In these tide pools, aliens make their presence known

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.  

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.

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 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.

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.

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.

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. 

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.

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 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 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.

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.

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.

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.

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).

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, 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.

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.

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

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 the water's surface toward the left.