Wainwright Lab

University of California, Davis

Month: April 2011

Waiting in the weeds: the Sargassum frogfish (Histrio histrio)

This blog is cross-posted on my personal website’s blog.

This week’s blog will focus on one of the things the Wainwright Lab does best, fish suction feeding. One of the current projects in the lab is comparing the kinematics of suction feeding from a large inter-specific group of fish. To do this, we film fish feeding on various prey (mostly zebrafish) with a high-speed video camera at a 1000 frames per second. Once the sequence has been captured the videos are digitized to obtain several kinematic parameters associated with suction feeding, such as gape and jaw protrusion. A product of this research is some amazing videos of a diversity of fish using suction to subdue prey. Throughout this blog I will post some examples of the fish we have filmed. Hopefully these videos will allow you to observe not only the diversity in fish species, but also the diversity in the kinematics associated with suction feeding.

The first video comes from a fish that made its debut at UC Davis’ 97th annual Picnic Day this last Saturday. The Wainwright lab had a small demonstration for the public, showing some videos of the various fish in the lab. Also present were two charismatic frogfish on display for the public. The public got to see the live fish and their feeding kinematics in slow motion from some videos. One of the fish on display was the sargassum frogfish (Histrio histrio) of the family Antennariidae. One of the unique features of Antennariidae is that the first dorsal fin ray is modified into a lure to attract prey. These fish tend to be sit-and-wait predators so the lure helps bring in food.

The sargassum frogfish can be found in most tropical seas living among the Sargassum weeds, hence its name; often near reefs. It is the only frogfish with a swim bladder, allowing it to float among the weeds. They can reach a size of 20 cm total length (from the most anterior tip to the edge of the caudal fin); and their color patterning is a mottled greenish/brown with numerous weedy dermal appendages. This patterning allows the sargassum frogfish to blend in to the weeds waiting for unsuspecting fish or shrimp. Unlike some of the other frogfish, this species swims more in open water, and some divers have noted their ability to jump out onto the floating weeds to avoid potential threats.

In terms of their feeding, these fish are mainly sit-and-wait predators, hiding among the weeds; however they do have the ability to swim after prey.  The lure in this species is small compared to some other frogfish.  Once the prey is close enough, the frogfish initiates its strike using suction to obtain the prey. Being a sit-and-wait predator these frogfish have little to no body displacement (you can see it is not swimming to capture the prey item in the video below), but you may also notice their mouth can displace a good distance in order to capture the prey. This clip is a mid-water strike, typical of this species. These strikes are also extremely quick. This video was filmed at 1000 frames per second, but is playing back at 10 frames per second; so the actual strike is much faster than the video playback.  One of the things we are interested in is the variation in kinematics of suction feeding associated with different ecologies. Have sit-and-wait predators evolved a greater mouth displacement compared to more active foragers? The data we are collecting will be able to shed light on some of these questions, and the sargassum frogfish is a good example of a sit-and-wait predation strategy.

Nocturnal dinosaurs

Yet another non-teleost blog, but an interesting example of functional morphology and phylogenetic comparative methods. I’m cross-posting this on my own blog, too.

Nocturnal dinosaurs. Wait a second! Is that right? Nocturnal (= night-active) dinosaurs? Yes, indeed. Contrary to what was commonly believed, many dinosaurs were nocturnal.  We have to change our perception of the dinosaur era.

All details about methods, results, and implications can be found in these two siblings papers in Science and Evolution (thanks to AAAS and Wiley for being so extremely helpful in coordinating this release!):

L. Schmitz, R. Motani, Nocturnality in dinosaurs inferred from scleral ring and orbit morphology. Science, published online 14 April 2011 (10.1126/science.1200043)

R. Motani, L. Schmitz, Phylogenetic versus functional signals in the evolution of form-function relationships in terrestrial vision. Evolution, published online 14 April 2011 (10.1111/j.1558-5646.2011.01271)

We make several important points in these papers: (i) Dinosaurs were not restricted to day-activity (diurnality) by any means, (ii) Activity patterns evolve following ecological characteristics (e.g., diet+body size, terrestrial or flying), (iii) Physics of the environment drive the evolution of shape, (iv) We have a great new tool for reconstructing ecology of extinct species.

So, why did everyone think that dinosaurs were diurnal in the first place? We think there are at least two (historical) reasons. First, several decades ago dinosaurs were portrayed as sluggish, “cold-blooded” animals. It seemed too unlikely that these animals were capable of being active at night, when ambient temperatures were cold compared to the day. Second, the idea of diurnal dinosaurs may have arisen because many (paleo)biologists were trying to explain why the majority of mammals is nocturnal – and the picture of dinosaurs occupying the diurnal niche, pushing the mammals into the dark just seemed to fit all too well.

