University of California, Davis

Category: coral reef fish

The eyes of reef fishes

[cross-posted on my personal blog, as well]

Peter and I recently published a paper in BMC Evolutionary Biology and today the final HTML and PDF versions have become available. BMC is an open access journal, so everyone can read the paper:

Schmitz, L. & P.C. Wainwright (2011). Nocturnality constrains morphological and functional diversity in the eyes of reef fishes. BMC Evolutionary Biology, 11: 338. html, reprint.

We have several really interesting results. The eye morphology of nocturnal reef teleosts is characterized by a syndrome that indicates good light sensitivity. Nocturnal fishes have large relative eye size, high optical ratio and large, rounded pupils. However, there is a trade-off. Improved optical light sensitivity comes at the cost of reduced depth of focus and reduction of potential accommodative lens movement. Diurnal reef fishes, which are released from the stringent functional requirements of vision in dim light, have much higher morphological and optical diversity than nocturnal species. Diurnal fishes have large ranges of optical ratio, depth of focus, and lens accommodation.

This paper is the first outcome of the analysis of a data set on the eye morphology of 265 species in 43 families of teleost reef fishes. It’s an enormous amount of data. All in all, we measured 5 traits in both eyes of 849 specimens, resulting in one of the largest data sets on eye morphology ever assembled. One aspect I would like to stress is that we measured eye morphology on fresh fish (in accordance to the UC Davis animal care protocol). The fixation process that preserved specimens went through may have altered eye morphology, so we wanted to avoid this potential problem.

I also would like to highlight one of the analyses in our paper. We assessed morphological diversity of diurnal and nocturnal species by calculating the combined variance of all shape axes of a principal component analysis. However, there are far more diurnal (n=211) than nocturnal species (n=54) in the data set, and ultimately the results may be biased because of uneven sampling. Inspired by a suggestion from David Bellwood we designed a simple rarefaction analysis. We randomly re-sampled 54 diurnal species (matching the number of nocturnal species) without replacement and calculated variance on PCs 2-5, and repeated this procedure 100, 000 times. This resulted in 100,000 PC analyses with the same number of diurnal and nocturnal species, with diurnal species randomly selected anew for each run. Then, we compared the distribution of nocturnal variances to the bootstrap distribution of diurnal variances. The results are convincing and, importantly, should no longer be biased by uneven sampling. The rarefaction is easily done in ‘R’; let me know if you are interested in the code.

Finally, here are thumbnails of all figures in the paper, with links to the corresponding to the high-resolution files. Feel free to use for teaching purposes.

Red lionfish: stunning and invasive

This week’s blog focuses on one of the most well recognized marine fish. Not Nemo, but the lionfish, specifically the red lionfish (Pterois volitans). Lionfish belong to the Scorpaenidae family, which includes rockfishes and Scorpionfishes. This particular species is native to the Indo-Pacific, where they can be found in lagoons and reefs in waters up to 50 meters in depth. They feed mostly on other fishes, shrimps and some crabs. One unique feature of these fish is their large pectoral fins. You can see in the video below how the pectoral fins are splayed out perpendicular to the body when initiating a strike. According to fishbase, they use these fins to trap prey in a corner, stun it and swallow it, with the aid of suction. Like most Scorpaenidae, red lionfish are venomous at the dorsal spines. However, despite this they are a common species found in home aquariums.

One thing that I was not aware of until recently about this particular species of lionfish is its invasiveness. Although red lionfish are native to the Indo-Pacific oceans, they were first documented in the Atlantic Ocean in 2000 off the coast of North Carolina, although there were some earlier reports of them off the coast of Florida in the 1990’s. The cause of invasion is most likely through the aquarium industry, whether accidental or not. While some juvenile red lionfish may make their way up to the New York area due to the Gulf Stream, their range in the Atlantic seems to extend to Cape Hatteras, North Carolina, with cold temperatures limiting further range expansion. For instance, laboratory experiments have shown they do not feed below 16C. Despite this northern limitation on their range expansion, their numbers seem to be increasing off the southeast coast of the US and into the Bahamas. In fact, they seem to be the first invasive marine species to become established off the coast of Florida. Much research has been conducted on this recent invasion, with reports documenting their effect on the recruitment of native species, and their feeding ecology in the new habitat, which seems to be primarily fishes.

The recent invasion of red lionfish to the Atlantic Ocean is just one of many examples of non-ntaive species becoming established in a new habitat and affecting the ecosystem. Florida alone has seen the establishment of several freshwater, brackish water and terrestrial species (such as pythons in the Everglades). This recent invasion has already spawned new research and will most likely provide years of research to understand the ecology of invasive species.  Who knows maybe a comparison of feeding kinematics will be done to determine the evolution of suction feeding in response to varying ecology.

This is cross-posted with my personal blog.

Inermia vittata: Camera Debut

Below is one of the first ever recorded high-speed video sequences of Inermia vittata, a zooplanktivore from the tropical western Atlantic.  We are using its first live appearance in the lab to see how the feeding kinematics of Inermia compare with that of other reef fishes.  Watch how far that upper jaw projects forward!

[youtube=http://www.youtube.com/watch?v=WOQ3US92Tt0]

One common name for this fish is the bonnetmouth, named after the appearance of the protruded mouth.  Like other reef zooplanktivores, Inermia appears qualitatively to be specialized at picking prey from the water column.  As you can see in the video, the mouth reaches forward, closing the distance to the prey while preparing to pull the prey closer with suction.

