens are probably long lived and it is possible that somewhere there are those that make the bristlecone pines seem young and have lasted as long as Pando, but who knows?  We have not explored and documented the world’s lichens very completely…or even fully understood the mechanisms of their partnership.  What is certain is that they are one of life’s most efficient colonizers: in areas such as the Atacama Desert and Antarctica, plants cannot grow unless lichen lived there previously (in fact I am going to include this post in my “invaders” category for just this reason). Lichens are also efficient at exchanging carbon dioxide for oxygen, and they are a critical link in the carbon cycle capable of fixing elemental carbon back into the soil and into the ecosystem.  When you look at a tundra landscape and savor the beauty of reindeer, mountains, and arctic birds, spare a thought for the ancient lichen, one of the first organisms on the land and still one of the most important.

Lichen slowly colonize a New England gravestone from the 1700's.

A Giant Clam, Tridacna gigas (Photo by Stig Thormodsrud)

Today we celebrate the world’s largest bivalve mollusk, the magnificent and world-famous giant clam (Tridacna gigas).  Native to shallow coral reefs of the South Pacific and Indian oceans, giant clams can weigh up to 500 lbs and measure 50 inches across.  Huge specimens can be very ancient and some have lived for more than a century. Giant clams are hermaphrodites: every individual possesses both male and female sex organs–however a clam is incapable of mating with itself.   They are broadcast spawners producing vast numbers of gametes which they release in response to certain chemical transmitter substances. During these spawning events (which usually occur in conjunction with certain lunar phases) a single clam can release over 500 million eggs in one evening.  Giant clam larvae then swim free among the plankton.  They pass through several mobile transition phases before settling down in one favorite home (as can be seen in the comprehensive life cycle drawing below).

Giant Clam Life Cycle (After H.P. Calumpong, ed. 1992 "The Giant Clam: an Ocean Culture Manual")

As usual for sea creatures, the giant clam has a troubled relation with humankind.  Fabulists have asserted that the great bivalves chomp down on divers for food or out of spite (the clams do slowly shut when harassed, but the movement is a defense mechanism and happens gradually).  They are considered delicacies on many South Pacific islands and naturally the insatiable Japanese pay a premium to eat them as “Himejako”.  Their shells also command a premium from collectors.  Across the South Pacific, giant clams are dwindling away thanks to overfishing, reef destruction, and environmental factors.

Divers with a Giant Clam: Bikini Atoll in the Marshall Islands

It is sad that the gentle and lovely giant clam is suffering such a fate (although aquaculture is now bringing a measure of stability to some populations).  In addition to being beautiful and useful to ecosystems, they are remarkable symbiotic creatures.  A unique species of algae flourishes in the mantle of the giant clam and the clam gains much of its energy and sustenance from these photosynthetic partners. The clam possesses iridophores (light sensitive circles) on its flesh which allow it to gauge whether its symbiotic algae is getting enough sunlight–and perhaps watch for predators.  It can then alter the transparency of its mantle flesh accordingly. According to J. H. Norton, giant clams have a special circulatory system to keep their symbionts alive and happy. The happy and beneficial relationship between a clam and its algae allows the former to attain great size and the latter to remain alive in the ever-more competitive oceans.  I have concentrated on writing about T. Gigas, but there are many other members of the Tridacninae subfamily which lead similar lives (although they do not attain the same great size).  To my eye they are all remarkable for their loveliness.

Tridacna Maxima (in a home aquarium)

Tridacna Derasa (in a home aquarium)

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