Well, that story isn’t all that simple and straight-forward anymore. Dinosaurs were nocturnal, too. And we don’t even know if the earliest mammals were actually nocturnal (Kenneth Angielczyk at the Field Museum and I are working on this right now). So, the assumed split in activity pattern between mammals and dinosaurs is certainly rejected. However, we do see a different pattern emerging. It appears as if the evolution of activity patterns is driven by ecology.

We found striking similarities between the Mesozoic and today’s biosphere. Large herbivores, just like living ‘megaherbivores’, were active both day and night, probably because of foraging needs (they just had to eat most of the time…), except for the hottest hours of the day when there was risk  of overheating.  Small carnivores such as Velociraptor were nocturnal hunters. Flying species, including early birds and pterosaurs (like Scaphognathus above, with scleral ring highlighted in blue color) were mostly day-active (although some of the pterosaurs were actually nocturnal). These ecological patterns are also found among today’s living mammals, lizards, and birds.

So, how can we tell? Eye shape is the key. Nocturnal animals roam around in low light during the night, and their eyes show characteristics that relate to improved light sensitivity. Eye soft-tissues rarely if ever fossilize, but many vertebrates (e.g., teleost fish, lizards, and birds) have an extra bone element within their eyes, the so called scleral ring. Notably, neither mammals, crocodiles, nor snakes have scleral rings [for more details I would refer you to the work of Tamara Franz-Odendaal]. The scleral ring, however, is present in basal archosaurs, pterosaurs, and dinosaurs, which enables us to reconstruct eye shape in these fossils..

If you look at a bird eye (photos below) you will see the pupil opening defined by the iris. The scleral ring, situated within the sclera, the ‘leathery’ layer of the eye, surrounds the iris. The shape of the scleral ring and eye socket closely reflects eye shape, enabling us to distinguish nocturnal and diurnal dinosaurs.

For example, if you compare the scleral rings below, the nocturnal species on the right, a potoo, has a much larger ‘aperture’, or inner diameter than the diurnal hawk compared to eye socket size. Of course, the differences are often more subtle, so we use a Discriminant Analysis (DA) to retrieve a quantitative prediction. Prediction from a DA are rarely perfect, but overall the DA performs very well.

The relation between form and function of the eye is another convincing example how physics of the environment, that is light levels, drive the evolution of shape. However, it is not just physics that influences the evolution of shape. Shared ancestry is an important factor that may blur the form-function relation. Simply put, closely related species are expected to be more similar to each other than more distantly related species, so shape alone could potentially be misleading. With Ryosuke leading this part of the study, we solved this issue and developed a new method (implemented in ‘R’) that makes it possible to account for this phylogenetic bias. A step-by-step guide to this analysis is provided in the Supplementary Information of the paper in Science (www.sciencemag.org/cgi/content/full/science.1200043/DC1). We hope that this method will be helpful for other studies dealing with inferences of ecology from shape.

Eye shape revisited

In the anticipation of the publication of the dinosaur paper I thought it may be timely to highlight one of my latest papers. [Please note that this blog is also cross-posted on ecomorph.wordpress.com, my personal webpage].

Schmitz, L. & R. Motani (2010). Morphological differences between the eyeballs of nocturnal and diurnal amniotes revisited from optical perspectives of visual environments. Vision Research, 50: 936-946. [doi:10.1016/j.visres.2010.03.009]

This paper was one of the first steps towards inferring activity pattern in extinct dinosaurs and pterosaurs. We needed to find out whether it is possible to distinguish night-active and day-active living species before we could make any quantitative inferences in fossils. It was known previously that activity patterns indeed influence eye shape (see, for example, papers by Margaret Hall, Chris Kirk, and Callum Ross), but a reliable method to make quantitative predictions was still lacking.

Here is the short version of our approach: The eyes of land vertebrates cope with highly different light levels, largely depending on the preferred activity time of the animal. Night-active (nocturnal) species experience much lower light levels than day-active (diurnal) species. Twilight-active (crepuscular) and day-and-night-active species (cathemeral) species are exposed to both high and low light levels. The problem with low light levels is that it is very difficult to form a good image.

Overall, then, these contrasting lifestyles (and light levels) should drive the evolution of eye shape, because nocturnals need to have eyes with very good light sensitivity, whereas cathemerals and especially diurnals can optimize other aspects of vision.

We tackled this problem by applying Discriminant Analysis, focusing on features of macro-morphology that are correlated with optical function. Indeed, different activity patterns occupy distinct areas in morphospace and are identified with high accuracies (see the image to the left, modified from the Vision Res. paper). Diurnals are represented by open symbols, gray symbols are cathemerals, and black symbols are nocturnals.

The great news for paleobiology was that this method not only works with eye soft-tissue dimensions like eye diameter, axial length, or lens diameter (which rarely if ever fossilize), but also with skeletal features (i.e., size of the orbit and scleral ring) of birds. These skeletal structures are still correlated with optical function, even though their shape is influenced by other factors as well (see, for example, Schmitz 2009, J. Morph.). So, potentially one may be able to retrieve some or most of the optical information recorded in scleral ring and orbit structure… more on this next week!

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