Crevice pools sparkle with color

Crevice Pools are the long, narrow pools characteristic of the upper intertidal reef flat areas, and also the shallow mid-tidal pools.

All the organisms described for the Crevice Pool will not necessarily be found in every pool. The volume and tidal elevation, as well as the nature of the pool's bottom (i.e. gravel, cobble, sand, bare rock) will be important. However, the more common organisms will be well represented.

Figure 1. Observing the diversity in a shallow crevice pool at Seal Cove.

Figure 2. Colorful purple coralline algae submerged in a tide pool.

In all but the highest pools some form of coralline algae occurs. These red algae are called "coralline algae" because they incorporate calcium carbonate into their cell walls, which gives the plant a rigid, coral-like texture. Corallines occur as pale, pastel encrusting patches or sheets on the sides and bottoms of tide pools, while other species grow upright from holdfasts and have joint-like regions along their length. Because of their crusty texture, coralline algae are tough fodder for most intertidal herbivores and as such, their presence in tide pools is more assured compared to the softer, more palatable species of algae.

Another plant that may be seen here is known as surfgrass (Phyllospadix spp.). Surfgrass is not an alga. It is a flowering plant like the terrestrial grasses from which it has evolved. As such, it requires soil into which it can sink its roots to acquire the nutrients needed for growth. Therefore, surfgrass only is found in pools with sediment on the bottom.

Figure 3. Unmistakable bright green surf grass Phyllospadix sp.

Figure 4. Curious tide pool sculpin blends into the background of the tide pool.

Figure 5. Broken back shrimp Heptacarpus.

The swiftest animals in the pool are probably the small fish known as tide pool sculpins (Oligocottus & Clinocottus spp. and other genera). These fish remain almost motionless on the bottom, and their mottled color pattern makes them difficult to detect. Suddenly they will dart out and then they can be followed. Another rapidly-moving, but less often seen animal is the small broken-back shrimp (Heptacarpus spp.). These shrimp can be quite numerous in lower pools, yet their disruptive coloration and their inactivity during mid-day make them "phantoms" of the tide pools.

Figure 6. The bright blue leg bands identify the common hermit crab Pagurus samuelis.

Figure 7. A small aggregation of the black turban snail, Tegula funebralis, seek shelter along a moisture-trapping crevice.

Figure 8. A small clone of the aggregating anemone, Anthopleura elegantissima.

Hermit crabs (Pagurus spp.) move around the pool in broad daylight. Hermits carry their protection with them in the form of an empty snail shell into which they will quickly retreat if bothered. Otherwise they scavenge about the pools in search of animal and plant debris that serves as their food -- they are very effective scavengers! Most of the larger hermit crabs will be housed in the empty shells of the black turban snail, (Tegula funebralis), which may be found alive in the tide pools as well.

Many other carnivores occur in tide pools. The stationary hunters, the anemones, include the aggregating anemone (Anthopleura elegantissima) and its much larger relatives, the giant green sea anemone (Anthopleura xanthogrammica), and the sunburst anemone (Anthopleura sola). Aggregating anemones are abundant throughout the Reserve and occur exposed on rocks as well as in tide pools. This anemone grows to a certain size and then divides in half, forming genetic clones that can cover several square feet of substrate. The giant green anemone is found in lower tide pools, and reaches a maximum size of 8 - 10 inches in diameter in the surge channels of the low intertidal. The sunburst anemone, distinguished by the sun-ray pattern of lines found on its oral disc, is only slightly smaller than the giant green, and individuals 6 - 8 inches across may be found in these crevice pools.

The proliferating anemone (Epiactis prolifera) a smaller anemone delicately patterned with white lines, is also found in tide pools. It ranges in color from cherry red to bright green. Unlike other anemones, which attach to the solid bedrock, the proliferating anemone is sometimes found attached to loose rocks, shells, or even blades of algae.

Figure 9. Two giant green sea anemones, Anthopleura xanthogrammica.

Figure 10. Strong radiating lines on the oral disc identify the sunburst anemone, Anthopleura sola.

Figure 11. Bright white lines on the column distinguish the proliferating anemone, Epiactis prolifera, from the other tide pool sea anemones.

Figure 12. The small but common six rayed sea star, Leptasterias hexactis.

Figure 13. The beautiful leather star, Dermasterias imbricata, is a predator of sea anemones.

Figure 14. The Pacific sea star, Pisaster ochraceus, occurs in both purple and orange varieties.

Figure 15. The sea bat or bat star, Asterina miniata, is a common omnivorous scavenger of tide pools.

Mobile predators, including sea stars, use the pools. Some sea star species have a truly intertidal distribution, while others appear to forage into the intertidal zone from below during high tide and retreat into tide pools if trapped in the intertidal zone when the tide recedes. The small, six-rayed sea star (Leptasterias hexactis), appears to remain in the intertidal zone and often shows up in the tide pools where it searches for small molluscan prey. The larger leather star (Dermasterias imbricata), may also be found in tide pools. This smooth-skinned star feeds on sea anemones. The ochre or Pacific sea star (Pisaster ochraceus) is a more generalized predator which will feed on a wide variety of invertebrate prey. Although capable of withstanding complete exposure during low tide, the Pacific sea star often comes to the tide pools.

The sea bat (Asterina (Patria) miniata), is also a common tide pool inhabitant. This orange, web-rayed sea star will eat almost anything and has an incredible reversible stomach to accomplish the task!

If a mid-level pool is fairly deep and contains small boulders and a little sediment, another group of animals may occur. Most prominent will be the red rock crab (Cancer antennarius), which may be quite large -- up to 4+ inches across the carapace -- although larger specimens are usually found in the lower intertidal subhabitats. This crab has black-tipped claws which it uses for crushing the shells of its prey, mollusks, and hermit crabs. Specimens of the red rock crab are typically found under rocks and burrowed into the sediment with only a portion of their carapace showing. At high tide they emerge and prowl the bottom. Another crab in this under-rock hiding place is the small pebble crab (Lophopanopeus bellus). This grab is grey to buff colored and also has black tipped claws.

Figure 16. The red rock crab, Cancer antennarius, is distinguished by its long antennae and red splotches on its ventral surface.

Figure 17. The pebble crab, Lophopanopeus bellus, will often “play dead” when uncovered.

Figure 18. The small claws of the kelp crab, Puggetia producta, are deceptive, it can deliver a sharp pinch!

If there is a substantial amount of algae in a pool, it may produce a spidery-looking kelp crab, (Pugettia producta). This crab ranges in color from green to dark brown and blends with algal cover. The young kelp crab feeds on algae and as it grows, it moves downward in the intertidal zone and takes on a more carnivorous diet.

A final under-rock denzien of tidepools is the small fish with the elongate eel body form, a prickleback (Xiphister sp.). There are several species of eel-like blennies that occur and these are generally 2 - 5 inches in length. Larger specimens move downward in the intertidal and into the subtidal. These small fish will also be found under exposed rocks throughout the middle and lower intertidal zones, as will the other under-rock tide pool species.

Figure 19. A small specimen of the prickleback eel, Xiphister sp.