I've noted some examples before (in posts here and here) from the bizarre world of deep-sea fish, where life gets really ugly (because where there's no light and no-one can see you, you can really let yourself go). I thought I'd put in a mention of what are arguably among the most bizarre of deep-sea fishes, the Saccopharyngiformes. I can't recall when I first came across an illustration of these incredible creatures, but they're not something you readily forget (the image above comes from Animal Diversity Web).
Saccopharyngiformes are deep-sea 'eels'. They're not real eels (i.e. they're not members of the order Anguilliformes), but they are closely related and like true eels are members of the clade Elopomorpha. Elopomorphs are united by a distinct planktonic larval form called a leptocephalus, with a leaf-shaped transparent form shown below in a photo from Wikipedia. Admittedly, this photo shows a true eel rather than a saccopharyngiform eel, but the general idea's the same - except that saccopharyngiform leptocephali have the unique feature that the myomeres (the muscle blocks) are V-shaped instead of W-shaped. Saccopharyngiformes have greatly elongate jaws, attached to the neurocranium by only a single condyle. Most of the other uniting features of the order are absences - no scales, no pelvic fins, no ribs (see Fishbase for a complete list).
There are four families of Saccopharyngiformes. The most distinctive family is the bobtail snipe eels of the Cyematidae (image above of Cyema atrum from Animal Biodiversity Web again). Cyematidae are relatively small creatures with a distinctly cut-off appearance. Only two adult species are known, but apparently the known diversity of leptocephali attributable to this family suggests the existence of more. The long jaws bend away from each other and so can't be closed against each other - a feature shared by the unrelated but superficially similar true snipe eels of Nemichthyidae in the Anguilliformes.
The family Saccopharyngidae is the most familiar in the order (relatively speaking, of course), containing the gulper eels. Gulper eels can be extremely long, up to 2m in length, but the greater part of this (2/3 to 4/5 of the length) is taken up by the exceedingly long and filamentous tail. The remainder is dominated by the head - specifically the jaws - giving the appearance that these creatures are all mouth. How exactly that gigantic mouth is propelled by such a slender tail seems somewhat mysterious to me, and I'd love to know just how gulper eels spend their time. The tip of the tail bears an expanded, usually luminescent caudal organ - is has been suggested that this is used for a lure to attract prey, but without life observations this is mere speculation. Male gulpers have reduced jaws and an enlarged olfactory system relative to females. Eurypharynx pelecanoides, the pelican eel (the subject of the picture at the top of this post) is similar to the gulpers, but is separated as its own monotypic family. Eurypharynx has an even larger mouth than the Saccopharyngidae - over half the preanal length in the former as opposed to less than 40% in the latter.
The most bizarre of all the Saccopharyngiformes (and that's saying something) are undoubtedly the one-jawed eels of Monognathus, shown above in an image stolen from Smith (2002). Monognathus are the deepest-living of all elopomorphs, and have been found at depths of 5400m. The head is greatly reduced, and the common name refers to the complete absence of the upper jaw. A single venomous pronged fang sticks forward from the skull where the upper jaw should be - doubtless this is used to impale prey, but as Smith (2002) notes, " their odd morphology and their near total lack of sense organs make it difficult to imagine how they function and survive in their environment". Like other Saccopharyngiformes, Monognathus have a distensible abdomen, the posterior part of which oftens extends in a pouch that may go past the anus.
Though fourteen species of Monognathus have been described, only a single mature male specimen has ever been recovered. This specimen differed significantly from females. The lower jaw was almost absent, the fang was blunted, the olfactory organs were greatly enlarged, a layer of spongy tissue covered the head and the dorsal and anal fins were enlongated behing the tail into a notched fin. Obviously the males completely stop feeding on reaching maturity, and become totally dedicated to finding a mate. Their short life-span as a result probably explains why specimens are so rare.
Smith, D. G. 2002. Families Cyematidae, Saccopharyngidae, Eurypharyngidae, Monognathidae. In: FAO Species Identification Guides for Fishery Purposes, The Living Marine Resources of the Western Central Atlantic, Vol. 2.