My Favourite Coevolutionary Relationship!

I have saved this relationship for now because it is one of my favourites!!

Has anyone wondered how crustaceans can sometimes remain hidden from sharks even though the sharks have the electrosensory ability to detect the electrical pulses made by the heart when it beats?? Well I wondered, so I did some research and found out some incredible things.

Firstly, it is due to coevolution. Secondly, it is due to the crustaceans’ ability to induce cardiac arrest when they feel threatened!!! For example, when a shadow passes overhead that could be a potential predator. This absolutely blew my mind when I heard about it. 

In this case it is predator-prey coevolution; the reason why it is coevolutionary is due to the sharks initially gaining an upper-hand with electrosensory detection, due to an electric signal produced each time the heart pulses; the crustaceans followed by allowing their heart to stop until a threat has past, therefore having the ability to hide from the sharks' electro-sensors (Burnovicz et al., 2009).

Neil Gribble, lecturer at JCU Cairns, did his PhD thesis on this topic, finding that crustaceans still retained the ability to see and catch potential prey even if their heart was “stopped”. There must have been a way for it to retain enough blood circulating to its eyes and brain in order to respond like this. He found that there is a tiny muscle just above the heart that contracts in a very subtle and even way so that enough blood is circulated to the eyes and brain to remain alert, but not enough to emit an electrical signal to other predators.

References:

Burnovicz, A., Hermitte, G., Oliva, D. 2009. The cardiac response of the crab Chasmagnathus granulatus as an index of sensory perception. Journal of Experimental Biology, 212, 313-324.

Image of crab sourced 14/5/14: http://marinebio.org/gallery/indonesia/

Image of shark sourced 14/5/14: http://www.xray-mag.com/Batoidea?page=3

Obligate Mutualism in Coevolution

For my blog this week the focus will be on a coevolutionary relationship that could date back to the Eocene epoch, 40 million years ago! It is also a major model system for the study of coevolving species interactions.

The yucca moth and yucca plant share an obligate mutualistic relationship, meaning that both organisms depend entirely on each other for survival. Yucca moths provide an important pollination service to the plant. In return, as well as providing a meal in seeds for the moth larvae, the plant allows with moth to lay its eggs in a deep place within the flower so that they are protected from predators (Pellmyr & Leebens-Mack, 1999).

Before a female moth lays her eggs she collects pollen from a flower; to do this she must scrape pollen from the anthers of the flower with specialised mouth parts and packs it into a ball, securing it under her head. After that she flies to another flower and climbs to the deepest part of the flower, opens a hole in the ovary and lays her eggs inside the ovary of the flower. She then climbs to the flowers’ stigma, retrieves the ball of pollen that she collected earlier and packs it into the tiny depressions that are within the style. Interestingly, before she moves on to a different flower, she marks it with a pheromone to notify any other moths that she has already laid her eggs here. This is done because if too many eggs are laid in the flower, the plant itself will abort the flower.

Both organisms show coadapted traits; mediated pollinator specificity due to structural adaptations in the flower; and specific behaviours for collection and deposition of pollen in the moth.

References:

Pellmyr, O., Leebens-Mack, J. 1999. Forty million years of mutualism: Evidence for Eocene origin of the yucca-yucca moth association. Proc. Natl. Acad. Sci. USA, 96(16), 9178-1983.

Image sourced 5/5/14: http://www.bobklips.com/earlyjuly2008.html 

Evolutionary Arms Race!

This week the topic of predator-prey coevolution is continued. The relationship that I will be talking about has resulted in an evolutionary arms race where the predator has developed incredible tolerance to its highly toxic prey!

The rough-skinned newt has over time concentrated an extremely potent neurotoxin within its skin; and through a series of genetic mutations and natural selection for increased resistance, the common garter snake has developed a resistance to the newts neurotoxin (Williams et al., 2003). The relationship is an evolutionary arms race because as the garter snakes’ resistance is continuing to increase, so is the levels of toxin that the rough-skinned newt can produce.

The studies show that the garter snake assesses the newt toxicity and its own resistance, rejecting newts that are too toxic to devour. The really interesting thing is that all of the newts that were rejected by the snakes survived attempted ingestion and attack, even if they had already been ingested for over 50 minutes!!

If the predator doesn’t survive the encounter with the toxic prey, natural selection for increased resistance cannot occur.

References:

Williams, B. L., Brodie Jr, E. D. & Brodie III, E. D. 2003. Coevolution of Deadly Toxins and Predator Resistance: Self-Assessment of Resistance by Garter Snakes Leads to Behavioral Rejection of Toxic Newt Prey. Herpetologica, 59(2), 155-163.

Credits for image located bottom right of image.

Predator-Prey Coevolution

Predator-prey coevolution is based on how a predatory organism may gain an advantage over its prey, which then triggers an evolutionary response from the prey to better avoid the predator; or vice versa.

My example of predator-prey coevolution this week is between broadheaded snakes and velvet geckos.

Broadheaded snakes are nocturnal and venomous, primarily feeding on nocturnal rock-dwelling velvet geckos. Geckos within the same geographical area of these snakes showed strong tendencies to avoid them by using chemosensory cues; detecting the specific scent of the broadheaded snake. It was found that if the snake’s scent was distributed all over the rock surface, the geckos were unlikely to enter the crevice. However, if the scent was only localised to a central part of the rock, the gecko would feel safe enough to enter the rock crevice as a retreat-site.

