NAKED IN THE OCEAN

It’s time to home in on the extreme low tides we get in our local marine environment during daylight hours in the summer. Get to the coast an hour before the low and spend a couple of hours looking in the shallow water and especially in pools left by the receding tide. If the pool is rich enough, just sit down and enjoy watching the invertebrates and fish, if there are any, go about their business.

But don't think you have to shed your clothes; the nudity here involves the animals you are watching. Among the animals that inhabit such places are the nudibranchs (= naked gills). These are mollusks that appear to lack shells, something like seagoing slugs; many have prominent external gills. Like slugs, they are members of the class Gastropoda, which includes the snails. Having dispensed with a shell gives the nudibranchs both constraints and opportunities.

The primary constraint, of course, would seem to be a much greater risk of predation without a hard shell into which to retire when disturbed. Along with this is the risk of desiccation when exposed at low tides, so you won’t find many nudibranchs sitting around on exposed rocks like you do some of the snails.

These are mostly small animals, ranging from 1 cm (Rostanga) up to 20 cm (Peltodoris), but because of their conspicuousness and beauty, they are more admired and sought by tidepoolers than some of the other invertebrates that share their habitat.

Prominent anatomical features are two tentacle-like structures toward the front that are called rhizophores and are probably chemical detectors. Toward the rear there may be a ring of gills. And projections all along the upper surface are cerata, the bright colors of which are probably involved in predator deterrence, a warning coloration that goes along with distastefulness.

The lack of a shell has provided selective pressure for nudibranchs to develop other ways of protecting themselves from predators, and one of the most effective is the ability to secrete chemicals, including sulfuric acid in some cases, that make them inedible. They are surprisingly predator-free, and fish have been seen to spit them out. Anything as conspicuous as some of these are would seem to be predator resistant!

One of the opportunities for such animals comes in the form of being free to be predators themselves. They often specialize in animals that aren’t eaten by many other predators, thus assuring them a reliable food supply. Most of their prey items are sessile, fixed to the rocks on which they crawl, but a few are able to capture motile animals as well.

Acanthodoris eats colonial ascidians (tunicates) and bryozoans. Triopha and Janolus eat arborescent (branched) bryozoans. Doris and Peltodoris eat sponges, especially the crumb-of-bread sponge (Halichondra), and how they avoid being pierced by the sponge spicules hasn’t been determined. These two species are called sea lemons. The tiny Rostanga eats red sponges, on which it may be perfectly camouflaged. Hermissenda eats hydroids and incorporates their toxic nematocysts into its cerata, thus affording it protection from predators. Dirona eats everything, including not only the same prey as the others but also snails and crustaceans.

The species shown here were found at two localities on the north coast of Oregon in May. All are common in Pacific Northwest waters.

Dennis Paulson

THE CHORUS OF THE CHORUS FROGS

Early every year in the Pacific Northwest a familiar sound rings out, telling us it is spring whether it seems that way or not. this is the “song” or advertisement call of the male Pacific Chorus Frog, Pseudacris regilla.

Most people recognize the call immediately as the generic frog call of movies in decades past. As this is a common frog (or at least used to be) in Hollywood, California, its call was incorporated into many a movie that needed frog calls as ambience. In fact the “ribbit, ribbit” sound has become the stereotype of frog calls.

These frogs come out of their winter dormancy very early in spring, when, to paraphrase Robert Burns, a young frog’s fancy lightly turns to thoughts of love. The only way a frog has to express itself in such a situation is to call . . . . and call . . . . and call. And that they do, with surprising strength.

When a male frog feels these stirrings, he heads for the nearest pond or marsh, usually in the evening but sometimes even during the day. On arrival, he jumps in the water and swims to what he considers a good position. Only the frog knows why it is a good position, but it probably provides a place to hold onto the vegetation and a place where he can be easily seen as well as heard. Interestingly, it is often the same male that calls first each evening.

He begins to call: ribbit, ribbit, ribbit, a creaky two syllables that carries at least a hundred yards or more on a quiet night. Another frog heads for the pond, either because hopping downhill in a moist environment will lead to water or because it homes in on the first frog. The second frog begins to call, perfectly insinuating its calls between those of the first. The two may sound quite different, so we hear ribbit, rabbit, ribbit, rabbit, ribbit, rabbit.

A third male begins to call, amazingly also able to insert its calls into the soundscape so they can be heard as distinct: ribbit, rabbit, robbit, ribbit, rabbit, robbit . . . the pace is speeding up, and there is no room for a fourth frog, but that one calls anyway. As the chorus swells, the individual voices become less apparent, even though the structure may still be there, but a female approaching the pond can easily distinguish the individual voices.

Females apparently choose males based on the vigor of their songs, and as the evening progresses, more and more males acquire a mate. The male clasps the female and stays with her while she looks for a place to lay her eggs. She finds such a place, lays a cluster of eggs, and the male fertilizes them.

The eggs hatch in a few days, and the tadpoles grow quickly on a diet of plant matter. After a few months, they finally absorb their tails and grow a set of limbs and are then ready to leave the pond. If they survive the year, they will return the following spring, and the pond will resound again with the chorus frog chorus.

Dennis Paulson