In the last few weeks we (team nudes) have been out collecting nudibranchs from the docks here at the Bamfield Marine Research Center. I was surprised by the diversity and profusion of life underneath these structures. I thought that with the constant movement of the motorboats along it, the level of disturbance would be too great for some of these fragile creatures to thrive. I was happily proven wrong.
Beneath, the docks are completely covered with a thick mat of orange and yellow tunicates. Some make patterns that look like flowers or crop circles. Various types of anemones puncture this layer, like towers, waving their tentacles, patiently waiting for something to come by. Floating down from the surface, like a canopy in suspension, are various types of algae that filter the light for the inhabitants beneath. On the more exposed corners we see agglomerations of dark, sharp edged muscles, treacherous for unprepared bare hands. Among these, stand tall elegant pillars with big red delicate pompoms overlooking the depths. These tubeworms are especially sensitive to the current and will retract swiftly to the approach of unwanted impositions. The inhabitants of this palace range from small chitons, skeleton shrimps and isopods to sea urchin, snails, jellyfish and many more. Big kelp crabs and Pisaster sea stars often make their way to this wonderland, from the greater depths, and terrorize these critters.
The most delicate, beautiful, prestigious, glorious, gorgeous, noble of these inhabitants are the nudibranchs. Our study project, the Opalescent nudibranch Hermissenda crassicornis, bares a multitude of small blue and orange projections (cerata) throughout its back. It possesses several identifiable lines along its iridescent, translucent mantle and is well known for its voracity. It mostly eats hydroids and tunicates as I have mentioned before but has also been seen eating muscle, bryozoans, anemones, and many more. This nudibranch is however far from representative of its entire clade. The Hooded nudibranch, for example, has lost its buccal mass and evolved as a suspension feeder. This organism looks extremely delicate as it undulates through the water column, trapping small critters in its veil and squeezing them into its mouth. The Hooded nudibranch is completely translucent and only has a few paddle shaped cerata along its back. As we took out this nudibranch into a bucket on the dock to look at it more closely, a delicious smell of watermelon diffused out of the water.
During our collection and field study under the dock we encountered other nudibranchs such as the Frosted nudibranch and Shaggy Mouse nudibranch. These nudies are all more beautiful than the next and seem to have very distinct behavior of their own. I love watching these and I hope to continue encountering different species in the field.
Harbo, R.M. 1949. Whelks to whales: coastal marine life of the Pacific Northwest. Newly rev. and expanded second edition. Harbour publishing.
Yesterday, we reached an impasse in our project. Using the fancy, confocal, microscope we were finally able to find and see cnidae (sequestered stinging cell) within the nudibranch’s cerata (body protrusion, extension of the gut)! However, we were not able to identify a difference between the cnidae. We were planning on investigating the difference in cnidae composition within the nudibranch’s cerata with exposure to a predator. If the nudibranchs chose to eat the hydroid (a source of cnidae) more often then the tunicate, we would expect to see a greater change in cnidae composition. This is impossible if we cannot identify the various types of cnidae! We counted approximately 400 cnidae per cerata, but since the size of the cerata varies, its capacity to hold cnidae also changes. Therefore quantifying the change in cnidae uptake using the total number of cnidae per cerata is likely meaningless and affected by too many variables other than predation presence.
During our pilot study and first feeding trials, we observed that bigger nudibranchs more often picked tunicate over hydroid while small nudibranch showed the opposite preference. Out of curiosity we decided to take pictures of each of the Hermissenda crassicornis and calculate their size. Measuring H. crassicornis food preference in the presence of a predator in respect to body size could give us some insight on a trade off between growth (nutrients) and defense in the development of H. crassicornis.
Studies suggest that tunicates may be more nutritious and more time efficient for nudibranchs to forage compared to hydroids. Small H. crassicornis (juveniles) have many predators including bigger H. crassicornis. We might expect that small nudibranchs would optimize their nutrient uptake and foraging efficiency to grow faster and quickly progress out of this vulnerable stage. This would result in more frequent feeding choice of tunicates over hydroids. However, there may be certain defense benefits in foraging in the bushy filaments of the hydroids as opposed to the exposed surface of the tunicates. These hydroids, in addition, provide cnidae that can be sequestered and used as defense. Therefore, hydroid seem be the optimal food and habitat preference of young nudibranchs. Large nudibranchs (adults) may not have as many predators and thus can afford to feed on the more exposed tunicates.
Studies on other organisms suggests that juveniles may be more specialized because they are less mobile and thus moving from one food source to the next could be very costly. Also, because of their small size they can potentially live and forage in the same location without depleting it. Many studies argue that small herbivore specialize on chemical rich plants to avoid predation. Could this be the case in the carnivorous H. crassicornis? Will our studies show that small nudibranchs feed more often on hydroid? Are physical protection and sequestered defenses more important at this life stage than pure nutrition? Stay tuned to find out!
