Tuesday, 28 June 2016

Sea Life growing on Floating Walkways in a Coral Sea Marina

The floating walkways in the Cairns Marina in tropical Australia are a new type of habitat that is not seen in nature.  Existing species have colonised but in new combinations to create what is known as a novel community, a community that does not occur in nature.   This community features a rich multi-coloured carpet of filter feeders.  These photos were taken by lying down on the walkway and holding a a low cost underwater camera in the water with my hand.  I would probably modify my technique if gropers were known live in the marina.

A garden of sponges, sea squirts and hydroids
The floating concrete walkways provide a habitat that is different from natural rocky surfaces in many respects.  This habitat is not exposed to waves or high and low tides as the walkways rise up and down with the tides and marinas are as protected from waves as possible.  Another difference is that waters in the marina are deep and there is very little connection between the floating walkways and substrate on the bottom of the harbour.  Indeed sediments fall away from the floating walkway community.  On natural coasts, sediment from the bottom is constantly being resuspended deposited on hard surfaces or the hard surfaces are being scoured by sand and waves.

Inlet of a tunicate
Possibly a coral - corals were generally absent
In nature, only fissures between boulders, deep rock pools on rocky headlands and the vertical sides of fjords have vaguely similar communities as these locations are protected from wave and sand blasting and from desiccation at low tide.  Forgetting fjords as we have none in north Queensland, marine life in fissures and rock pools can be subject to wild swings in temperature and salinity as due to very hot days, cold nights and intense rainfall events.   In contrast the life attached to walkways enjoys relatively constant temperatures and salinities.  Even when the same creatures occur in other habitats, they are often larger when they are attached to walkways.

Ferny growths are hydroids, a relative of coral

Fish are surprisingly uncommon as perhaps they do not like the shallow depth of the water and mobile invertebrates such as crabs and shrimps are also fairly uncommon.  Even macro algae is uncommon which is surprising given that the walkways suspend their denizens in the bright sunlit surface waters and bath them in nutrient rich flowing waters.   The community is completely dominated by sessile filter feeders.  As to what species they are, it is a real struggle to find out as very few people know how to classify these species and resources are hard to find.  

A white sponge and a tube worm
A flat oyster with mouth ajar surrounded by green seaweed

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