Sunday, April 13, 2008

The Stoplight Loosejaw

In the book "Astonishing Animals" there is an incredible painting of a fish with jaws that look like something out of an H. R. Giger painting. The authors of the book noted that one of the animals in the book was an invention, and I began to wonder if the stoplight loosejaw was too bizarre even for a deep sea fish. A quick search in Google revealed the phony (the entry before it) and the existence of this incredibly bizarre fish. Even more troubling is the internal anatomy, which is some ways is even more strange.

The genus Malacosteus belongs to the order Stomiiformes and the family Stomiidae, which includes barbeled dragonfish. I can't resist sharing the larvae in the genus Idiacanthus. Anyways, the genus Malacosteus is usually stated to be monotypic (with M. niger) although it is possible that the one record of M. indicus does represent a distinct species and not a junior synonym (Sutton 2005) [see Post-script]. Malacosteus forms the subfamily Malacosteinae with the genus Aristostomias (Herring et al. 2005) and they do appear to be roughly similar in appearance (see Fishbase for the 200+ other species in Stomiidae). So while the basic appearance of the stoplight loosejaw isn't totally unprecedented, there still are some rather remarkable features. The head is capable of being "thrown back" thanks to unossified vertebrae, the jaws take up a quarter of the fish's length and subsequently this species has one of the relatively largest gapes of any fish (Sutton 2005). There are elongated teeth on the jaws, recurved gullet teeth and a lack of gill rakers, gill teeth or an ethmoid membrane (the "floor" of the mouth (Sutton 2005). Seeing Shouten's painting in Astonishing Animals makes this anatomy a lot more apparent and this photo was the best I could find online. I'll get back to the jaws and feeding because there are other strange features that deserve mention.

The "stoplight" portion of the name is in reference to a large red postorbital photophore. The production of red light is highly unusual amongst fishes and is only present in the closely related Aristostomias and the more distantly related barbeled dragonfish Pachystomias (Douglas et al. 2000). Apparently amongst animals only the beetle Phrixothrix is also capable of this (Sutton 2005). Even more amazing is that this feature seems to have evolved individually in the different fish genera, and the structure of the photophore in Aristostomias and Malacosteus is radically different (Herring et al. 2005). The photophore of Malacosteus seems to be clearly derived from the ones that operate in the more conventional blue-green spectra, and unlike the other species the red flashes are not visible to the human eye (Herring et al. 2005). Deep sea fish are not capable of seeing wavelengths longer than blue, excluding of course these species (Douglas et al. 2000).

Aristostomias and Pachystomias converge more so in possessing very long-wavelength sensitive pigments that have shifted towards red (Douglas et al. 2000); Malacosteus of course does something different. For one thing it uses a form of chlorophyll as a pigment, more specifically bacteriochlorophyll derivatives (Douglas et al. 2000). Since vertebrates aren't known to secrete chlorophyll derivatives, Douglas et al. theorize that it is somehow taken from green photosynthetic bacteria which are not known from the open ocean. The diet of Malacosteus is highly unusual and the stomach content was noted by the authors as possibly containing the pigment. I've always wondered how complex molecules are sent through the digestive system and incorporated in a specific part of the body, but it does seem reasonable.

Sutton 2005 further comments on all these unusual systems and ties it in with the unusual diet. The feeding morphology suggests that it feeds on very large prey, but M. niger feeds heavily on copepods, even at around the maximum size (24 cm or just under 10 inches). The feeding of this species has not been observed and Sutton comments that the lack of an ethmoid membrane and gill rakers seem problematic for eating small prey. Malacosteus may even be successful enough to select its prey (assuming there isn't a retention artifact) and it seems likely that there is an as-yet unknown feeding mechanism involved. Sutton also pointed out that the eyes are binocular-facing, quite unlike any other dragonfish, (see fig. 4B) and it may be related to the feeding habits. The stoplight loosejaw has apparently abandoned the vertical migration and feast-or-famine strategy of its family by maintaining itself on copepods and the rare large food item at one depth. Since the copepods may also provide the pigment, this, in Sutton's words, is a "chicken-or-the-egg" scenario.

It seems that there is a great deal to learn from this species, or genus*, and I'm still wondering exactly how something with jaws quite separate from the rest of the head eats copepods. I'm not entirely sure if it does get the pigment it uses to see copepods from the copepods it eats, but for now there just doesn't seem to be another plausible mechanism.


Refs:


Douglas, R. H. et al. 2000. Long-wave sensitivity in deep-sea stomiid dragonŽ sh with far-red bioluminescence: evidence for a dietary origin of the chlorophyll-derived retinal photosensitizer of Malacosteus niger. Phil.Trans. R. Soc. Lond. B 355, pp. 1269-1272

Herring, Peter J. et al. 2005. Red bioluminescence in fishes: on the suborbital photophores of Malacosteus, Pachystomias and Aristostomias. Marine Biology 148: 383–394

Sutton, Tracey T. 2005. Trophic ecology of the deep-sea fish Malacosteus niger (Pisces: Stomiidae): An enigmatic feeding ecology to facilitate a unique visual system? Deep-Sea Research I 52, 2065–2076



*Post-Script:

Not one minute after I published this post, I stumbled on revised Malacosteus taxonomy:

Kenaley, Christopher P. 2007. Revision of the Stoplight Loosejaw Genus Malacosteus (Teleostei: Stomiidae: Malacosteinae), with Description of a New Species from the Temperate Southern Hemisphere and Indian Ocean. Copeia (4), pp. 886–900


I am not sure how I looked over this with Google scholar. This is the first taxonomic revision in nearly 75 years. Three species are currently recognized, M. indicus (from the Challenger expedition) and M. danae (after the research vessel). M. choristodactylus was also named and is apparently a synonym for M. niger. Kenaley examined 450 specimens of Malacosteus to resolve these taxonomic uncertainties. Apparently various specimens of M. indicus, M. danae and M. niger all describe one taxa, M. australis or the Southern stoplight loosejaw. It appears to have the same bizarre photophore and chlorophyll-related derivations. The paper very extensively described the anatomy of the species and is recommended for anyone further interested in that bizarre jaw.

1 comment:

Caitling said...

The etymology of lantern jaws I understand, but stoplight jaws?

Well alright then, I guess anything that glows red has at least 1/3 of a reason to be named after a stop light.