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.
Hydroids?
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
Go into any Asian food shop and you can buy bags of dried shrimps or paste made from these shrimps, known as Jelly Prawns in Australia, so it is quite surprising that I cannot find much information on the net about them. This week (10 June 2016), a swarm of shrimps formed in the swash zone of Holloways Beach and I was able to make a video of them.
Every few minutes the swarm erupts from the surface, with shrimps flicking in all directions. These eruptions seem to be mainly chain reactions where shrimps falling back in make other shrimps jump. A single small toadfish swimming close to the swarm can trigger the eruption which then spreads across the swarm.
The swam appears as a reddish band in the water
Usually the prawns are completely clear but swarms are dull red
The prawns leap into air on the slightest hint of danger
The swarm was thickest around the mouth of Barr Creek (near Cairns), which was possibly less saline that usual due to unusual winter rains. According to a paper published by Kemp in 1917 "The species of Acetes are found gregariously swimming in great numbers in mid-water or near the surface. They are apparently met with only in coastal waters; they occur near the shore in the open sea, and are frequently common in estuaries and backwaters. They are often found in water of low salinity, and occasionally in places where it is quite fresh, but penetrate little if at all beyond the reach of tidal influence. The species are fished commercially in India and Japan, the small size of the individuals being evidently compensated by the great abundance in which they are taken."
As the swarm hugs the breaking waves it is a dense swirling mass of shrimps, sand and bubbles
A fishes eye view of the edge of the swarm
I have seen only three swarms in thirty years, however that is not to say that the shrimps are uncommon. Rather they are probably observed only occasionally. Conditions had been calms for several days in a row in Far North Queensland at a time when Tasmania was enduring some of its worst ever bad weather and floods. Usually the shores are being pounded by wave after wave and it can even be difficult to launch a surf ski as being hit by large and continuous closely spaced breaking waves can stop even a strong young man. In the unusual calm weather, the fish have the advantage over the shrimps and it appears that the shrimps crowd into the shallows for protection as fish are rightly very timid about entering shallow water. The swarm at Barr Creek was present for at least three days and would have been 1 m wide and 10 m long, however the entire beach and adjoining Machans Beach had an abundance of shrimps which could be seen flicking from the surface to escape fish.
As the swarm builds, up it becomes very dense and I suspect that oxygen depletion occurs. Small fish continually nip at the edges of the swarm and keep it compact. Whilst the shrimp can survive the oxygen depletion, they appear to become sluggish and when a slightly larger wave breaks in deeper water and within the swarm, it can swash large numbers of the sluggish shrimps up the beach where they are stranded. The seagulls think it is Christmas but as there are not so many seagulls in the north, they are quickly sated and just stand around and try to digest as quickly as possible so that they can some eat more.
Jelly Prawns washed up by waves for bands along the beach
Under the water, there are silver flashes from fish nipping at the swarm. The only fish to swim around in the swarm were three-banded toad fish and possibly striped grunter. On the bottom were juvenile shovel nose sharks (rays). On previous occasions I have seen 1.5 m long shovel noses cruising up and down 3 m off the beach when the shrimps are present. At Ellis Beach I have even seen at least three fully grown manta rays feeding on a swarm. In an attempt to make contact with a manta ray, I stood in the knee deep water but the rays know you are there and swim around you gracefully whilst taking in the goodness.
An article in Fishing World provides more information on Jelly Prawn biology.
