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.

The northern Hawaiian coast

Geographic regions of the Big Island. Map source

The northwestern district of Kohala and the Hamakua coast were among the final regions we visited while making a loop around the Big Island. Like Kau to the south, these areas less populated than the Kona and Hilo coasts, scattered only with small towns, farms, and impressive vistas of open space. There are a series of green valleys sculpted into the northeast-facing coast of Hamakua, largely inaccessible except by hiking.

From the North Kohala side, at the terminus of state road 270, is Pololu Valley. It appeared to have an estuary of sorts, with a sediment-laden chocolate stream that wound back into the valley. The mouth of the stream had a shallow connection to the ocean because of the buildup of large cobbles on the beach. Visible to the east were cliffs and a few offshore islets. The wind was strong here, stirring up numerous white caps out at sea. To me, the whole scene was reminiscent of portions of central California in some degree, like the rough coastal gem of the Big Sur area. 

The northwest coast of Hamakua from the overlook
above Waipio Valley. A waterfall nearly reaches
the beach.

Waipio Valley is also among this chain of coastal valleys. The valley used to be a prosperous settlement, both during Polynesian rule of the Hawaiian Islands and more recently. Hawaiians referred to it as the ‘Valley of the Kings’ (Barth 1995). A devastating tsunami in 1946 obliterated the area, and resettlement has been sparse. From Waipio Valley, there is a trail that heads northwest to the next coastal valley (Waimanu Valley) which would be a treat to backpack some day.

Down in Pololu Valley, the shore was rough with wind, waves, and a beach covered in cobbles of lava rock worn smooth by the surf. There also was a significant amount of debris – bottles, fishing buoys, decaying buckets. These items may have traveled thousands of miles to land on the shores of Hawaii. With some help from the kids, we gathered up a number of items to remove them from the beach.

Pollution is a distinctly visible reminder of humankind’s impact to our most cherished natural areas. It is particularly unwelcome on our beautiful coastlines. Yet, visibility aside, it is probably one of the lesser human impacts to the oceans. Habitat destruction, overfishing (which disrupts entire food webs), and changing climate are perhaps more pernicious threats to ocean biodiversity and function.

Pololu Valley.

A black cobble beach at Pololu Valley and the cliffs of the Hamakua coast in the distance.

Flying for hours over the vast Pacific, the immensity of the global ocean is made apparent. Individually, we are barely specks on this immense globe, so it is perhaps natural to assume that our species can have little impact on this globe. Yet evidence from every line of science points otherwise. When tsumani debris bearing living organisms arrives on the shores of Oregon from Japan (as I was able to observe firsthand a few years ago), it becomes apparent just how close supposed distant places are to each other. Our collective human impacts on our planet have become pervasive.

The kids helped gather marine debris washed ashore at Pololu Valley.

Coastal topography near Pololu Valley, from 1995 USGS topographic map, Honokane, HI.

Reference

Barth S. 1995. The Smithsonian Guides to Natural America. The Pacific. Smithsonian Books, Washington D.C.

Wai'opae pools

Wai'opae pools among basalt bedrock and boulders.

The Big Island has about 6 Marine Life Conservation Districts, marine reserves established by the state of Hawaii that variously protect coastal marine organisms from harvest. A short stretch of coastline containing the Wai’opae pools is the only MLCD on the eastern end of the island. The site is southeast of Hilo, near the town of Kapoho, tucked behind a few coastal residences.

At Wai’opae the dark lava is spread in a wide flat bench that creates a broad area of pools between emergent rocks. Some of the pools are shallow and might be properly considered “tide pools”, but others are deeper and create permanent subtidal habitat. Partly isolated from each other, each pool is a small reef ecosystem.

Purple ember parrotfish, Scarus rubroviolaceus followed by an endemic saddle wrasse, Thalassoma duperrey among convict tangs.

Closeup of coral polyps.

 Saturday I spent several hours exploring five pools, ranging in size, depth and species composition. My son Ben located the first, a deep pool with a few corals and a moderate number of fish located far inshore from the breaking waves off shore.

Later I spent a little time in a round pool not wider than 5 or so few meters across, with black basalt enclosing a cozy aquarium. One side of the pool was shallow, the other deep, but each with a number of brown plate corals. The colonies of this species form rounded shelves against the rocks like shelf fungi that grow on trees or fallen logs in a temperate forest.

The last pool I explored was the largest and had the most obvious active connection to the ocean, indicated by a strong current present at the southeastern end. I was already cold at this point in the afternoon, but I probably ended up staying another hour in this pool. The axis of the pool was like a shallow V-shaped valley, along which I swam back and forth several times. Fish abundance in the pool was remarkable, and included wrasses, butterflyfish, large purple and greenish parrotfish, and a dense school of striped convict fish accompanied by several large black colored fish that swam with their smaller associates. Blue needle fish were common in the surface water, creating a ring around me, but keeping some distance.

Unidentified blenny perched in coral in a shallow pool.

The pools I observed did not have continuous coral cover. In fact, at some had just a few percent cover. Rather, corals appeared as discreet colonies, with bedrock, volcanic rubble, or fuzzy algal turfs occupying the rest of the space. They seemed absent from the shallowest areas, perhaps because these are exposed a low tides. Lobe (Porites sp.), lace, and plate corals were common. In the pools I found few species of larger macroalgae, but notable exceptions were a single siphonous green alga (?Caulerpa sp.) shaped not unlike an immature bunch of green grapes, and dichotomously branched thin blades of an attractive red foliose alga. Larger invertebrates included beautiful slate pencil urchins and two species of holothuroideans (sea cucumbers).

A school of convict fish.

Green siphonous macroalga, perhaps Caulerpa sp.

Plate coral.

Needlefishes swimming near the water surface.

Hawaiian whitespotted toby, Canthigaster jactator.

The common saddle wrasse, Thalassoma duperrey. Many wrasses change sex over
the course of life. This fish is an "early stage" male or female. "Supermales" have a
white vertical mark behind the reddish band.

References

Hoover JP. 2014. Hawai’i’s Fishes. A Guide for Snorkelers and Divers. 2^nd ed. Mutual Publishing, Honolulu.

Krupp D. 2010. Hawaiian Coral Index Page: http://krupp.wcc.hawaii.edu/BIOL200/hawcoral/corindex.htm

Mahaney C, Witte A. 1993. Hawaiian Reef Fish. Blue Kirio Publishing.