Hunters in the Deep Sea

Longnose lancetfish Alepisaurus ferox, photographed by Paulo De Oliveira.

For today's post, I'm going to tackle the Alepisauroidei. The exact scope of this clade of fishes has changed a bit between authors; here, I'm focusing on the restricted sense used by Sato & Nakabo (2002). In contrast, Davis & Fielitz (2010) used 'Alepisauroidei' in a broader sense that combined the Alepisauroidei, Chlorophthalmoidei and Giganturoidei of the former authors; if I have to refer to this larger clade, it'll be as 'Alepisauroidei sensu lato'.

Tedious definitionising aside, the Alepisauroidei sensu stricto include the living families Scopelarchidae, Evermannellidae, Alepisauridae and Paralepididae. All members of these families are predators in the mesopelagic zone of the ocean, below the level of the light. Members of the Alepisauridae (lancetfishes) and Paralepididae (barracudinas) are elongate, reaching lengths of over a metre in the case of the lancetfishes. Many mesopelagic fish migrate closer to the surface at night, and Bond (1996) refers to lancetfishes being caught by anglers standing on the shore during spring in the Pacific Northwest of North America. The predatory nature of the alepisauroids, as well as something of their general appearance, can be inferred from the common names given to many of them: as well as the barracudinas already mentioned, there are the sabretooth fishes of the Evermannellidae, and the daggertooth Anotopterus pharao.

An array of alepisauroids, from here. Species shown are: (Paralepididae) (1) Stemonosudis rothschildi, (2) Lestidium atlanticum, (3) Lestrolepis intermedia, (Evermannellidae) (4) Coccorella atlantica, (5) Evermannella indica, (Scopelarchidae) (6) Scopelarchus analis.

Alepisauroids show many of the adaptations common among deep-water fishes, such as the presence of bioluminescent organs (absent in Alepisauridae) and thin-walled, distensible stomachs allowing the immediate engulfment of large prey items. In the evermannellid genus Coccorella, the caecum of the intestine has become expanded to the extent that part of it actually extends into the animal's head and can be seen in the base of the oral cavity (Wassersug & Johnson 1976). The members of the family Scopelarchidae are known as pearleyes due to their enlarged, dorsally-directed tubular eyes (also present in the Evermannellidae) that presumably increase their ability to detect the limited light filtering down from above (and, more importantly, from the bioluminescent organs of other mesopelagic animals). Another notable adaptation to the mesopelagic environment present in all alepisauroids is that they are simultaneous hermaphrodites: each individual has fully functional male and female reproductive organs. In an environment where the usual scarcity of food items means that species exist at very low population densities, simultaneous hermaphroditism means that any other individual of your species is a potential mate. Simultaneous hermaphroditism is also found in other members of the Alepisauroidei sensu lato, making it the largest clade of vertebrates utilising this reproductive strategy (Davis & Fielitz 2010).

Specimen of daggertooth Anotopterus vorax, photographed by Peter Marriott.

A comprehensive investigation of the molecular phylogeny of alepisauroids was published by Davis & Fielitz (2010). Evermannellids and scopelarchids were resolved as successive sister groups to the other alepisauroids sensu stricto, suggesting that their tubular eyes may have arisen independently (as corroborated by their absence in the evermannellid genus Odontostomops; alternatively, tubular eyes could have been lost in other alepisauroids). The Alepisauridae were nested within an apparently paraphyletic Paralepididae. The clade as a whole was suggested by molecular dating to have diverged some time in the Early Cretaceous, a result in concordance with the known fossil record.

REFERENCES

Bond, C. E. 1996. Biology of Fishes, 2nd ed. Saunders College Publishing.

Davis, M. P., & C. Fielitz. 2010. Estimating divergence times of lizardfishes and their allies (Euteleostei: Aulopiformes) and the timing of deep-sea adaptations. Molecular Phylogenetics and Evolution 57: 1194-1208.

