Secrets of the Rock Pools

The quest to uncover the secrets of rock pools continues…

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Have you ever wondered exactly what is living in the pretty little rock pools scattering the beach? You hear the crunch of littoral snails underfoot and gaze out upon a veritable starfield of similar creatures: little blue and grey shells in hunkering packs on every available rock, others clustered like refugees at the edges of pools. Now and then you may see the silvery flash of a fish darting for cover. A crab may scuttle under a rock, or bump into another crab and simply stand there, wondering what to do. As you peer deeper, you begin to make out the long red spines of urchins, the mottled blue and green backs of sea stars, and a lonely sea hare drifting amongst pink fronds of coralline algae.

But life in the rock pools is tough. Temperatures are extreme, salinity ranges wildly from hypo to hyper in the space of a rain shower, the waves constantly hammer, hammer, hammer, throwing in new cell mates and dragging out the old, pushing in sand and cold water, offering respite and obscuring shelters in the same breath. Birds circle, and humans flounder clumsily over generations of nerites, and big crabs drive the small crabs to the lesser hunting grounds, and small crabs strip away generations of nerites that the humans couldn’t get to.

Just what are the limitations of life in this war zone? What stops a blenny from living in two inches of water? Is it the hypersalinity? The temperature? Predation from birds? Or is it simply a lack of space? Similarly, why will a green anemone attempt to take root in half a centimetre of water in the midst of a minefield of Pacific oysters?

I wanted to know. And, because no one tried to stop me, I picked 11 rock pools out on a bluff at my local beach to study with the intensity of Charles Darwin observing a barnacle. If you haven’t caught it, you can read my first post of Crab Bluff here, or what the hell, jump right in, the water is warm (-er than ambient temperature.)

High tide at Thirroul Beach averages 1.525 metres above the datum. Low tide averages 0.46 metres above the same datum. Average water level is 0.99 m, meaning anything less altitudinous than 0.99 m is inundated more often than not, and anything above 0.99 m is exposed more often than it is inundated. All pools surveyed on the Bluff remain above the waterline even during high tide.

The Pools

#1 Little Crater

1

Volume: 0.24 L

Temperature: Warmer than ambient.

Exposure: This pool could be considered submerged at high tide; water constantly pushes over Big Pool into a water course encompassing Little Crater and Dry Triangle. This stream then flows across the back of the Bluff onto the beach.

As the water drops, less and less water flows over this pool, until the stream narrows and dries to isolate Little Crater. After isolation, water in this pool quickly evaporates, reducing its small volume even further.

Species:

This tiny pool is devoid of sheltering algae. What life there is clusters around the overhanging crater lip.

Molluscs:

  • Little blue periwinkles (Austrolittorina unifasciata)
  • Pyramid periwinkles (Nodilittorina pyramidalis)
5_littlecrater
What little there is of Little Crater, with scattered little blue periwinkles.

#2 Dry Triangle

2

Volume: 211 L

Temperature: Cooler than ambient up to the point of isolation. Shallow depth means it warms rapidly.

Exposure: As with Little Crater, this pool could be considered submerged at high tide. Water constantly pushes over Big Pool into a water course encompassing Dry Triangle. The pool is shallow with low walls and so the water in it is constantly changing. Outlets are to #3 Left Twin and in the tidal stream running down the back of the Bluff.

As the tide recedes, this pool still receives water, but is changed far less often. Volume is still above that of isolation. Ninety minutes from low tide, the pool is isolated from incoming water. It continues to output a small amount to #3. Eventually the pool is drained completely, and remains dry until it is restored by the rising tide.

Species:

Prone to being drained, there is no macroalgae in this pool.

Molluscs:

  • Pacific oyster (Crassostrea gigas)
  • Striped conniwink (Bembicium nanum)
  • Little blue periwinkle (Austrolittorina unifasciata)
  • Pyramid periwinkle (Nodilittorina pyramidalis)

Crustaceans:

  • Swift-footed crab (Leptograpsus variegatus)

 

#3 Left Twin

3

Volume: 105 L

Temperature: Offers sustained respite from high ambient temperatures.

Exposure: At high tide this pool receives regular water from the tidal stream. #2 runs into this pool even after the tide has receded. Additional water is provided by large waves. Left Twin outputs into Right Twin, which then flows down the back of the Bluff. As the tide reaches average height, inlets and outlets to this pool stabilise, leaving it isolated.

Species:

On original observation, this pool was deep with a rocky substrate, harbouring numerous small algae. In early December a storm pushed sand into the pool, halving its volume and obscuring most of the algae. Young crabs still find it a convenient hiding spot on their hunting ventures.

Molluscs:

  • Black nerite (Nerita atramentosa or N. melanotragus … orange and black operculum suggests melanotragus)
  • Pacific oyster (Crassostrea gigas)
  • Pyramid periwinkle (Nodilittorina pyramidalis)
  • Striped conniwink (Bembicium nanum)
  • Little blue periwinkle (Austrolittorina unifasciata)

Crustaceans:

  • Swift-footed crab (Leptograpsus variegatus)

Annelids:

  • Spirorbid worms (Spirorbis sp.)
20161222_131221
Orange and black operculum on Nerite sp. Note cute spirorbid worm spirals on shell.

