Passing into deeper water from the eelgrass beds found in the shallow nearshore environments of many embayments of the American San Juan Islands, the highly organic muddy sand substrate is typically replaced by a less organic or “cleaner” mixture of sand and silt. Such a transition is certainly the case in Cowlitz Bay of Waldron Island. I can verify that the silty-sand substrate continues to, at least, a depth of 50 m (165 feet). Except for emergent rocky outcrops, this habitat type is likely characteristic of all the deeper water of Cowlitz Bay and the nearby San Juan Channel and Boundary Passages.
The tidal ranges that distinguish this region, coupled with its geography, mean that high tidal currents are the norm, and the volume of tidal water movement is immense. All of this, added to the dense, rich plankton found in those waters creates a habitat that is probably nearly optimal for suspension feeders. As a result, virtually all of the hard subtidal real estate is occupied some sort of organism specialized to grab food or nutrients from the water moving past them. Subtidal rocky substrates are often characterized by dense populations of suspension-feeding epifaunal sea cucumbers. And, although it may seem unlikely, some of the unconsolidated, silty-sand, habitats are also dominated by dendrochirote holothurians, albeit in this case these cases they are infaunal, not epifaunal. Infaunal sea cucumbers dominate the subtidal Cowlitz Bay benthic environment below 10 m.
Pentamera individuals extending from the bottom of Cowlitz Bay, 11 July, 1977. The abundance of the adult animals exceeds 20,000/m2 (about 0.2 m2) is visible.
Although a few other species are rarely found, the vast majority of these suspension-feeding, infaunal cukes belong to a few species of Pentamera. The individuals belonging to the different species are relatively similar in size, shape, and coloration making them effectively indistinguishable in the field by non-specialists, so I will refer to them all as Pentamera. Living buried in the sediments they feed by extending a small portion of the oral end of the body above the sediments. This exposes just a bit of the animal, primarily the mouth, and its surrounding crown of highly-branched feeding tentacles.
White, and only about 2 or 3 cm long, these relatively small sea cucumbers are often found in beds so very dense that in the summer, the benthic sediment appears snow-covered due to the many tentacles visible. In the clear water of the late autumn and winter plankton-free periods, these holothuroids do not feed. Presumably quiescent, they remain buried under the sediment surface. During these seasons, the habitat looks relatively barren; with only scattered larger animals, such as individuals of tube anemones, Pachycerianthus fimbriatus, orange sea pens, Ptilosarcus gurneyi, snake-skin stars, Luidia foliolata, sunflower stars, Pycnopodia helianthoides, or weather-vane scallops, Patinopecten caurinus being evident to the casual observer.
With beginning of the diatom bloom starting in February, smaller life “returns to”, or more correctly, becomes evident again on the benthos. The sediment becomes covered completely with a thick and rather ugly, dense dark brown film, consisting of several species of microalgae, primarily diatoms and dinoflagellates.
By early March, many turbellarian flatworms of several visually distinctive types are commonly found gliding over the brown algal film and sediments. These small worms, each only a few millimeters long, may be distinguished by their differing shapes and color patterns. Although common, at least in the spring, virtually nothing is known of their natural history. Shortly after the worms become common, small caprellid amphipods, otherwise known as “skeleton shrimp”, seem to appear out of nowhere and are soon found covering the diatom film. These small, about a centimeter (0.4 inch) long, animals reproduce rapidly and soon reach abundances around 1 animal per square centimeter, or a density of 10,000 animals per square meter. As they become common, pelagic predators, such as ctenophores and chaetognaths, may be observed grabbing copepods off the bottom and swimming back up into the overlying water.
By the middle of March, the spring plankton are in full bloom and the Pentamera are beginning to feed. By moving up and down in the sediment, the resulting bioturbation soon destroys the diatom film, and the sediment becomes relatively clean again. Snake-skin stars, Luidia foliolata, are common in this habitat where these sea cucumbers are their principal prey. Caprellid amphipods, Caprella gracilior, and small hermit crabs are often seen on the aboral surface of the stars. The Luidia-sized, star‑shaped, feeding depressions, along with the small piles of regurgitated remains attest to the star’s feeding habits. Pycnopodia helianthoides is also commonly found in these beds and may also feed on the sea cucumbers. Some aspects of the natural history of Luidia in this habitat will be discussed in subsequent post.
Individuals of the large, up to 15 cm (6 inches) in diameter, weathervane scallops, Patinopecten caurinus, rather rare elsewhere in the San Juans, are found not uncommonly in these cucumber beds. They are found lying in shallow, somewhat bowl-shaped, depressions probably created over time by the scallops’ feeding currents which might gently displace and excavate the sediments. Eaten by the sunflower star, the scallops will swim in response to being touched by the predator. They are not particularly vigorous swimmers, however, nor do they seem to start swimming immediately, thus they could be captured relatively easily. Their shells are a common feature in this habitat, so presumably some predators are capturing them. These large shells, either living or dead, provide about the only hard substrate in these habitats, and are often covered with barnacles, algae, or occasionally attached bryozoans or hydroids.
The tube-dwelling anemone, Pachycerianthus fimbriatus, is particularly common in this habitat, and becomes very abundant just below the dense Pentamera beds in the more silty habitats of the steeply sloping areas. Pachycerianthus individuals may be colored either gray or a dark brown to maroon. These do not appear to represent separate species, and the different colors have no known significance. Close examination of the anemones will show some very small epifaunal, possibly stenothoid, amphipods visible as small dots moving over the anemone’s body and tentacles. During the spring and early summer periods of dense plankton, it is possible to watch the Pachycerianthus catch copepods, and other small crustacean zooplankton, with their long tapering, thin, tentacles.
