Sexy times in the tidepools

It seems that most years, the Memorial Day weekend brings some of the lowest spring tides of the year, and 2017 certainly fits the bill. I’ve been out for the past two days, heading out just as the sun is starting to rise, and already I’ve seen enough to whet my appetite for more. And with plans for the next few days, I’m pleased to say that my dance card is completely full for this tide series. There are a lot of stories building out there!

At this time of year everything is growing and reproducing. Many of the larvae I’ve seen in the plankton have parents that live in the intertidal; makes sense that those parents should be having sex now. Barnacles, for example, copulate when the tide is high. I’ve seen them go at it in the lab, but never in the field, as they don’t mate while emersed. This morning I interrupted a pair of isopods locked in a mating embrace, and they swam off, still coupled together, when I disturbed them. Other animals were much less shy. Lifting up a curtain of Mazzaella to see what was underneath, I spotted a small group of dogwhelks (small, predatory snails). I can’t be certain, but suspect they were having an orgy.

A short distance away I found the inevitable result of the dogwhelk orgies.

Each of those urn-shaped objects is an egg capsule, containing a few dozen developing embryos. After the snails copulate the mating individuals go their separate ways. The females lay these egg capsules in patches in the mid-intertidal, usually on a vertical surface under the cover of algae to minimize the risk of desiccation.

For many years now, some of my favorite animals have been hydroids. I worked in a hydroid lab as an undergraduate, and this is when I fell in love with the magic of a good dissecting microscope. A whole new world became visible, and I found it easier than I ever imagined to fall under the spell of critters so small they can’t be seen with the naked eye. I still do.

Hydroid colonies come in a variety of forms, shapes, and colors. Most of them are small and cryptic, resembling plants more than any ‘typical’ animal, and aren’t easily seen unless you’re looking for them. One intertidal species, however, is pretty conspicuous even to the casual tidepool visitor or beachcomber. It often gets torn off its mooring and washes up on the beach.

A hydroid colony is the benthic polyp stage of the standard cnidarian life cycle. The polyp represents the clonal phase of the life cycle and reproduces by dividing to make several copies of itself. In a colony such as a hydroid, the polyps remain connected to each other and even share a common digestive system. The polyps don’t reproduce sexually. That function is reserved for the medusa stage of the life cycle. Some hydroid colonies produce free-swimming medusae, and others hang onto reduced medusa buds or structures so un-medusa-like that they’re called gonangia. Aglaophenia is a hydroid that houses its sexual structures in gonangia that are located on the side-branches of the fronds.

Here’s a closer view of a single frond of the Aglaophenia colony. I had to bring it back to the lab to look at it under the scope.

The gonangia look like leaves, or pages of a book, don’t they? After working a low tide I’m always hungry, and when the lows are early in the morning I’m often cold and sleep-deprived as well. That’s my excuse for not dissecting open one of the gonangia to see what’s inside.

Even the algae are getting into the act of reproducing and recruiting. This spring I’ve noticed a lot of baby bullwhip kelps (Nereocystis luetkeana). Nereocystis is one of the canopy-forming kelps in subtidal kelp forests along our coast, but every year some recruit to the low intertidal. However I don’t remember seeing so many baby Nereocystis thalli in the tidepools. The smallest one I saw this morning had a pneumatocyst (float) the size of a pea! In mature thalli, the float might get as big as a cantaloupe.

Nereocystis doesn’t usually persist or get very large in the intertidal. It is more common to see detached thalli washed up on the beach than to see a living bullwhip kelp longer than about 2 meters in the intertidal. Whether or not this particular nursery area results in an established population remains to be seen. I’m betting ‘No’ but could very well be proved wrong. Only time will tell.