#4 Smoking Gun

4

Volume: 85 L

Temperature: Warms rapidly after isolation.

Exposure: Inundated at high tide from backwash across the Bluff, this pool is otherwise relatively higher than the rest and so protected from most inlets. Still, rough wave action sees a total water change of this pool at least once per minute.

Two hours after high tide, no regular inlets remain, and there is only one small outlet to Dry Triangle.

Species:

While shallow and readily overlooked, this pool contains a good deal of encrusting and stringy algae, and supports a surprising amount of life.

Molluscs:

  • Black nerite (Nerita sp.)
  • Little blue periwinkle (Austrolittorina unifasciata)
  • Pyramid periwinkle (Nodilittorina pyramidalis)
  • Striped conniwink (Bembicium nanum)
  • Zebra winkle (Austrocochlea porcata)
  • Variegated limpet (Cellana tramoserica)
  • Flamed limpet (Notoacmea flammea)
  • Pacific oyster (Crassostrea gigas)

Crustaceans:

  • Swift-footed crab (Leptograpsus variegatus)

Annelids:

  • Spirorbid worm (Spirorbis sp.)

 

#5 High Triangle

5

Volume: 99 L

Temperature: With no spray to refresh it, temps reach ambient and above.

Exposure: High Triangle is the most isolated pool tested. Its west banks are dry even at the peak of high tide; it’s a safe place to stand and not be doused. Water, even spray, does not reach is regularly. Silty sediment on the pool’s bottom and persistent crab skeletons suggest only in rough weather and rain does this pool receive new water.

Species:

Any algae in this pool is on the micro scale. A few hardy periwinkles are all that survive here. On hot days they clustered above the pool’s edges. The empty white carapaces of crabs drift along the silty bottom.

Molluscs:

  • Little blue periwinkle (Austrolittorina unifasciata)
  • Pyramid periwinkle (Nodilittorina pyramidalis)

 

#6 Big Pool

6

Volume: 8998 L

Temperature: Cooler than ocean – possibly due to high evaporation levels from Big Pool.

Exposure: Hammered and inundated by violent wave action at high tide, this huge pool has outlets into the ocean on its east side and a tidal stream on its west side. Rock forms a natural funnel at this point of the Bluff and white water jets dramatically over Big Pool.

Two hours after high tide, far less is received and output. The water level remains about 5 cm above the pool’s algae line. On Big Pool’s surface, the only hint of its violent past are in the foam and shredded bladderwrack.

Species:

Big Pool is home to all sorts of interesting creatures washed in from the sea. Large amounts of coralline algae, sand and rocky crevices provide a multitude of lifestyle options, and creatures here are both visitors and regulars.

Arthropods:

  • Honeycomb barnacle (Chamaesipho tasmanica)
  • Rosy barnacle (Tesseropora rosea)

Molluscs:

  • Warrener (Turbo undulatus)
  • Mulberry whelk (Morula marginalba)
  • Black nerite (Nerita sp.)
  • Tent shell (Astralium tentoformis)
  • Zebra winkle (Austrocochlea porcata)
  • Striped conniwink (Bembicium nanum)
  • Little blue periwinkle (Austrolittorina unifasciata)
  • Variegated limpet (Cellana tramoserica)
  • Sea hare (Aplysia sp.)

Anemones:

  • Green anemone (Aulactinia veratra)
  • Shellgrit anemone (Oulactis muscosa)
  • Pacific oyster (Crassostrea gigas)

Echinoderms:

  • Tuberculate urchin (Heliocidaris tuberculata)
5_bigpool
Big Pool at high tide. Ride those waves!

#7 Stream Pool

7

Volume: 9.6 L

Temperature: Struggles to remain cool after isolation.

Exposure: Although this pool is high above the water, waves channelled against the Bluff jet into it, changing its volume several times a minute. It outlets to Big Pool.

Two hours after high tide, Stream Pool receives far less water and is not inundated, though it still receives regular small doses from wave action.

Species:

This narrow crevice is thickly grown with Pacific oysters. A few very small urchins also hide out amongst the regular smattering of littoral snails.

Molluscs:

  • Pacific oyster (Crassostrea gigas)
  • Black nerite (Nerita sp.)
  • Zebra winkle (Austrocochlea porcata)
  • Striped conniwink (Bembicium nanum)
  • Little blue periwinkle (Austrolittorina unifasciata)
  • Variegated limpet (Cellana tramoserica)

Anemones:

  • Shellgrit anemone (Oulactis muscosa)
  • Green anemone (Aulactinia veratra)

 

5_anemone

Shellgrit anemone, full of sand and grit.

#8 Elephant Pool

8

Volume: 162 L

Temperature: Pool temp measured in the afternoon was cooler than the ocean temp measured in the morning. Could be due to evaporation, or cooler waters pushing onto the beach after initial ocean temperature was measured.

Exposure: Elephant Pool is the most exposed of any pool tested. It receives intense wave action during high tide, with water constantly pouring in and out of this deep pot of a pool, often violently.