These slightly deeper habitats where Pachycerianthus is most common, ranging downward from about 10m (33 feet) in depth, have a silty sand substrate. Pentamera are found in these regions, they are just not as abundant as they are in the dense assemblages in shallower water. Individuals of the orange sea pen, Ptilosarcus gurneyi, are well represented in these deeper habitats, and although they are not as abundant here as they are in the dense sea pen beds of the lower Puget Sound region, they are nonetheless found relatively frequently. Occasionally, a different type of pennatulacean, a sea whip, may be found. In the genus Virgularia, these whips are narrow pennatulaceans, with short “leaves”. At least two species within this genus found in our waters and they are not terribly difficult to distinguish in the field. The species found in Cowlitz bay is small, tan to whitish, with small “leaves” and is seldom over 15 cm (6 inches) in height. The feeding zooids often appear to arise from directly from the central stalk. The other species, found in other areas, such as Lopez Sound, is larger and more robust, pink to orange, and often reaches heights of 50 or more centimeters. This species has larger relatively distinct “leaves” with the gastrozooids on them.
Several nudibranch species are also found in these areas, most of which are probably preying on the cnidarians. The largest and most evident of these are individuals of Dendronotus iris. These are amongst the largest local snails; in this area they often reach lengths exceeding 25 cm (10 inches) which is probably due to the high abundances of their preferred prey, the Pachycerianthus anemones. They approach the anemones by slowly crawling under the tentacle crown, to where the anemones extend from their tube. They, then, reach up rapidly, bite, and hang on to either a mass of tentacles or even the anemone’s column. Generally, the Pachycerianthus rapidly withdraws into its tube when it is bitten, and in these cases, it often pulls the predator in with it. Sometime later, the Dendronotus iris often crawls out of the now empty tube, and may set off in search of another anemone. The nudibranch may, at times, lay its loosely coiled egg masses attached to the Pachycerianthus tube, bits of shell, or just bits of the sediment.
Other nudibranch specimens are found in the area, and they can be relatively common at certain times of the year. Dendronotus albus specimens will be found occasionally, preying on those few hydroids that are found attached to the shell fragments or other hard substrata present on the sediment surface. These nudibranchs are slender and may reach lengths of about 10 cm. The basic ground color is white, but the tips of the branched cerata are tipped in orange. Individuals of another dendronotid, Dendronotus albopunctatus, are often abundant in the spring. These animals are brown to pink and freckled with small light dots. They only reaches lengths of 2 to 3 cm (up to about 1.5 inches), but they are recognizable by their somewhat “oversized”, relatively large, “front” cerata, which are often about a centimeter in length. Little is known of the natural history of this species, although it is likely a predator on small cnidarians.
Dendronotus albus is a not uncommon, small, about 3 cm, (1.2 inches) long, nudibranch in habitats such as those found in Cowlitz Bay. It eats hydroids, as this individual was doing when photographed
Acanthodoris brunnea, about 2 cm (0.8 inch) long, photographed on the sediment of Cowlitz Bay. Reported to eat bryozoans, this dorid species is found on muddy-sand, a habitat notably lacking in bryozoans. In this region and habitat, it is likely eating something other than bryozans.
Acanthodoris brunnea is another nudibranch species that is somewhat common at times in this habitat; little is known of its natural history. These animals are small dorids, roughly the same size as Dentdronotus albopunctatus, reaching lengths of 2 to 3 cm (up to about 1.5 inches). Their basic coloration is brown; the individuals are covered with distinctive relatively large papillae on the back. This species is considered to be predatory on bryozoans, but that is unlikely in this region as bryozoans are exceedingly rare in this habitat.
Also found in these areas are pennatulid-eating nudibranchs in the genus Tritonia. The most abundant of these are individuals of the small white Tritonia festiva, described in the earlier post on sea pen beds. Here, as well, T. festiva individuals seem to prey on Ptilosarcus. Individuals of the larger, orange nudibranch, Tritonia diomedea, are also occasionally seen in these areas. They seem to prefer the larger Virgularia as prey.
Large shelled gastropods are relatively rare in this particular habitat, although several smaller species can be very abundant. Perhaps the largest commonly found gastropod, and certainly one of the most beautiful, is the wentletrap, Epitonium indianorum. These animals are often found buried near to the bases of the tube anemones upon which they feed. As with most snails, wentletraps have a feeding organ called a radula; unlike the “classic” gastropodan radula which functions something like a rasp, filing off pieces of tissue, the wentletraps’ radulae are highly modified and look like an inverted thimble lined on the inside with sharp teeth. A wentletrap crawls up to the anemone and pokes the anemone with its radula everting the “thimble” in the process. This turns the radula inside out, which in turn, carves a circular hole in the tissues on the side of the anemone. The lacerated tissues are eaten, and the snail extends its proboscis which has the radula on its tip through the hole and proceeds to use the radula to cut up and eat other internal anemone tissues. These snails reach lengths of 3 cm or more, and don’t seem to move much once they have found an anemone to feed on. It is recognized by the distinct axial ribs, the rounded aperture, and the relatively high spire.
One cephalopod can be relatively common in the lower slope areas, the Pacific Bob‑Tailed Squid, Rossia pacifica. This small benthic squid lives buried in the bottom during the day. If a diver is careful, they can sometimes see the slight depression that the Rossia occupies, and then can make out the eyes watching him. The hole for the siphon is generally visible and if approached carefully, one can see the regular breathing movements of the mantle. Rossia pacifica reaches lengths of about 10 cm, and seems to live about a year or eighteen months. They have an interesting, stereotyped, escape response which I have described, briefly, in a previous post. This small squid preys on small shrimps, crabs, and fishes, and is a nocturnal hunter.
Well, that’s enough for now…