Sato, T., & T. Nakabo. 2002. Paraulopidae and Paraulopus, a new family and genus of aulopiform fishes with revised relationships within the order. Ichthyological Research 49 (1): 25-46.

Wassersug, R. J., & R. K. Johnson. 1976. A remarkable pyloric caecum in the evermannellid genus Coccorella with notes on gut structure and function in alepisauroid fishes (Pisces, Myctophiformes). Journal of Zoology 179 (2): 273-289.

Living Larvae and Fossil Fish

Marshall's macristium larva, from Rosen (1971).

Before anything else, a-few-days-belated birthday wishes to Tetrapod Zoology, which has now been going in one form or another for three years. Darren Naish, the author of Tetrapod Zoology, also notes that the number of palaeontology blogs being written that aren't afraid to be technical has increased significantly in recent times - "I don't know if it seems arrogant to think that Tet Zoo was a driving force behind this uber-nerd movement, but I like the idea that it was, so will stick with it". I don't know about other sites, but Darren Naish can pretty much take sole credit (or blame, whichever way you want to look at it) for inspiring yours truly to publish my own ramblings. Of course, I haven't Darren's ability, and I've never achieved his level of following (I'm still waiting for my invite to join ScienceBlogs ;-) ).

In my last post, I briefly alluded to the recent discovery that what have been thought to be three separate species of fish in three different families are, in fact, different life cycle stages (larva, adult male and adult female) of a single species. As remarkable as this discovery is, it can't be called completely incredible - it simply highlights just how little we know about many marine animals. In animals that undergo significant metamorphic changes over the course of development, it is not surprising that the connection between stages should not be initially recognised*. What really struck me about the affair were the low numbers of known specimens - of the three "families" involved, only 65 specimens of "Megalomycteridae" (the adult males) have ever been collected. "Mirapinnidae" (the larvae) are represented by only 120 specimens, while "Cetomimidae" (the adult females) tip the scales at about 600 specimens. To put that into a bit of perspective, the other day I was counting my way through a vial of harvestmen that included some 200 specimens from a single collection.

*Indeed, for those of you familiar with marine invertebrates, this is the reason behind the latin-derived terms for many invertebrate larvae - nauplius, cypris, cercaria. These forms were all initially described as distinct taxa, and after they were recognised as larvae of other taxa their past generic names persisted as terms for that stage in the life cycle.

Because Ed Yong at the link above has already done a bang-up job of explaining the cetomimid situation, I thought I'd dig into the vaults a little and bring up an earlier situation where a family of fish became written off as larvae - the Macristiidae.


Bathysaurus mollis. Photo from here.

"Macristium chavesi" was described by Regan in 1903 from a single specimen collected off the Azores in the North Atlantic (Rosen, 1971). It should be noted that "Macristium" was recognised from the start as a larval form, but supposedly of adults as yet unknown. Initially, Regan regarded Macristium as related to Bathysaurus, a genus of deep-water predatory fish currently in the Aulopiformes, but in 1911 he separated it as its own family that he suggested was related to the Alepocephalidae (other deep-water predators, but now in an entirely different order, the Osmeriformes). Regan's Macristium specimen was in dreadfully poor shape - the lower jaw was damaged, part of the upper jaw was lost entirely, and only one fin (a pectoral) had remained reasonably intact. A second macristiid specimen, in better condition, would not be recognised until 1961, when Marshall described a specimen collected by the ship 'Discovery' in the Bay of Biscay. Marshall's specimen was long and slender, with remarkably elongate fins. On the basis of the new specimen, Marshall reclassified Macristium once again, as a member of the Ctenothrissiformes.

