Look at the head and jaws of a halfbeak:
The lower jaw appears very elongate, reaching as much as ten times further than the upper jaw. What is the function of this bizarre anatomy? Is it used in feeding? Is it used in locomotion? If so, how?
Halfbeaks (Hemiramphidae) are related to needlefish, flying fish and sauries. They are slender-bodied fish that spend most of their time swimming right up at the surface of the water where they are expert at hiding from predators. They mainly occupy warm and temperate inshore marine habitats, though there is also a radiation of freshwater forms. The marine species feed on drifting pieces of plants and zooplankton. The freshwater species and most inshore marine forms also eat drifting surface insects.
Once you get past the very strange appearance of these fish and closely inspect the head you find that the long lower jaw is not actually a lower jaw but instead an absurdly long chin (have a look at the diagrams below from Montgomery & Saunders 1985). The toothed lower jaw is short and matches the toothed upper jaw. The halfbeak is actually a very long extension of the part of the mandible that is below, and in front of the teeth – a chin. The structure is smooth and not armed with teeth. This is our first clue that the structure is not an elaborate jaw used in prey capture. If it’s not part of the feeding apparatus, then how does it function?
One idea that has some support is that the long chin is part of a specialized sensory devise. Montgomery & Saunders (1985a) showed that there are a series of lateral line pores along the length of the lower jaw, with neuromasts in between these pores. They argued that this long structure, equipped with the lateral line pits, may function in prey detection. The species they worked with feeds at night on large zooplankton and they showed it is capable of detecting live plankton in total darkness. In effect, they argue that the halfbeak functions as an extension of the lateral line system that may aid these fish in nocturnal attempts to locate moving prey animals in midwater habitats.
This is a fascinating proposal that may help us understand the function and evolution of the halfbeak. But the system requires more attention before we fully understand how it works and how it evolved. It would be valuable to model the potential sensory advantage of the long, sensitive chin. Does it convey a major performance advantage over fish that lack such a device? What was the evolutionary sequence here? Did the long chin evolve as a specialization of the lateral line system, or did the long chin evolve first for a different function, and then become co-opted for use as a scaffolding for the sensory system? And, what role does the long lower jaw play during prey capture? Such a large structure must interact with prey during feeding. Are there other, secondary functions of the long chin? What do you think?
Montgomery, J.C. & A.J. Saunders. 1985. Functional morphology of the piper Hyporhamphus ihi with reference to the role of the lateral line in feeding. Proc. Roy. Soc. Ser. B. 224:197.
Saunders, A.J. & J.C. Montgomery. 1985. Field and laboratory studies of the feeding behavior of the piper Hyporhamphus ihi with reference to the role of the lateral line in feeding. Proc. Roy. Soc. Ser. B. 224:209.
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?