Constant Reproduction in the Tropics by Kirsten Steinke

Collecting Chthamalus spp. in my field site, Punta Culebra  

It’s a little known fact that barnacles have the largest penis to body size ratio in all of the animal kingdom, but did you know that they have an almost continuous rate of reproduction in the tropics? As an REU participant working in the Collin lab my research experiment focuses on a species of barnacle in the Chthamalus genus and aims to answer the question of whether or not there is a reproductive cycle within this continuous-brooding species. These little guys have a shell that is on average 10 mm in diameter and they like to hang out in the upper reaches of the intertidal zone so they can avoid competition with the larger Balanus species that dominate the lower intertidal zone and tide pools. This study compares the differences in reproductive cycles (i.e. percent brooding) between individuals found in the mid and high intertidal zones—at the extremes of their habitat.

At the study site in Punta Culebra I climb over the rocks to get to the four different sites where I collect my organisms. I am a slave to the tides and must do my collection during low tide when the barnacles are easily accessible! Most days during low tide the weather has been just shy of blazingly hot. However, sometimes I am afforded a nice break and get to scrape barnacles off the rocks when it’s dumping rain! When I am done collecting, I head on over to the lab to examine their broods. There are a couple things I look for: 1) Do they have a brood—notable by an orange lamella that encompasses the embryos found on the dorsal surface of the barnacle 2) their current stage of embryo development and 3) the relationship between brood size and barnacle size.

Chthamalus spp. has a shell that ranges in color 

from light grey to black.

My results thus far show a more distinct cycle from species living in the upper intertidal zone, while the individuals living in the mid-intertidal zone exhibit fluctuation in percent brooding over time with no obvious pattern. Since the start of my experiment in mid-June, the Chthamalus spp. in the upper intertidal zone have had a consistent rate of reproduction where ~80%-100% of individuals were brooding in a given day. The exception to this being on Monday, July 6 where only half of the barnacles collected in the upper intertidal had broods. There are many reasons this could have occurred, however my primary hypothesis is that it was due to the high amplitude tide that occurred over the weekend. It could have been just what the barnacles were waiting for to release their embryos in hopes of offshore settlement!

There is another high amplitude tide scheduled for the beginning of August and I’m excited to see if this drop in percent brooding occurs again! Because I am only in the Collin lab for ten weeks it is impossible to observe their long-term reproductive cycle, but it would be very interesting to repeat this experiment in the dry season to see how the cycles differ from the current wet season. Other aspects to my experiment include the effects of competition and temperature on brood size…but that’s another story!

Although my barnacles are not the most charismatic bunch, I find them fascinating. The environmental stressors that these barnacles are subject to are also impacting the reproductive success and life cycles of a multitude of animals living in the intertidal zone. A better understanding of what factors limit the reproduction and fitness of invertebrates like Chthamalus spp. is becoming exponentially important as the threats of global warming continue to grow and change the marine environment.