To attempt to hide its scent from the gecko, the broadheaded snake will hide in a rock crevice and remain sedentary for weeks so that it can minimise the spread of its scent over the rocks forming the crevice and therefore no longer being considered a threat by the gecko.

An interesting thing about this relationship is that it only occurs within sympatric populations. The geckos from sympatric populations could also detect the different scent of a small-eyed snake that does not eat geckos; however, in response to this chemical cue the geckos did not change its behaviour or its retreat-site choice. Additionally, geckos from allopatric populations did not show the same avoidance of the rock crevices containing a broadheaded snake, nor did they show any apparent detection of scent made by small-eyed snakes (Downes & Shine, 1998).

References:

Downes, S. & Shine, R. 1998. Sedentary snakes and gullible geckos: predator-prey coevolution in nocturnal rock-dwelling reptiles. Animal Behaviour, 55(5), 1373-1385.

Image 1: http://animal.memozee.com/view.php?tid=3&did=19871 sourced 7/4/14

Image 2: http://www.pittwater.nsw.gov.au/environment/animals_and_plants/native_fauna_management_plan/threatened_fauna_species_list/broad-headed_snake sourced 7/4/14

Mutualistic Coevolution Continued...

As promised, the topic of mutualism in coevolution continues!! Yay!! This week is about the extraordinary lengths that plants go to ensure their flowers are pollinated.

Flowers produce a store of nectar (corolla) within them as a reward for pollinating organisms transferring their pollen from one flower to another. What happens when pollinating insects evolve their proboscis so that they can reach the nectar store within the flower without having to land?

To reduce the risk of being disadvantaged, flowers began to increase the depth at which they store their nectar; forcing the insects to get close enough to acquire pollen. This process is constant as the insects continually increase their proboscis length and the flowers continually deepening their corolla to accommodate (Anders Nilsson, 1988).

A prime example of this coevolution is seen in the photo below of a Morgan’s Sphinx moth extending its phenomenal proboscis into a Comet Orchid, which has an equally phenomenal corolla depth.

References:

http://news.discovery.com/earth/plants/top-10-evolutionary-tricks-for-pollinators-1310112.htm - date sourced 31/03/14

Anders Nilsson, L. 1988. The evolution of flowers with deep corolla tubes. Nature, 334, 147-149.

Mutualistic Coevolution

Mutualistic coevolution is something that I am going to focus on over the next couple of weeks. It is where species coevolve into a mutualistic symbiotic relationship.

I am choosing to discuss the mutualistic relationship between clownfish and anemones because although it is common knowledge that clownfish can live unharmed in an anemone, it is not common knowledge what the reason for this is.

Does the protection come from the mucous of the anemone itself? Or, does the clownfish alter its own mucous coating to allow it to occupy the anemone unharmed?

Brooks and Mariscal (1984) tested these questions by exposing a clownfish to a constructed surrogate anemone for a period of time before exposing to a real anemone, observing how long it takes for the clownfish to acclimate. They concluded, due to the rapid acclimation rate, that it was the clownfish altering its own mucous during acclimation to form protection from the anemone.

Different subspecies of clownfish along with different subspecies of anemones can show preference to each other, and each subspecies of clownfish can share its own mutualistic bond with its particular subspecies of anemone. For example, ocellaris clownfish will only occupy the magnifica anemone. The reason for this is unknown, possibly due to convenience of not having to alter its mucous to a new anemone.

 

References:

Photo: http://www.colormaniacs.com/blog/?m=201003 - date cited 26/03/2014

Brooks, W. R. & Mariscal, R. N. 1984. The acclimation of anemone fishes to sea anemones: Protection by changes in the fish’s mucous coat. Journal of Experimental Marine Biology and Ecology, 80(3), 277-285.

Types of Coevolution

This week, rather than focus on a specific coevolutionary interaction, I am going to go through the five different modes of coevolution that have been suggested by various scientists so far.

I know, I know... not quite as interesting as my previous blog! However, I think it’s necessary to chat about it so that we all know what types of species interactions are likely to occur in each particular mode.

The five different modes of coevolution are as follows: Gene-for-gene, Specific, Guild, Diversifying and Escape-and-radiation (Thompson, 1989).

Gene-for-gene coevolution is most likely seen in species interactions of plants and pathogens with the perception that each gene affecting resistance in the host population is matched by a specific gene affecting virulence in the parasite population.

Specific coevolution is most likely seen in all interactions with selection pressures that are reciprocal, however uncommon in competitive interactions. This type of coevolution has an assortment of possible outcomes; divergence, convergence and ‘evolutionary arms races’ being the main three.

Guild coevolution, or diffuse coevolution, is seen in all species interactions and is a helpful experimental tool for discerning how groups of species within communities link and change together; Showing that evolutionary interactions could be more extensive than a pair of species.

Diversifying coevolution is most likely seen in seed-parasitic pollinators & plants, hosts & symbionts that regulate movement of host gametes, and maternally inherited symbionts & hosts. In some cases this form of coevolution may result in reciprocal speciation.

Escape-and-radiation coevolution is most commonly seen in species interactions involving hosts and parasites and is a more explicit form of how guild coevolution could involve both speciation and adaptation.

As I mentioned earlier, not quite as interesting but hopefully informative :)

Reference:

Thompson, J. N. 1989. Concepts of coevolution. Trends in ecology & evolution, 4, 179-183.