Even at low tide, this pool continues to receive water, and is always at capacity. Its only outlet is the ocean.

Species:

Generous algae grows provide shelter and food for the curious inhabitants of this pool, all of whom seem to be regulars. In particular, a pair of large elephant snails hide amongst the sea grass at the bottom, while crabs and urchins tuck themselves into a deep crevice.

Anemones:

  • Green anemone (Aulactinia veratra)

Arthropods:

  • Swift-footed crab (Leptograpsus variegatus)

Molluscs:

  • Elephant snail (Scutus antipodes)
  • Zebra winkle (Austrocochlea porcata)
  • Black nerite (Nerita sp.)
  • Striped conniwink (Bembicium nanum)
  • Little blue periwinkle (Austrolittorina unifasciata)
  • Variegated limpet (Cellana tramoserica)

Echinoderms:

  • Tuberculate urchin (Heliocidaris tuberculata)

 

#9 Big Crater

9

Volume: 229 L

Temperature: Is able to resist some heat from the sun after isolation. This measurement, taken at nearly low tide, suggests the pool would reach ambient temperature before regular cool spray was restored.

Exposure: Regularly sprayed in high tide, where it sits above capacity with only a small outlet draining to the ocean. This pool’s depth and sides give it some protection from wave action. Pool #10, High Half-Crater, also flows into Big Crater.

Two hours after high tide, this pool is close to its isolated volume. It receives only spray, and retains a small volume outlet to the ocean.

Species:

The deep bowl of this pool is a nursery for snail eggs. Young crabs hang out at this pool in the manner of kids at a skate park, having been pushed back from the Bluff by their old folks.

Arthropods:

  • Swift-footed crab (Leptograpsus variegatus)

Molluscs:

  • Little blue periwinkle (Austrolittorina unifasciata)
  • Black nerite (Nerita sp.)
  • Zebra winkle (Austrocochlea porcata)
  • Pacific oyster (Crassostrea gigas)
20161110_131018
A curious Zebra winkle.

#10 High Half-Crater

10

Volume: 68 L

Temperature: Approaching ambient after isolation.

Exposure: Despite this pool’s proximity to the ocean, it is readily isolated by its height and orientation in the rocks. It receives spray only in high tides and rough weather, and more regular inlets across the Bluff from the back of #11, Neptune’s Crater. Here large waves increase #10’s volume and change its water regularly.

Two hours after high tide, the pool’s outlets to #9 Big Crater have subsided, and it receives little new water.

Species:

A small Neptune’s necklace grows here; otherwise algae is microscopic and the pool is superficially barren.

Molluscs:

  • Little blue periwinkle (Austrolittorina unifasciata)
  • Zebra winkle (Austrocochlea porcata)
  • Black nerite (Nerita sp.)

 

#11 Neptune’s Crater

11

Volume: 4 L

Temperature: While small, this pool’s proximity to the ocean keeps it cool. The shade from the many Neptune’s necklaces might also help.

Exposure: This pool rests in a wet site near the ocean edge of the Bluff, where it is doused several times a minute even in receding tides. As tide recedes, waves tend to strike the Bluff wall and then shower down on this pool. Action is intense and disruptive but not violent.

Two hours after high tide, the outlets to #10 and the ocean have subsided, and the inlets from waves are less regular; perhaps not even once a minute for a decent douse of spray.

Species:

Neptune’s necklace grows thickly here, fronds obscuring the surface of the small pool and providing plenty of cover for its inhabitants.

Anemones:

  • Green anemone (Aulactinia veratra)

Molluscs:

  • Black nerite (Nerita sp.)
  • Zebra winkle (Austrocochlea porcata)
  • Striped conniwink (Bembicium nanum)
  • Pacific oyster (Crassostrea gigas)

Crustaceans:

  • Swift-footed crab (Leptograpsus variegatus)

Arthropods:

  • Isopod ~

Annelids:

  • Spirorbid worm (Spirorbis sp.)

 

Conclusions:

12

Regular inlets do a lot for a pool. Those pools receiving regular wave and spray action are closer in temperature to the ocean, while those more isolated at the back of the Bluff or higher up can be even hotter than the air temperature.

This means one, that the degree of isolation is a vital component of temperature. Two, that isolation determines what species may survive and flourish in a pool. Also interesting to note is juvenile crabs seemingly pushed back away from the edge of the Bluff, itself a wall of good hiding places, to hunt in the landward pools.

  • Are these young crabs displaced from fertile hunting grounds by larger crabs?
  • Why are larger periwinkles (Austrolittorina unifasciata and Nodilittorina pyramidalis) found in the hottest and most isolated pools – is this to prevent competition with their young?
  • What role is played by salinity and water quality in determining species variation and distribution in the pools?
  • The ocean provides a stable temperature: at night as well as day. How does pool temp behave at night? More day testing is also needed for better results.
  • A cooler daytime pool seems to provide better species diversity, as does accessibility to the ocean. Is this actually because of more stable temperatures, or due to the likelihood of creatures being pushed into the pool by waves?
  • What is the role of substrate – rock, sand, algae – in keeping a “local” population in a pool?

Join us next time for more rock pool action on Danger Ball Z!

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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