Boiler Bay

Boiler Bay rocky intertidal (north of the
small sandy cove). Is this the boiler from
which the site gained its name?

Although I lived in Oregon for several years (including a year and a half on the coast), I did less exploration of the rocky intertidal during those years than I would typically do during the same time span living in California. Oregon’s exceptionally scenic coastline features dunes, sandy beaches, rocky shores, and numerous estuaries. It was the latter ecosystem that I explored the most, working up and down the coast to research Oregon’s tidal wetlands.

For my last rocky shore visit of this month’s spring tide series, I wandered about the shores of Boiler Bay, a small cove north of Depot Bay on the central Oregon coast. A short steep trail leads to a small sandy beach at the head of the cove, with rocky intertidal benches to the north and south. Boiler Bay is apparently a well studied area, as evidenced by bolts and other obvious scientific interventions along the shoreline. I believe it may be a study site for Bruce Menge’s long-term study of Oregon’s rocky intertidal communities. South of the cove there was even a collection of dozens of small pools cut into the bedrock, too uniform in size and location to not be the hard work of a former research project, perhaps the sweat and tears of a graduate student dissertation from years past.

Various evidences of scientific interventions to study the rocky shore at Boiler Bay.

After observing for a few hours, an obvious ecological story of the Boiler Bay rocky intertidal is that of plant versus herbivore. Along the rocky benches south of the cove, large areas – dominated by purple urchins (Stronglyocentrotus purpuratus) in the thousands – are clearly claimed by herbivore. I don’t ever recall seeing intertidal urchins in such high densities anywhere else I’ve wandered on the west coast. The power of the urchins to shape this segment of coastline has even formed a geologic imprint: many of the urchins rest in small cavities carved into the rocks, the gradual work of spines that have eroded the mudstone over however many generations.

Small bull kelp in a tide pool at Boiler Bay.

Large fleshy seaweeds are prolific in other areas, away from the urchins. This means they are more abundant at higher shoreline elevations where urchins probably can’t tolerate the prolonged exposure to air, or in lower tidal areas where urchins perhaps aren’t able to gain high densities for other reasons. The large kelp Saccharina sessile grows in abundance higher on the shore out of reach of the grazing hordes, while Alaria marginata and Nereocystis luetkeana (bull kelp) occur in low intertidal pools or other fortuitous havens of safety. The elegant bull kelps typically occurred in aggregations as small to medium plants that will never reach the much larger stature that plants forming thick forests offshore will attain.

There is one notable kelp species that seems more impervious to the grazing menace of the purple urchins: Costaria costata. Having various common names like seersucker or five ribbed kelp, this striking species consists of a single, highly ruffled brown blade growing out of the base of the plant. Within the extensive urchin enclaves of the low intertidal at Boiler Bay, Costaria was present in remarkable abundance. The plants were typically small, and often tattered, but it was basically the only large seaweed common in the “barrens” where the purple urchins ruled.

Costaria costata sporophytes with abundant purple urchins in the low intertidal zone.

Costaria’s secret may lie in part from being an annual species. With quick growth in the spring, some plants may escape herbivory long enough to produce spores to continue the life cycle. Another possibility is that Costaria is a less preferred food source for the hungry urchins. Alternatively, competition might limit Costaria to urchin-dominated areas. In areas where perennial kelps like Laminaria setchellii or Pterygophora californica are present, perhaps the annual Costaria seldom gains the upper hand in competition with the established plants for space and light.

One important ecological force structuring coastal rocky shore ecosystems in Oregon however, lays obscured in history. Before being hunted severely to near extinction, the sea otter of the northeastern Pacific was a key predator of urchins, keeping grazers in check and promoting kelp abundance. This example of a marine “trophic cascade” was described decades ago in work done by ecologist Jim Estes. In a trophic cascade, healthy predator populations keep herbivore abundance low which favors primary producer populations. Today however, sea otters are essentially absent from Oregon’s coastline, so a key link in the historic web of connected coastal species is missing. Are Oregon urchin populations much higher today than they were several hundred years ago when otters were present? Whatever combination of mechanisms underlies the spatial patterns of distribution of urchins and Costaria, the observations suggest an interesting story.

