Study nature

Seen in the WHOI-MBL library

"Study nature, not books" - Louis Agassiz

The sign at right hangs in the entrance to the WHOI-MBL library on Water Street in Woods Hole. The sentiment seems odd for a library, but it hearkens back to the early days. Louis Agassiz is one of the fathers of marine research in the United States, who for years lead field schools for students and scientists in Woods Hole. His message to his fellow professors was clear: discoveries are made outdoors.

I've spent a considerable amount of time with books over the past couple of weeks, as I try to work my way through the samples I collected in the Arctic. As you might recall, I got a lot of specimens of Bouillonia cornucopia, a hydroid that colonized my fouling panels. I had observed the hydroids to have gonadal tissue between their two rings of tentacles, and I thought the small spherical structures were eggs. But while I was certain the pink, branching tissue was involved in reproduction, the eggs never looked quite right. They were too lumpy. Maybe they had been fertilized and already undergone the first cellular divisions, but even that didn't seem right. I wasn't quite sure what to make of the lumps.

"Eggs" that had broken off of my hydroid specimens.

So I went to the library. Old taxonomic literature does a fantastic job of describing and diagramming every structure of an organism in detail, so I was hoping someone had figured out why hydroid eggs were lumpy. I pored over old papers, read everything I could find about the reproduction of athecate hydroids, and I was right. Someone had figured out why the eggs were lumpy.

Because they're not eggs at all. They're a different life phase entirely.

Back up. Hydroids are cnidarians, which means they're related to corals, anemones, and jellyfish. All cnidarians are built the same basic way, with a central mouth that leads to a blind-ended gut and tentacles surrounding the mouth. Cnidarians have basically two main body forms - a polyp (anemone-shaped thing, tentacles facing up) and a medusa (jellyfish-shaped thing, tentacles facing down). Most cnidarians actually have both a polyp and a medusa phase in their life-cycle, and hydroids definitely fall into this category. The polyps live attached to hard surfaces like docks and boat hulls, and they bud off medusae that drift in the water column. Medusae produce eggs and sperm, which combine to become larvae, which settle and develop to become polyps again. The polyp makes the medusa, which makes the polyp, and around again. It’s a two-phase life-cycle.

The head of a Bouillonia cornucopia polyp, photographed
from above (oral side). You can see the central mouth, the
oral tentacles, the fluffy pink gonophores, and then the outer
(aboral) ring of tentacles. 

So here's what I learned in the library: in some hydroids, the medusa is reduced. Some species have medusae that stay attached to the polyp and never actually drift in the water column. Other species have medusae that don’t even look like medusae because they’ve lost all of their defining features. They just look like lumps. (Can you guess where this is going?) The lumpy "eggs" I was seeing in my specimens were actually the "medusae" in this species' life-cycle. They're called gonophores because they're so reduced and featureless, but they're derived from the medusa form. They produce eggs and sperm, which combine to become larvae, which give rise to more polyps, which make more "medusae," and around again.

I sent some of my B. cornucopia specimens to a colleague who specializes in analyzing reproductive tissues. She's planning to embed the specimens in wax and slice them open to see what the cellular structures look like inside the gonophores. I'm really looking forward to seeing what she finds and learning more about this species!