Thursday, 23 November 2017

A Mystery

ascidia paratropa - Glassy
When you have dived in Northern Europe and on the West Coast of North America, you more or less expect to encounter similar species, and even for amateurs like us we are not surprised that Alaska’s Giant King Crabs are roaming westwards, pulverizing less aggressive species and taking over in Russia and Norway. These animals share familiar habitats and it’s not surprising that Northern climates feature very similar species – especially those that move around. BUT – when you encounter a glassy tunicate (pictured) you have to wonder: “How did it get here?”

How did it make it through the equator? How did it make it through the torrid waters that stretch so far on either side of the equator? We certainly didn’t see any Glassies in Australia’s Coral Sea or even in the temperate waters of New Zealand.

 In our last dive at Caleta Damien, we saw not just a glassy tunicate (or ascidia paratropa) but another ascidian or tunicate called halocyntia aurantium or Sea Peach. They’re both quite amazing – the Glassy looking just like something that could be a distorted but lovely water glass – and the Peach quite the opposite – gnarled and mis-shapen like a plastic Halloween face that has first had ears and nose pulled out of place and then been left forgotten in an oven to partially melt. Both of these animals do not have the ability to move. So how did they get here?

David Behrens gives us a clue about all invertebrates when describing the dispersal of certain nudibranchs (- his specialty-) “the overwhelming majority have been distributed by natural means”. Some species of animals have a much greater tolerance for variety of temperatures and of prey animals. They can flourish while other similar creatures are excluded. “Phenomena such as El Nino events (when the ocean experiences a drastic increase in temperature) can provide … distribution of the species.”

Sometimes, it IS our influence that brings new animals from across the equatorial divide. The prey of some species can be organisms fouling the bottom of a boat, or in the bilge water of ocean-going vessels.   If the animal being brought into new water can survive, find appropriate prey and reproduce, it will have found a new home. Some are unwanted, so every country has strict regulations to reduce any impact a vessel like ours can have on their territorial waters. Neither the Glassy nor the Sea Peach are shown in the ‘Benthic Fauna of Chilean Patagonia’ but no doubt they find the climate, water and food here as agreeable as the waters across the equator and up North.

REFINEMENTS of the Tunicate/Ascidian family:

Eating and excreting with the same orifice? UGH! The glassy tunicate and its family are quite remarkable. They’re a more complex form of life because they have TWO orifices - unlike the anemone and most other marine invertebrates which have only the one. This earns it a higher number (16) in our Linnaeus-based numbering system as compared to the anemone (3.2) Andy Lamb explains:  “Each solitary tunicate has two siphons: an in-current one that brings in the food-laden water and an ex-current one that expels the filtered product.”

halocyntia aurantium - Sea Peach

Sex: Andy continues: “Nearly all tunicates are hermaphroditic, meaning that each adult specimen has both male and female organs. However, the Peach avoids the disadvantage of self-fertilization by releasing the eggs and sperm at different times.”

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