Ctenothrissiformes is a small order of four genera known from England and Lebanon. The type genus, Ctenothrissa resembles Macristium in its elongate fins, but differs from it in being fairly deep-bodied. All three ctenothrissiform genera had one other significant difference from Macristium - they are known only from fossil deposits laid down in the Cretaceous (Patterson, 1964). If Macristium was indeed a member of the Ctenothrissiformes, it was a living survivor of a group long thought to be extinct. As it turned out, though, it was not to be. The relatively few features cited by Marshall as uniting Macristium and Ctenothrissiformes varied from the superficial (fin shape) to the non-existent (supposed similarities in jaw structure). When Berry and Robins described a third macristiid specimen from the Gulf of Mexico in 1967 as a new species, Macristiella lucens, they were sceptical of Marshall's interpretation.


Specimen of Bathytyphlops marionae. Photo from here.

The resolution of the macristiid mystery came in the early 1970s. A third specimen of Macristium was collected by the ship 'Chain' in the mid-Atlantic, allowing Rosen (1971) to convincingly relate it through various meristic characters such as vertebral count, anal position, etc. to members of what is now the order Aulopiformes, and specifically Bathysauridae. Specimens of "Macristiella" from the Pacific Ocean were identified by Okiyama (1972) as belonging to the genus Bathytyphlops in the Ipnopidae (also Aulopiformes). Finally, Johnson (1974) demonstrated using similar characters as in Regan (1971) that a specimen of Macristium from the Gulf of Mexico was assignable to the adult species Bathysaurus mollis. It is perhaps one of ichthyology's great ironies that Regan, as it turns out, had gotten it right in the first place.

As for the Ctenothrissiformes, it may not be a natural group even with the exclusion of Macristium. As indicated in Rosen (1971), while ctenothrissiforms are seemingly related to the modern acanthomorphs (spiny-finned fishes), the characters uniting them as a group are probably all primitive, and Patterson (1964) demonstrated that the genera show different mixtures of primitive and derived features for acanthomorphs. Rosen suggested a relationship to the Beryciformes, but certain features such as the absence of spines in the fins exclude Ctenothrissiformes from the Acanthomorpha (Patterson, 1964). Recent studies suggest that the "Beryciformes" may be a paraphyletic grade near the base of the acanthomorphs (Li et al., 1999), and perhaps the "Ctenothrissiformes" are themselves a paraphyletic outgroup to Acanthomorpha as a whole.


Part and counterpart fossils of Ctenothrissa from Lebanon. Photo from here.

Postscript: They sure don't write scientific articles like they used to. Hay (1903), writing in the American Naturalist, gave an introduction to the diversity of fossil fishes from the Cretaceous of Lebanon (including Ctenothrissa):

To the palæontologist the earth's crust, in its breadth and thickness, is a burial ground from which he may exhume the remains of the animals and plants that once lived on its surface or in its waters. The words of Bryant, spoken of the races of men, might truthfully be applied to other living things,

                     "All that tread
The globe are but a handful to the tribes
That slumber in its bosom."

But there are spots were the carcasses have been sown thicker and have been better preserved than elsewhere; and to such places the scientific birds of prey, who seek for, and must usually be satisfied with, fragmentary bones, and imprints of skeletons, and scattered scales and teeth, are gathered together; and, fed on such booty, they have visions of the swarms of animals, fat, sapid, and comely, that once populated the earth.

REFERENCES

Hay, O. P. 1903. Some remarks on the fossil fishes of Mount Lebanon, Syria. American Naturalist 37 (442): 685-695.

Johnson, R. K. 1974. A Macristium larva from the Gulf of Mexico with additional evidence for the synonymy of Macristium with Bathysaurus (Myctophiformes: Bathysauridae). Copeia 1974 (4): 973-977.

Okiyama, M. 1972. Morphology and identification of the young ipnopid, "Macristiella", from the tropical western Pacific. Japanese Journal of Ichthyology 19 (3): 145-153.

Patterson, C. 1964. A review of Mesozoic acanthopterygian fishes, with special reference to those of the English Chalk. Philosophical Transactions of the Royal Society of London Series B, Biological Sciences 247 (739): 213-482.

Rosen, D. E. 1971. The Macristiidae, a ctenothrissiform family based on juvenile and larval scopelomorph fishes. American Museum Novitates 2452: 1-22.