The evolutionary relationship of Inermia to other species has been tricky to resolve because it is very similar in appearance and behavior to other zooplanktivores such as fusiliers (Lutjanidae).  However, molecular analysis shows Inermia to be nested within the grunts (Haemulidae), which typically feed on benthic invertebrates.  A look at the pictures below will show how much different Inermia appears from a typical grunt and how similar it looks to the distantly-related fusilier.

boga boga bonnetmouth boga

Our new star, Inermia vittata

Doubleline fusilier

A fusilier, nested within the snappers (Lutjanidae)

French grunt

A close relative of Inermia

Why does Inermia look so different from a typical grunt, and why does it look so similar to a distantly related species?  Perhaps the feeding mechanisms captured in these videos can help to resolve this evolutionary anomaly.

An optical illusion?

Zooplanktivory is one of the most distinct feeding niches in coral reef fish and many morphological traits have been interpreted as adaptations to feeding on plankton in the water column above the reef. One of these traditional hypotheses is that zooplanktivorous fish have larger eyes for sharper visual acuity. A larger eye usually has a longer focal length and thus is expected to produce a better-resolved image.

Peter and I tested this hypothesis with a data set on eye morphology of labrids (wrasses and parrot fishes):

Schmitz, L. & P.C. Wainwright (2011). Ecomorphology of the eyes and skull in zooplanktivorous labrid fishes. Coral Reefs, 30: 415-428. reprint.

Labrids are a species-rich clade of reef fish with enormous morphological and ecological diversity. We sampled a total of 21 species, with three independent origins of zooplanktivory: Clepticus parrae, the Creole Wrasse (photo: fishbase.org), Halichoeres pictus, the Rainbow Wrasse (photo: wetwebmedia.com), and Cirrhilabrus solorensis, the Red-eyed Fairy Wrasse (photo: fishbase.org).

To our surprise we failed to find any indication of larger eyes in zooplanktivores. We tried several methods, including phylogenetic residuals of eye diameter on body mass and evolutionary changes in eye size along branches leading to zooplanktivores, but zooplanktivorous labrids did not show any signs of having larger eyes than other trophic specialists. Instead, we suspect that the notion of large eyes in zooplanktivorous labrids is an optical illusion evoked by a size reduction of the anterior facial region, which makes the eye look bigger.

However, we did find other features interpreted as adaptations to zooplanktivory in labrids. Both Clepticus parrae and Halichoeres pictus have a large lens for given axial length of the eye, related to better visual acuity, a round pupil, possibly an adaptation to search a three-dimensional body of water for food, and longer gill rakers to help retain captured prey.

Our results are quite interesting in that they highlight the importance of many-to-one-mapping in form-function relations. There often is more than one possible pathway to perform a function. In labrids, increase in eye size to improve visual acuity apparently is not part of the evolutionary response. But, let’s see what we can find in other groups!

Mysteries in Fish Functional Morphology 1. Unicorn surgeonfish


Welcome to “Mysteries in Fish Functional Morphology”. From time to time I plan on posting descriptions of some of the most fascinating and perplexing unsolved mysteries in fish functional morphology. My hope is that some of you will find these entertaining, but if you think you might have the answer to any of these mysteries or if you have any comments about them, please feel free to post a comment here.    My first installment is the cephalic horn that is found on the forehead of four species of surgeonfish in the genus Naso (N. annulatus, N. brachycentron, N. brevirostris & N. unicornis). The top photo to the left is N. brevirostris (courtesy of Bruce Yates – UnderwaterRelections.com). Below that, a group of N. annulatus. Four species also have a rounded, bump-like protuberance that is less extreme in size and shape (N. tuberosus, N. tonganus, N. mcdadei & N. vlamingii). A picture of N. tonganus illustrating the bump is at the bottom to the left.  Remarkably, according to a recent treatment of the phylogenetics of this group (Klanten et al. 2004. Molecular Phylogenetics & Evolution 32:221), the elongate, slender horn appears to have evolved three times among the living species.  What is the function of the cephalic horn?  One possibility is that the structure is used in mate selection or intrasexual displays – in other words, it evolved as a consequence of sexual selection. For me, this is the by far most appealing hypothesis I have heard, but unfortunately, the structure is not sexually dimorphic, as far as I know. Although this does not rule out the sexual selection hypothesis (sexually selected traits can have equal expression in the two genders), it is a strike against the idea.  Nevertheless, Arai & Sato (2007. Ichthyological Research 54:49) observed that the horn of male N. unicornis and the hump of male N. vlamingii were used in quick color-changing displays to females (unicornis) and other males (vlamingii) and these authors favor the sexual selection hypothesis.    Many years ago I heard another idea about function – that zooplanton-feeding species use it to help them gauge distance to their prey in the midwater, where there are few positional and distance reference points. This is an interesting idea that has not been tested to my knowledge. But, one complication here is that among the Naso species with the elongate horn, two are benthic herbivores and two feed on plankton and on the benthose. This indicates that a function in plankton feeding is unlikely to explain all occurrences of the structure.  So, is this fabulous structure the work of sexual selection or an adaptation for feeding on small plankton in midwater? Or, is there another explanation?  What do you think?

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