The Chitons' Strand
There is a wild coast at the antipodes, a shoreline that could recall that of French Brittany if its beaches were not so dark. Every day, the Tasman Sea uncover a strand made of magmatic rocks, overlooked by an archetypal volcano named Taranaki by the Maori, Mount Egmont by the Kiwis. The southwestern tip of Aotearoa, the North Island, harbors one of the most enigmatic landscapes in New Zealand.
From the famous Route 45, it takes about an hour of walking on rough paths and through vast pastures to reach, away from all, a small bay exposed northwest where accumulates pebbles and smooth basaltic rocks. The slope is mild, revealing at low tide an immense harvesting zone constellated with shallow puddles. The flats furthest away from the shore are mainly colonized by the brown algae Durvillaea antarctica and Homosira banksii, while the emerged areas are uniformly covered with green microalgae. At first sight, the area appears to be poor in species, with visible malacofauna limited to a few highly eroded Cellana radiata (Nacellidae) and several small colonies of Melagraphia aethiops (Trochidae) and Nodilittorina cincta (Littorinidae). But hidden by the day under the stones, hundreds of Polyplacophorans await the darkness to graze their algal pasture.
The density of the population is truly exceptional, and I have been able to count up to 27 individuals under a pebble of thirty centimeters in diameter. And at least 8 different species of Polyplacophores are present on an area of bout 35 hectares, cohabiting in the same biotope. Nocturnal low tides are conducive to observation, especially from two hours after dusk to the hour before dawn. There is also increased activity and greater movement during windless and moonless nights. The other classes of molluscs are very little represented in the area, both in terms of species and density. I could observe three natural predators of the Chitons: the starfish Coscinasterias calamaria which picks up the juveniles from their substrate, the small reef heron Egretta sacra that dislodges the exposed specimens with its pointed beak, and the Muricidae Haustrum haustorium that was caught punching the belt of a visibly weakened individual.
Here are the Polyplacophora species harvested in Cape Egmont, followed by a few observations:
(Quoy and Gaimard, 1835): the species is the most present in the bay, which is also the least bothered by exposure and light. Specimens are generally larger and eroded in the upper part of the intertidal zone.
(Gray, 1843): Maybe a smooth form of S. pelliserpentis as evoked by Boyle in 1967. Present in the north of the zone, in the puddles sheltered by large rocks at the limit of low tide.
Gray, 1828: the second species in terms of population, shortly after S. pelliserpentis. Stays immersed when it is at rest, but accepts short exposures during its moves. Of very variable hue, but most often green or brown. Blue specimens are the rarest, albinos excepted. Originally endemic species of New Zealand, but was accidentally introduced in Tasmania (Furlani, 1996).
(Gray, 1843): I could observe only three specimens of this great species (67 mm for the largest) which remains discrete. Always very eroded, the valves are hidden under several millimeters of calcareous crusts and red algae. During the day, stays very deep under the layers of pebbles.
Rochebrune, 1881: fairly common species in shallow puddles with little algae. Never observed by day, the species probably chooses the underside of the large non-movable rocks to hide. The patterns are variable.
(Quoy and Gaimard, 1835): easily identifiable with the clumps of silk that adorn the belt, this common species is often exposed.
(Quoy and Gaimard, 1835): only two specimens of this very beautiful species crossed the beam of my flashlight. The first was found in the brown algae on a puddle, the second under a piece of immersed wood, wedged between pebbles.
Carpenter in Pilsbry, 1892: a single specimen caught alive during the day under a smooth stone together with several C. glaucus. Moves faster than other species.
Iredale, 1914: in my opinion the most elegant species of the area. Unfortunately, I could observe only one specimen by night, moving quite fast on a smooth pebble at the bottom of a puddle.
I look forward to the comments of the "sea cradles lovers", wishing that the Chitons' strand will bring for a long time its share of discoveries and wonders.
Ashby, Edwin, “Further notes on New Zealand Chitons”, Transactions and Proceedings of the Royal Society of New Zealand, Volume 60, Auckland, 1930
Burghardt, Laura et Glenn, A gallery of Worldwide Chitons, “Selected Chitons of New Zealand”, http://home.inreach.com/burghart/newzea.html, consulté le 6 juin 2009
Collectif, Collections Online, “Family Chitonidae”, Te Papa Tongawera, Museum of New Zealand, http://collections.tepapa.govt.nz, consulté le 6 juin 2009
Dell, Richard Kenneth, A key tothe common Chitons of New zealand, Tuatara vol. 4, Wellington, 1951
Spencer, Hamish G., Checklist of the Recent Mollusca Described from the New Zealand Exclusive Economic Zone, “Key to New Zealand Molluscs, Polyplacophora”, Otago, 2003
Spurgeon, Andrew, New Zealand Mollusca, “Species Checklist, Class Polyplacophora”, http://www.mollusca.co.nz, 6 June 2009