THE PACIFIC NORTHWEST IS SLUG COUNTRY

With our very wet climate (favorable for terrestrial mollusks) and our relatively acid soils (not so favorable for forming snail shells), we furnish great habitat for slugs.

The ones most of us see are garden slugs (Arion ater). This slug, native to northern Europe, comes in two forms, a black and a reddish one. Long thought to be subspecies of one another, Arion ater ater and Arion ater rufus, they have recently been split as two separate species Arion ater and Arion rufus. Unfortunately for the field worker, both of them come in a great variety of colors and can only be distinguished by dissection or molecular analysis.

So we’ll just call them all garden slugs. They have proven to be very successful imports to our region but aren’t well liked by gardeners because of their predilection for garden plants. In fact, it’s the easiest thing in the world to go online to find out how to get rid of slugs. I am choosing to extol their virtues, perhaps the most important one just to familiarize people with slugs.

Our big native slug is the banana slug, Ariolimax columbianus. After a European species that grows to a foot (30 centimeters) in length, ours is the second largest in the world, reaching lengths of 25 centimeters. Four of these in a cup would weigh a pound! Rarely are such monsters seen, though; most that we encounter are in the range of 10-15 centimeters, just a bit larger than the much more familiar garden slugs.

But walk into a mature wet forest, and if it’s a moist day, you are likely to find lots of banana slugs. They come in a variety of colors, from white to plain yellow to heavily spotted with black. There must be some genetic differences among these color types, as often all the ones you see in one spot look about the same.

The big hole on the right side of these slugs is the pneumostome (breathing hole). An active slug shows two eyestalks above that detect light or movement and two tentacles below that are chemosensitive. They have mucous glands all over the body that keep them protected from dehydration and that can lay down a trail for easier locomotion.

Most slugs are herbivorous, feeding directly on plant tissue (garden slugs) or on detritus and mushrooms (banana slug). The leopard slug (Limax maximus), a large pale brown, black-spotted species that is also introduced in our region, feeds on other slugs as well as detritus and garden plants. In turn, garter snakes and ducks eat a lot of slugs, apparently able to combat the mucous that protects them from many other predators.

Slugs are hermaphrodites, both sexes present in the same animal, and when they mate each one contributes sperm to the other. The lack of separate sexes may make sense in slow-moving animals that might have trouble finding a mate. In this case, every slug encountered would be a potential mate, not every other one!

Dennis Paulson

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