Sponge in a low intertidal pool.

Large red urchins, Stronglyocentrotus franciscanus, were also relatively common at Boiler Bay, though their numbers are dwarfed by their smaller purple cousins. These grazers tended to be underwater (e.g., in pools), perhaps less tolerant of being exposed to air than their congener. Other intertidal invertebrates included beds of mussels, chitons, a few species of sea stars, and aggregating and solitary anemones. I observed no nudibranchs, but did see a few bright yellow sponges in the low intertidal and a lovely crab which I may try to identify when I am back in the company of a good reference book. 

Low intertidal crab at Boiler Bay.

Notes on some sea urchins of Hawaii

Map of the Big Island with intertidal and subtidal sites I
visited this fall. Map modified from this source.

Sea urchins were one of the prominent groups of marine invertebrates I encountered while exploring the intertidal and shallow subtidal of the Big Island’s coastline. Click here for a checklist of species published by the Bishop Museum. Some notes on a few common species:

1. The slate pencil urchin, Heterocentrotus mamillatus. This was the most striking of the species I found on the Big Island. Thick brick-red spines emerge from the body which is whitish or deep red. This species was present in both the low intertidal and the subtidal, including in coral reefs where it was often found in crevices. I was common along the Kona side of the Big Island.

Heterocentrotus mamillatus. Left: low intertidal urchin from Miloli'i. Right: subtidal animal at Old Kona Airport.

2.  Echinometra mathaei. This species was the most abundant overall in my coastal visits, commonly occurring in both intertidal pools and on subtidal reefs in high densities. The test diameter and spine length are both on the order of a few cm. Body color varied from a pale green to pink color. This species reminded me of Strongylocentrotus purpuratus, the common purple urchin of coastal California waters, in terms of size and morphology, and by its tendency to hide in small holes in the rocky substrate, perhaps sheltering there from predators.

Echinometra mathaei. Left: At Miloli'i; right: at Wai'opae.

The collector urchin, at Puako Bay.

3. The collector urchin, Tripneustes gratilla. Typically a subtidal species, it is purple to black overall, but often has at least some whitish and reddish spines. This species was common at Puako Bay where living coral cover appeared sparse, but there was significant cover of encrusting red coralline algae. It was also present at Honaunau Bay, Keauhou, and the Old Kona Airport on the Kona side of the island. The collector urchin is distributed in tropical waters of the Pacific and Indian Oceans.

4. The helmet urchin, Colobocentrotus atratus. This is a common intertidal species, usually black in color. Most of the spines on the body have been reduced to armor-like plates, while those near the sides are elongated to form a skirt around the animal. This species appears to favor more wave-swept coastlines and the overall shape probably minimizes hydrologic drag on the body. This species is probably a major intertidal herbivore, suggested by the barrens of pink corraline algae I often found in the vicinity of the animals. I observed this species at Ka Lae, Miloli’i, the Old Kona Airport, and Waikui Beach in the south Kohala District.

Colobocentrotus atratus. Both photos are from the intertidal at Keauhou, south of Kona.

5. Echinothrix calamaris, the banded urchin. This is a larger black urchin with menacing spines, occurring as either an all-black morph, or with white and black-banded spines. From my observations, the black morph was much more common on the Big Island. I found this species from intertidal tidepools to the subtidal. It was common at Waikui Beach and the shoreline near the Old Kona Airport. Also present at Honaunau Bay.

Two color morphs of Echinothrix calamaris. Left: subtidal at Honaunau Bay; right: subtidal at Waikui beach.

 

Close-up of Echinometra mathaei, with tube feet visible among the spines (white arrows).

Subtidal slate pencil urchins in coral at Waikui beach.

Reference

Stender K, Yuko K. 2014. MarinelifePhotography.com.