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In these tide pools, aliens make their presence known

Up close, starfish, sea slugs and shrimp look like aliens. The California Academy of Sciences tracks their population near San Francisco.

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Stephen Shankland
A tiny baby sea star, Dwarf mottled Henricia (Henricia pumila), seems to wave from its perch on my fingertip.
1 of 27 Stephen Shankland/CNET

Baby dwarf mottled Henricia sea star

I photographed this tiny baby starfish, a dwarf mottled Henricia (Latin name Henricia pumila) as it seemed to wave from its perch on my fingertip. I took the shot during a California Academy of Sciences survey of tide pools of Pillar Point, south of San Francisco on a citizen science excursion to track sea creatures at the site every two weeks.  

Leather star arm
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Leather star arm

A curled arm of a leather star (Latin name Dermasterias imbricata) at Pillar Point. On the underside are some of its hundreds of tube feet, and on the red patches on top are transparent papulae the sea star uses like fish gills to extract oxygen from water.

Cockerell's dorid (Limacia cockerelli) nudibranch
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Cockerell's dorid (Limacia cockerelli) nudibranch

A Cockerell's dorid (Limacia cockerelli) nudibranch creeps over a rock in a Pillar Point tide pool. This nudibranch, about an inch long, has two red-tipped scent receptors called rhinophores on the upper right. Around its perimeter are orange-tipped protrusions called cerata. Across its rump are red-tipped frills called branchia the slug uses to extract oxygen from the water. The term "nudibranch" translates to "naked gills" and refers to this external respiration system.

California Academy of Science researchers Alison Young, left, and Rebecca Johnson use out tape measures to mark one of the six plots at Pillar Point they survey every two weeks during low tide. The surveys are part of the science museum's citizen science program.
4 of 27 Stephen Shankland/CNET

Measuring the survey plot

California Academy of Sciences researchers Alison Young, left, and Rebecca Johnson use tape measures to mark one of the six plots at Pillar Point they survey every two weeks during low tide. The surveys are part of the science museum's citizen science program.

Sunburst sea anemone by ultraviolet light
5 of 27 Stephen Shankland/CNET

Sunburst sea anemone by ultraviolet light

I photographed this sunburst sea anemone by ultraviolet light. It's not actually the anemone that fluoresces, but instead algae and single-celled creatures called dinoflagellates that live symbiotically with the animal. Its mouth is in the center; at the outer rim are purple-tipped feeding tentacles.

A great blue heron hunts in the calm waters of Pillar Point Harbor.
6 of 27 Stephen Shankland/CNET

Photographing nudibranchs

The California Academy of Sciences researchers and volunteers who came along on the survey photographed creatures they found for the iNaturalist service. They're photographing a Chan's dorid, a nudibranch with the Latin name Hallaxa chani. For tide pools, waterproof cameras help.

A shrimp -- likely a broke-back shrimp of the family Heptacarpus -- glows green under ultraviolet light atop a bed of seaweed.
7 of 27 Stephen Shankland/CNET

Glowing shrimp

A shrimp -- likely a broke-back shrimp of the family Heptacarpus -- glows green under ultraviolet light atop a bed of seaweed.

An orange-peel dorid (Acanthodoris lutea)
8 of 27 Stephen Shankland/CNET

Orange-peel dorid

An orange-peel dorid (Acanthodoris lutea), a small nudibranch. You can see two scent receptors called rhinophores, darker protrusions toward the top of the photo. The whitish tuft at the opposite end is a branchial plume used to extract oxygen from the water. 

Citizen science tide pool search at Pillar Point
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Citizen science tide pool search

Alison Young, lower left, points to a nudibranch she found in a tide pool at Pillar Point, about 4 miles north of Half Moon Bay, California. Behind her, from left to right, are Jane Kim, Thayer Walker and Katie Bertsche.

Sunburst sea anemone
10 of 27 Stephen Shankland/CNET

Sunburst sea anemone

A brilliant blue-green sunburst anemone (Anthopleura sola) is a solitary sea anemone. It's a carnivore that eats crabs, mussels and small fish.

Chan's dorid, a nudibranch
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Chan's dorid, a nudibranch

Chan's dorid, a nudibranch with the Latin name Hallaxa chani, creeps around a tide pool at Pillar Point. It's rare to see this species in this location.

Alison Young photographs a brilliant sunset just after low tide at Pillar Point about 15 miles south of San Francisco. The rocks of Pillar Point just out into the Pacific Ocean toward the left; the Mavericks surfing competition takes place in the waves toward the right when winter storms create enormous swells.
12 of 27 Stephen Shankland/CNET

Pillar Point sunset

Alison Young photographs a brilliant sunset just after low tide at Pillar Point about 15 miles south of San Francisco. The rocks of Pillar Point jut out into the Pacific Ocean toward the left. The Mavericks surfing competition takes place in the waves toward the right when winter storms create enormous swells.

