Friday 2 November 2018

Volcanic Reefs of Mission Beach


At Mission Beach are some unique rocky reefs of volcanic origin. Local volcanoes produced a very fluid lava that spread across the landscape in long streams. One of these streams created Clump Point, a finger of land which juts almost a kilometre into the sea. On aerial photos, other lava flows are easily identified including a few that are partially or fully submerged. Old lava flows are also hidden two metres below the tourist trampled white sand of Mission Beach.

Pelicans resting on a lava flow, a few hundred metres from shore
This post looks at a pillow lava reef which was five kilometres out to sea. This rocky reef probably breaks the surface far more than it is submerged. There is no beach and no terrestrial vegetation, only a hectare sized patch of black rocks. It is very hot and not very interesting unless you are a biologist. Even then, you want to study this place properly the first time so that you do not have to come back. Surrounding the reef is a sandy seabed and the margins of the reef have a hedge of sargassum. Coral is not to be seen.

This lava reef is five kilometres from shore

The edge of the old lava flow has sargassum and is surrounded by a sandy bottom
When you stand and stare long enough, you realise that this reef has a rich diversity of algal turf grazers. Fish, crabs and snapping shrimp battle for territories of turf. Blennies would attack and drive off much larger crabs. Crabs with heavy nippers would become frustrated with trying to clip the turf and would push their faces into the turf to chew it off directly. Even snapping shrimps would graze the turf using their larger nippers like shears with long, thin, lightly built arms darting over the surface picking up bits and pieces.

This snapping shrimp was cropping algal turf with the big nipper and picking at it with a skinny green nipper

A common hairy crab - Pilumnus vespertilio


A swimming crab grazing with its mouth

Another type of hairy crab grazing with its nippers
Large, heavily built crabs consume molluscs, both oysters and many gastropods that would normally assume the grazing role. Oysters were limited to exposed edges where water flow would be better and predator access more limited. Only a few neritas and cowries were on the basalt reef.

Ozius truncatus
A black finger crab is able to crush strong sea shells
On a nearby reef trochus shells were present. The nearby reef was a different geology, possibly a contact zone rock which was melted by the lava flow, then re-solidified. Corals also performed much better on this substrate.
Trochus shell in pocket in rocky reef

The underside of the same shell

A rocky reef that is probably contact zone rock, rather than basalt.

Volcanic features such as dykes and cliffs of solidified ash are present on nearby islands and on the mainland. At the bottom of one of these cliffs was a rock shelf which was only just inundated at high tide. On this shelf was a very shallow pool and a few rocks, fallen from the cliff. It was a particularly nasty place, intensely hot and saline. Yet when I looked, most of the stones were surrounded by a wide black halo. I also had the feeling that something was hiding behind one of the stones. I walked around the stone about five times and didn’t see anything but could hear frantic splashing. The only way to find out was to lift and remove the rock. A few crabs ran off but there were several large gobies stranded in a film of water too shallow for them to swim in. Apparently this species chooses to live in this habitat, a tiny area of the very worst aquatic habitat I could find. I love finding exotic creatures in places were nothing should be able to survive.

Dusky Frill Gobies in very shallow water on a rock platform 

The rock I lifted to see the fish

Wider view of platform of compressed volcanic ash