Nature Watch: Rockin’ the Low Tides

Author: Steve Ellis | 06/27/24

About the author: Steve Ellis is a naturalist and Land Trust member who enjoys sharing his love of the natural world. His blog series, “Nature Watch,” will appear each month in “Habitchat,” chronicling native plants and wildlife you can expect to see during that particular time of year. We thank Steve for sharing his passion, illustrating the importance of island conservation.


Summer is the perfect time to explore our islands’ beaches. Not only is the weather congenial, but low tides in this season tend to be during daylight hours.

There are four major beach types: sandy, mudflat, rocky and cobble/gravel. Some beaches are a mixture of these various types.

Rocky shores offer the greatest diversity and abundance of species. There are more surfaces to cling to and niches to shelter in than found on sand or mudflats.

As with all beach types, rocky shores are affected by the twice-daily tidal rise and fall. This action creates life zones such as encountered on mountain slopes. The higher on the beach, the harsher are the living conditions.

Middle and low tide zones offer the most stability for life. Blue mussels, aggregate anemones, whelks {snails}, and various chitons populate the middle zone while purple shore crabs, sea urchins and nudibranchs live in the low tide zones.

Various fish dwell here too. Look for tidepool sculpin, gunnels and pricklebacks.

Aggregating Anemone by Martha Ellis.

One of the more intriguing species is the northern clingfish. Growing to six inches in length, they have a flattened, round head and tapered body. Their pelvic fins form a suction disc that anchors them to favored resting spots. Their diet consists of worms, amphipods and other small invertebrates. You can spot clingfish in tidepools and on the sides of rocks.

Aggregating anemones are a standard feature of rocky shores that include some sand. When covered by water, they display pale green tentacles tipped in pink or purple. As the tide recedes, they pull in their tentacles and resemble large green olives with a bit of pimento ringing the opening. The name aggregating describes the colonies they form under good conditions.

Finding the right tide is key if you want to experience this habitat. One hour before the lowest tide to one hour past is the best time to be on the beach. Pick a day with a minus two or three tide.

Sea lettuce and other algae can be very slippery. Rocks with barnacles are sharp and unforgiving to human skin in a fall. Watch your step!

Proper beach etiquette is crucial. Please resist the urge to look under every rock, as many creatures are killed by uncaring beachgoers. Carefully replace any rock you pick up, so the animals and algae can continue to live. Step carefully – that crunch you hear comes from the shells of barnacles.

Rocky shores can remain vibrant with life if we treat them and the surrounding waters with care.

Sea lettuce can be found on any rocky beach below the high tide zone. Photo by Martha Ellis.

Flora: Sea Lettuce  (Ulna species)

The two seaweeds in the Ulna species are the most common green algae found on rocky marine habitats. They grow throughout the intertidal and subtidal zones.

Sea lettuce grows from a holdfast secured to a rock or other algae. It lacks the stipe (stem) of kelp and other seaweeds. It grows instead as thin, lettuce leaf-like sheets that measure up to seven inches in length and one foot across.

This alga grows by absorbing minerals and carbon dioxide from sea water and photosynthesizes using sunlight. The holdfast remains throughout the year, but winter storms break apart the fronds.

Many small invertebrates shelter in sea lettuce. In addition, this alga is eaten by sea urchins, some snails, and by purple shore crabs. We once watched a small flock of Brant geese devouring large “beakfuls” of sea lettuce.

Look for sea lettuce on any rocky beach below the high tide zone.

Purple shore crab hatching occurs between April and July. Photo by Martha Ellis.

Fauna: Purple Shore Crab (Hemigrapsus nudus)

The most abundant crab species on rocky shores is the purple shore crab.

Adults are two inches wide and have a square-shaped carapace (body shell). Their colors range from purple or reddish brown to olive. The legs are hairless and the claws have white tips. Males tend to be bigger overall but females will have a wider abdomen.

Purple shore crabs are tough. They tolerate changes in salinity and can survive in or out of the water. Fluctuations in air or water temperature are also handled easily.

Mating occurs in December and January. Soon afterwards, females will lay 400 to 36,000 eggs, with larger females producing bigger clutches. Hatching follows between April and July.

Part of the beach “clean up crew,” purple shore crabs feed on detritus, diatoms, amphipods, snail eggs, sea lettuce and carrion. Most of this feeding is accomplished at night.

Gulls and other seabirds prey on purple shore crabs. Other predators include fish and larger crab species.

Purple shore crabs can be found near the high tide line down to low tide. They scuttle between rocks and may be found in tide pools. A large part of their day is spent hiding under rocks. Please allow them these daytime sanctuaries and don’t remove their preferred rocks.

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