Bathylychnops exilis

{{Short description|Species of fish}}

{{Speciesbox

| image = Bathylychnops_exilis.gif

| image_caption = Javelin spookfish

| taxon = Bathylychnops exilis

| authority = Cohen, 1958Cohen DM (1958). Bathylychnops exilis, a new genus and species of argentinoid fish from the North Pacific. Stanford Ichthyological Bulletin 7, 47-52.

}}

Bathylychnops exilis, the Javelin spookfish, is a species of barreleye found in the northern Pacific and in the eastern Atlantic Ocean near the Azores where it is found at depths of around {{convert|640|m|ft}}. This species grows to a length of {{convert|50|cm|in}} SL.{{FishBase |genus= Bathylychnops|species= exilis| month = February | year = 2012}}

The species is notable for unusual protuberances that grow from its eyes, which each have "a well developed lens"Pearcy WG, Meyer SL, Munk O (1965). A 'Four-Eyed' Fish from the Deep-Sea: Bathylychnops exilis Cohen, 1958. Nature 207, 1260-1262. and a retina, and which have led to the species being called a "four-eyed" fish. They have two spherical eyes that are dorsally directed and two secondary eyes that are ventrally directed.Partridge, J. C., et al. “Reflecting Optics in the Diverticular Eye of a Deep-Sea Barreleye Fish (Rhynchohyalus Natalensis).” Proceedings: Biological Sciences, vol. 281, no. 1782, 2014, pp. 1–9. JSTOR, http://www.jstor.org/stable/43601696. Accessed 18 Mar. 2023. It has been suggested that the purpose of these extra eyes is detection of threats from below, since these eyes point downwards.Dawkins R (2009). The Greatest Show on Earth: The Evidence for Evolution. London: Bantam Press. The Javelin spookfish have a distinct and advantageous secondary globe to their eyes. They use mirrors to collect and focus light, instead of a lens to divert the path of light. This feature provides great benefits in their chances for survival.{{Cite journal|last1=Pearcy|first1=William G.|last2=Meyer|first2=Samuel L.|last3=Munk|first3=Ole|date=September 1965|title=A 'Four-Eyed' Fish from the Deep-Sea: Bathylychnops exilis Cohen, 1958|url=https://www.nature.com/articles/2071260a0|journal=Nature|language=en|volume=207|issue=5003|pages=1260–1262|doi=10.1038/2071260a0|pmid=5884641 |s2cid=4193190 |issn=1476-4687}}{{Cite journal|last1=Stein|first1=D. L.|last2=Bond|first2=C. E.|date=1985|title=Observations on the morphology, ecology, and behaviour of Bathylychnops exilis Cohen|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1095-8649.1985.tb04022.x|journal=Journal of Fish Biology|language=en|volume=27|issue=3|pages=215–228|doi=10.1111/j.1095-8649.1985.tb04022.x|issn=1095-8649}}