Crab Bluff

At low tide on a Thursday evening, Keen Wasabi and Salty (and every other dog on the beach) joined me for chicken sandwiches and rock pool-measuring at Thirroul Beach’s Crab Bluff.

I knew I wanted to do a population survey – I wanted to know every living species on Thirroul Beach. If only it were that simple.

As a landlubber all my life, it took me weeks of loitering at the Bluff to figure out just what I was looking for in a population survey. I could see that different species of sea snails preferred different areas of the beach – striped Zebra tops (Austrocochlea porcata) remaining submerged at low tide, while the nodular Pyramid periwinkles, (Nodilittorina pyramidalis) hung around at the dry edge of the spray zone.

However, what I was mainly aware of was that I knew nothing. I didn’t even know what I didn’t know, because I didn’t know anything. I wasn’t sure what was a limpet or a barnacle or a rock oyster or an urchin or an anemone. And the seaweed seemed important – but what kind of plant grows in saltwater? I knew so little in those early weeks that I inhaled information, spending hours browsing sites like the Australian Museum’s zoological resources, sea shells of Australia and endless Wiki pages on tides, littoral zones and marine snails.

At last I felt equipped with a basic knowledge set. I spent a few days at the south end of the beach past the pump house, out on Crab Bluff, which is above the tide in all but the heaviest storms. Rock pools on the Bluff each had different animals and algae, some webbed with Neptune’s necklace, others shallow and dotted with infant periwinkles and conniwinks. Still others held big red urchins, swift-footed crabs, elephant snails and gobies.

I had a vague idea that it may have been proximity from the ocean that determined what lived where. Out of the hundreds of pools on the Bluff, I settled on 11 that are close to each other, display a variety of depths, volumes and distances from the ocean, and that can be accessed even at high tide.

crabbluff
Map of pools chosen on Crab Bluff.

Next I needed data. Lots of it. Data about the pools, the beach, and the animals that lived there. While some of the information, such as tide times and heights and ambient temperature, could be sourced online, the bulk of info I was going to have to collect myself. To do that required narrowing down and focussing on what I wanted to do, resulting in a series of actionable steps.

  1. Choose pools based on proximity to ocean and variety of life.
  2. Measure pool area and depth to calculate volume. Note substrates.
  3. Measure pool temperatures and compare to air and ocean. Note species.
  4. Conduct water testing for pH, ammonia, nitrate, phosphate and salinity. Count populations.
  5. Compile results and draw conclusions about what’s there and why it’s there, and how it interacts with other species.

Splitting the steps this way meant I could gather information across different days, giving me time to reflect on what I’d learnt and what needed improvement. It would also mean that I could modify my hypothesis: if proximity to the ocean turned out to be a dead end, I could add additional testing for salinity, temperature and other factors that I may not yet be aware of.

But there’s no time like the present. The pools were selected, a yard stick made from a length of bamboo, and the aid of Salty and Keen sequestered.

We met on the Bluff as the sun sank below the escarpment. Salty and I shared a chicken sandwich. The tide was low, leaving the pools isolated, and crabs hung thick on the wall above the water. Keen took up pen and paper as I wrestled Salty for the yard stick.

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Salty offers Keen instructions on proper yard stick use.

Results for pool measurement:

# 01 Tiny crater

Shape: Circle, 13 cm diameter
Surface area: 132.73 cm 2
Average depth: 1.8 cm
Max depth: 2.5 cm
Volume: 238.91 cm3 (0.24 L)
Distance from ocean: 5.0 m
Substrate: rock

# 02 Dry triangle

Shape: Triangle, 116 x 83 x 126 cm
Surface area: 4682.56 cm2
Average depth: 4.5 cm
Max depth: 9.5 cm
Volume: 21071.52cm3 (211 L)
Distance from ocean: 5.0 m
Substrate: sand on rock, small pebbles

#3 Left twin

Shape: Rectangle, 110 x 53 cm
Surface area: 5830 cm2
Average depth: 18 cm
Max depth: 31.5 cm
Volume: 104,940 cm3 (105 L)
Distance from ocean: 6.3 m
Substrate: sand

#4 Smoking gun

Shape: Triangle, 154 x 147 x 173 cm
Surface area: 10644.48 cm2
Average depth: 8 cm
Max depth: 15 cm
Volume: 85155.85 cm3 (85 L)
Distance from ocean: 5.5 m
Substrate: rock, black lichen

#5 High pool

Shape: Square, 180 x 110 cm
Surface area: 19800 cm2
Average depth: 5 cm
Max depth: 9 cm
Volume: 99000 cm3 (99L)
Distance from ocean: 4.5 m
Substrate: rock, sediment

#6 Big pool

Shape: Rectangle, 610 x 295 cm
Surface area: 179950 cm2
Average depth: 50 cm
Max depth: 65 cm
Volume: 8997500 cm3 (8998 L)
Distance from ocean: 1.1 m, 2.7 m
Substrate: Sand, rock, small rocks

#7 Stream pool

Shape: Rectangle, 80 x 30cm
Surface area: 2400 cm2
Average depth: 4 cm
Max depth: 6.5 cm
Volume: 9600 cm3 (9.6L)
Distance from ocean: 4.5 m
Substrate: rock

#8 Elephant pool

Shape: Rectangle, 36 x 85 cm
Surface area: 3060 cm2
Average depth: 53 cm
Max depth: 54 cm
Volume: 162180 cm3 (162 L)
Distance from ocean: 0.5 m
Substrate: sand

#9 Big crater

Shape: Semi-circle, diameter 230 cm
Surface area: 20773.78 cm2
Average depth: 11 cm
Max depth: 34 cm
Volume: 228511.58 cm3 (229 L)
Distance from ocean: 1.3 m
Substrate: Rock, small rocks

#10 High half-crater

Shape: Semi-circle, diameter 135 cm
Surface area: 7156.94 cm2
Average depth: 9.5 cm
Max depth: 13.5 cm
Volume: 67990.93 cm3 (68 L)
Distance from ocean: 1.2 m
Substrate: Rock, lichen

#11 Neptune’s crater

Shape: Rectangle, 40 x 25 cm
Surface area: 1000 cm2
Average depth: 4 cm
Max depth: 4 cm
Volume: 4000 cm3 (4 L)
Distance from ocean: 0.5 m
Substrate: Sand

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Big Pool, with a view towards the back of the Bluff, and the oft-inundated crab wall.

While these are only the initial observations for the Bluff, it was the beginning of many discoveries. Much of it is rehashed ground – the littoral zone is readily accessed and people have been doing so for the last 150 years, even in Australia. But for me it’s new. For Salty, it’s new. For Keen, even though he has always been a beach bum, there exists this entire other world just below the waterline. A world fraught with battles and courtship, a world where tiny creatures overcome enormous odds in the everyday fight for survival.

Join Salty and I again soon as we peer upon this miniature world, and discover some damned strange things washed up on the beach. Here’s an exclusive preview:

Dangerpus: “Just what is that weird white lumpy thing, Salty?”
Salty: “…”
Dangerpus: “It looks like some mass of anemones – or maybe sea spaghetti!”
Salty: “S:(”
Dangerpus: “Let’s bag it and take it back to the lab.”
Salty: “!”

*Over and out*

Part 2 here: Secrets of the Rock Pools

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Author: Anneque D. Machelle

Anneque "Dangerpus" Machelle (rhymes with ranger wuss) is a rebel and a rogue from way out west. Strictly banned from interactions with other human beings, she spends her days amongst molluscs, dogs and lizards, whom she counts as her closest friends.

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