Bay ghost crab
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Bay ghost crab

Unusually, both male and female bay ghost crabs have one enlarged claw. They live in the sand. We retrieved this one using a "slurp gun" that sucks up a sample of low-tide mud and sand for easier scrutiny of what's in the muck.

Slurp gun
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Slurp gun

Alison Young of the California Academy of Sciences uses a "slurp gun" to find creatures that live in the low-tide sand and mud.

An ochre sea star (Pisaster ochraceus) curls into a pocket on a loose rock at the Pillar Point tide pools south of San Francisco.
15 of 27 Stephen Shankland/CNET

Ochre sea star

An ochre sea star (Pisaster ochraceus) curls into a pocket on a loose rock at the Pillar Point tide pools south of San Francisco.

California Academy of Science researcher Alison Young measures a tide pool survey plot.
16 of 27 Stephen Shankland/CNET

Wading for science

California Academy of Sciences researcher Alison Young measures a tide pool survey plot.

A leather star, Latin name Dermasterias imbricata, flexes its arms at low tide to keep itself mostly underwater.
17 of 27 Stephen Shankland/CNET

Leather star

A leather star, Latin name Dermasterias imbricata, flexes its arms at low tide to keep itself mostly underwater.

Up close, you can see this leather star's transparent papulae, protrusions it uses to extract oxygen from water like fish gills.
18 of 27 Stephen Shankland/CNET

Leather star papulae

Up close, you can see this leather star's transparent papulae, protrusions it uses to extract oxygen from water, like fish gills.

Two nudibranchs, small and often colorful sea slugs, inhabit a tide pool at Pillar Point. At left -- just next to the fingertip -- is a Cockerell's dorid (Limacia cockerelli). At right is an orange-peel dorid (Acanthodoris lutea).
19 of 27 Stephen Shankland/CNET

Two nudibranchs

Two nudibranchs, small and often colorful sea slugs, inhabit a tide pool at Pillar Point. At left -- just next to the fingertip -- is a Cockerell's dorid (Limacia cockerelli). At right is an orange-peel dorid (Acanthodoris lutea).

Gumboot chiton, aka wandering meatloaf
20 of 27 Stephen Shankland/CNET

Gumboot chiton, aka wandering meatloaf

This gumboot chiton is the largest example of a type of armored mollusk called a chiton. Gumboot chitons, affectionately called wandering meatloafs, are the largest chitons, growing to more than a foot long, but this one was about a third that long. It's got a sort of skin called a girdle that covers its eight dorsal plates, tinted red because of red algae it eats.

Katie Bertsche logging a find
21 of 27 Stephen Shankland/CNET

Katie Bertsche logging a find

Katie Bertsche, who works with the Science at Cal program at the University of California, Berkeley, photographs a find at a Pillar Point tide pool with her phone.

A great blue heron hunts in the calm waters of Pillar Point Harbor.
22 of 27 Stephen Shankland/CNET

Great blue heron

A great blue heron hunts in the calm waters of Pillar Point Harbor.

A great blue heron hunts in the calm waters of Pillar Point Harbor.
23 of 27 Stephen Shankland/CNET

Dead Dungeness crab

A Dungeness crab shell rests on the sandy beach near Pillar Point.

This seemingly feisty six-rayed star, a member of the Leptasterias family, lost one of its arms, now just a small nubbin.
24 of 27 Stephen Shankland/CNET

Baby six-rayed star

This seemingly feisty six-rayed star, a member of the Leptasterias family, lost one of its arms, now just a small nubbin.

A juvenile black-crowned night heron wades through low-tide seaweed near Pillar Point.
25 of 27 Stephen Shankland/CNET

Juvenile black-crowned night heron

A juvenile black-crowned night heron wades through low-tide seaweed near Pillar Point.

A larger nudibranch, Heath's dorid (Geitodoris heathi), was about 3 inches long. It looks like a scrap of yellow leather, with two brown-tipped rhinophores at its head end toward the left and pume of gray-tipped branchia at its rear toward the right.
26 of 27 Stephen Shankland/CNET

Heath's dorid

This nudibranch, Heath's dorid (Geitodoris heathi), was about 3 inches long. It looks like a scrap of yellow leather, with two brown-tipped rhinophores at its head end toward the left and plume of gray-tipped branchia at its rear toward the right.

A sunburst sea anemone glows under ultraviolet light. The ultraviolet light itself is reflected on on the water's surface toward the left.
27 of 27 Stephen Shankland/CNET

Sunburst sea anemone and ultraviolet light

A sunburst sea anemone glows under ultraviolet light. The ultraviolet light itself is reflected on the water's surface toward the left.

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