Dreamers that will Give You Nightmares

            Using a natural, bioluminescent lure is a successful method of attracting prey, but the lures come in many different forms. The family Oneirodidae, or dreamers, use another variation of a modified dorsal spine as a lure, but it appears much shorter than that of the average anglerfishes. In addition to their unique illicium, members of Oneirodidae have several other adaptations giving them a highly unique and slightly terrifying appearance.

                                 

                                 Fig. 1 Bulbous Dreamer, Oneirodes eschrichtii  -                   (http://fishesofaustralia.net.au/home/family/161)

            Like the Ceratioid seadevils, Oneirodidae fishes appear deep black in colour, diminishing their visibility in water. With only the bright glow of the illicium, other fish are instantly drawn toward the sneaky predators. Based on the scary appearance of the various Oneirodidae fishes, it should not be surprising that they act as predators.

Fig. 2 A female Longhead Dreamer, Chaenophryne longiceps, collected in the Tasman Sea on the 2003 NORFANZ Expedition (http://fishesofaustralia.net.au/home/family/161)

The fishes are typically found between 11-29 centimetres and fall primarily on the shorter end of the spectrum (Jónsson and Palsson, 1999), but regardless of their smaller size, there is something threatening about their appearance. Their globular bodies and short caudal and pectoral fins look like an ineffective means of locomotion, but they somehow thrive as ambush predators.

Fig. 3 Haenophryne melanorhabdus SL 52 mm with an attached parasite 15 mm long (http://tinyurl.com/mtwtwrk)

            Oneirodidae fishes are clearly doing well in means of survival, as they have been found throughout a variety of waters. The first species was captured in 1973 off the west coast, but have also been captured with bottom trawl in the Víkuráll area and the southeast coast of Iceland (Jónsson and Palsson, 1999). Of the deepsea anglerfishes, the family Oneirodidae is the largest and most diverse family within the suborder Ceratioidei, but is also the least well known (Pietsch, 1973). The different fish within this family may hold another key to discovering information on how and when the evolution of deepsea fishes occurred, but the elusive nature of Oneirodidaes poses a challenge in studying their traits and behaviour.

The (Sea) Devil is in the Details

While anglerfishes are recognized for having unique features, the most well-known attribute pertains to the natural tool many of the fishes use for feeding. This tool is a modified dorsal spine that forms a lure and is used to attract prey. It is common among many varieties of Lophiiformes, like Ceratioids, but is not identical from one species to another.

Fig. 1 The angling fish can look quite different from each other (http://tinyurl.com/kcbhj3t)

Ceratioids, or seadevils, are widely recognized due to a combination of their luminescent angling lures, black skin pigment, and fang-like depressible teeth (Anderson and Leslie, 2001). 

Fig. 2 Deep-sea ceratioid anglerfish (http://tinyurl.com/klnpl9o)

The lure is not a trait exclusive to Ceratioids, as it is found in most of the living 323 Lophiiform species, but within the Ceratioids, males lack the luring apparatus (Quigley, 2014). In addition to the lack of a lure, males also lose their normal jaw teeth during metamorphosis, and instead develop a set of denticles to grasp onto prospective mates (Quigley, 2014).  The males are obligatory sexual parasites (Anderson and Leslie, 2001) with well-developed large eyes, and oversized nostrils that aid in detecting potential mates through recognition of a species-specific chemical attractant emitted by females (Quigley, 2014). Ceratioid fishes are noted for their sexual dimorphism, with males displaying dwarfism, a denticular apparatus, an absence of specific traits, and total structural reorganization (M.P. et al., 2016). The strong differences between the two genders make Ceratioids easily distinguishable from their bottom-living Lophiiform relatives, like Lophius piscatorius (Quigley, 2014).

Fig. 3 Longray Seadevil, Ceratias holboelli (http://tinyurl.com/kdfw3yr)

The adaptations of Ceratioids have allowed the anglerfishes to survive in various waters, with sightings recorded in all oceans to both subpolar regions (Anderson and Leslie, 2001) and just one sole finding in Russian waters (Kharin and Milovankin, 2007). Due to their relative abundance, high species diversity, and trophic position as top primary carnivores, ceratioid anglerfishes hold high ecological importance (Quigley, 2014). 
Although they are not the most commonly discussed of the anglerfishes, seadevils clearly have an array of noteworthy features that separate them from similar fishes, and aid in their survival. Not only are Ceratioidei hardly limited by water conditions, but their reproductive strategies transform their ability to survive into an ability to thrive.

References

      Anderson, M. and Leslie, R. (2001). Review of the Deep-Sea Anglerfishes (Lophiiformes: Ceratioidei) of Southern Africa. Ichthyological Bulletin, [online] 70, pp.1-29. Available at: http://bit.ly/2qzoUIt [Accessed 18 Apr. 2017].

      Kharin, V. and Milovankin, P. (2007). A new occurrence of the Deepsea Anglerfish Cryptopsaras couesii (Osteichthys: Lophiiphormes: Ceratiidae) in Russian waters. Journal of Ichthyology, [online] 47(1), pp.112-113. Available at: http://bit.ly/2pSyHx7

      M. P., R., Jacob, V., K. S., S., V. N., S., M., H. and M., S. (2016). Three new records of rare deep-sea Anglerfishes (Lophiiformes: Ceratioidei) from the Northern Indian Ocean. Marine Biodiversity, [online] 46(4), pp.923-928. Available at: http://bit.ly/2qyZNW4 [Accessed 18 Apr. 2017].

      Quigley, D. (2014). Ceratioid Anglerfishes (Lophiiformes: Ceratioidei) in Irish Waters. Sherkin Comment, [online] (58), p.7. Available at: http://bit.ly/2p3YuOD [Accessed 18 Apr. 2017].

      Fig. 1: Pietsch, T. (2013). The angling fish can look quite different from each other. [image] Available at: https://m.simplyscience.ch/kids-tiere-pflanzen/articles/auf-tauchstation-24659.html [Accessed 18 Apr. 2017]. http://tinyurl.com/kcbhj3t

      Fig. 2: Luminescent Labs (2013). Deep-sea ceratioid anglerfish. [image] Available at: http://blog.luminescentlabs.org/post/61812091341/deep-sea-ceratioid-anglerfish-this-beautiful [Accessed 18 Apr. 2017].

      Fig. 3: Bray, D. (2017). Longray Seadevil, Ceratias holboelli. [image] Available at: http://fishesofaustralia.net.au/home/family/180 [Accessed 18 Apr. 2017].