Week 2; Discovery, disappointment and delight

This week has been an interesting mix of nudie discovery, disappointment and delight! As the second week of our Directed Studies project began, Ondine, Paige and I were enthusiastic about getting started on facing the greatest challenge of our project; identifying and counting cnidae1 within the cerata2 of our nudibranchs.

Cnidae are located in the cnidosacs at the tips of the cerata of the nudibranch

The discovery:

Hermissenda crassicornis has the ability to drop their cerata2 when threatened by a predator, much like a lizard drops its’ tail. Once dropped, these cerata continue to move and writhe around to distract the would-be attacker, allowing H.crassicornis to escape. This mechanism allowed us to spend many hours previous to this week ‘plucking’ cerata from our poor unsuspecting nudies and meticulously making them into permanent slides (whilst the cerata tried to escape off the slide) with glycerol and nail polish. This method was taught to us by our friend Susan, who discovered this technique after likely countless hours of wrestling with these sneaky, sticky suckers! So at the beginning of this week we finally took our precious slides to the special confocal microscope and to our delight were able to locate the cnidae! We could see them clearly, we could count them, they were identifiable in all the slides we had made! It was the best success we had achieved to date!

The disappointment:

……..and they all looked exactly the same. There was no way we could identify a difference between them.

Unfortunately, if we wanted to look at the change in cnidae composition (ie. Which cnidae were present and how they changed over time) we needed to be able to see a difference between them! So, our fantastic successes of the day had now led to us having to rethink our entire experimental design. Oh.

Cnidae within the nudibranch cerata… all looked indistinguishable from each other.

Luckily, Paige, Ondine and I had the ambitious desire to make out project a ‘multi-layered cake’ (ending in some kind of leaning-tower-of-Pisa-cake). This allowed us to see what we could make of our cake (study) if we removed the cnidae aspect. Straight away an observation we had made when collecting our nudies came to the rescue.

We had originally been interested in whether predation pressure influenced feeding preference, suggesting a trait mediated interaction3 between the nudibranchs and their predators may be occurring. However when we were collecting the nudies from the dock we made the observation that many of the smaller H.crassicornis were found on hydroids, whilst only the larger H.crassicornis were collected from tunicates. Could this suggest a change in dietary preference with size and development as well? It seemed logical enough, smaller nudibranchs may be more vulnerable to predation and therefore prey on hydroid in order to also benefit from the physical protection of living inside its complex structure and the defence offered by sequestering its cnidae? Maybe as they grow older predation pressure decreases allowing them to widen their dietary range? So we started researching.

Sea hare Aplysia californica

The sea hare4 Aplysinia californica is small herbivorous invertebrate that, like H.crassicornis, often lives on / in its’ prey. Penning (1990a,b), studied the feeding preference of this organism in relation to its development. The study indicated that when in its’ juvenile state, A.californica has a diet comprised solely of the red alga Plocamium cartilagineum (very rich in chemical defences and quite structurally complex) and is able to sequester these chemicals for its own defences. However as A.californica grows larger, its diet widens and is able to feed and grow on a broad range of algae. This change in diet may be due to a number of factors, but may represent a very common trend, even in relatively short lived organisms.

Plocamium cartilagineum, the sole food source of juvenile Aplysia californica.

The delight:

Excellent!

So, what does this mean for us? Well, we were no longer going to spend hours with our heads down a microscope, instead we could cut our trial down shorter (no need for long amounts of time to allow cnidae sequestering), and all we had to do extra was measure our nudies to examine the effect of size on feeding preference!

 

More snorkelling, shorter trials, no microscope time, no worries!

Well, that wasn’t so bad after all.

Keep updated for the results!

 

 

Hermissenda crassicornis, David Hall photography.

 

 

1:      Cnidae; Microscopic cnidarian stinging cells (found in anemones, hydroids, corals, and all cnidarians)

2 :    Cerata; (external projections of the digestive tract of nudibranchs. Like pretty coloured leaves on the back of H.crassicornis.

3:     Trait mediated interactions; Also referred to as non-consumptive effects, describes the interactions between predators and prey in which the   predators can influence and change the behaviour and physiology of their prey purely by their presence. This can be reflected in many ways, from certain species decreasing body mass in presence of predator effluent, to species being found in separate areas to affective competitors (other prey for the same predator).

4:   Sea hare; Sea hares are medium to large soft bodied marine gastropods with a small internal shell made of protein. Although their taxonomy is currently under debate, they are currently considered closely related to nudibranchs, both being within the same suborder Opisthobranchia.

References

Greenwood, P., 2009, Acquisition and use of nematocysts by cnidarian predators. Toxicon , 54(8), 1065-1070

Pennings, S., 1990a, Multiple factors promoting narrow host range in the sea hare, Aplysia californica. Oecologia, 82(2), 192–200.

Pennings, S., 1990b, Size-related shifts in herbivory: specialization in the sea hare Aplysia caliifornica, Journal of Experimental Marine Biology and Ecology, 142, 43-6 1.

Werner, E. & Peacor, S., 2003, A review of trait-mediated indirect interactions in ecological communities. Ecology, 84(5), 1083–1100.

http://www.nudipixel.net/taxonomy

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