Hoover, R., Armour, R., Dow, I. & Purcell, J., 2012, Nudibranch predation and dietary preference for the polyps of Aurelia labiata (Cnidaria: Scyphozoa), Hydrobiologica, 690: 199-213
Pennings, S.C. 1990. Size-related shifts in herbivory: specialization in the sea hare Aplyda calijbmica Cooper. Journal of Experimental marine Biology Ecology, 142: 43-61
Trowbridge, C.D. 1991. Diet specialization limits herbivorous sea slug’s capacity to switch among food species. Ecology, 72: 1880-1888.
STRESS EFFECT IN KLEPTOMANIAC NUDES, A SERIOUS CHANGE IN APPETITE.
I have always been fascinated by nudibranchs. What first caught my attention are their intricate and diverse morphologies as well as their astonishing colors. Later on, I learned that many nudibranchs are capable of sequestering cnidae provided by their prey and incorporating them into their body without setting them off. Cnidae are stinging, ejective cells produced by Cnidarians, a phylum of invertebrates that includes anemones, hydroids and jellyfish. These nudibranchs are able to release these cnidae as a defense mechanism against their own predators.
Since than, I have wished to one day study them. I am now at the Bamfield Marine Sciences Center and I intend with two friends and the help of the brilliant researchers here to unravel maybe a small fraction of the mystery behind this mechanism. Here are some of these stunning animals I have had the privilege to encounter while diving.
After very preliminary brainstorming, my directed studies group and I generated several questions concerning the defense mechanism of Hermissenda crassicornis, the commun opalescent nudibranch. Our first question was related to its food preference and whether it prefers foods with cnidae or not. Secondarily, we wondered whether certain cnidae are more potent than others, thus providing a better defense against predators. Finally, we wondered whether H. crassicornis changed diet according to the predation pressure in order to better protect itself.
We started off by reading various papers to funnel our questions into something coherent for our project. In 2012, Hoover et al. demonstrated a feeding preference in H. crassicornis for A. labiata polyps (moon jelly) and Obelia occidentalis (hydroid). H. Crassicornus is a generalist and thus can eat various food types. However, if limited to only one food type, it seems to only be able to survive on tunicates. This is surprising because tunicates do not have cnidae and thus are not a beneficial food against predator. This is interesting and suggests a tradeoff between different food properties (nutrition vs cnidae).
This study, however did not look at cnidae within these prey choices and so we continued our research by looking at a study done by Frick (2003) on Flabellina verrucosa, a different specie of nudibranch. The results in this paper showed an increase in F. verrucosa uptake of specific types of cnidea when exposed to a predator. From these results it was inferred that certain cnidae must be more effective against predator. Also, it was hypothesized that these specific cnidae are not only sequested through a change in diet but also through a selective mechanism within the individual’s body. This again demonstrates that nudibranchs can distinguish among the types of food in respect to the quality of their cnidae. This paper was extremely valuable seeing as it provided background information in respect to F. verrucosa. However, we still wanted to know whether the same mechanism applied to H. crassicornis.
Further along our research we found that H. crassicornus is able to regenerate cerata. Cerata are projections of the body containing the ends of digestive tract, which differentiate into pockets. These pockets hold the sequestered cnidae. Cnidae are only released for protection and do not seem to degrade. Thus, with this study in mind, we speculated on the possibility of quantifying the turn over rate of cnidae in the H. crassicornus cerata when exposed to a predator.
In the end, we had even more questions than at the start but we were able to narrow down our directed study project to: the effect of predation pressure on H. crassicornis food preference and cnidae uptake. We hope this will help us better understand the mechanism behind the sequestering of cnidae in respect to diet and predation pressure. Also, if everything fails, we will at least have learned how to carry out a research project and continued being mesmerized by these breathtaking creatures.
Frick, K, 2003, Response in nematocyst uptake by the nudibranch Flabellina verucosa to the presence of various predators in the Southern Gulf of Maine, Biological Bulletin, 205, 367-376
Hoover, R., Armour, R., Dow, I. & Purcell, J., 2012, Nudibranch predation and dietary preference for the polyps of Aurelia labiata (Cnidaria: Scyphozoa), Hydrobiologica, 690, 199-213
Welcome water lovers, this is a space where you may share and discuss all of your water discoveries, adventures and dreams. I will mainly be discussing my directed study project in at the Bamfield Marine Sciences Center and would love to hear your feedback. Thank you in advence for keeping all content appropriate.