#cutlassfish

The glossy cutlassfish has no scales, instead reflective guanine crystals on their skin. These microscopic crystals scatter light, like a vertical mirror, making it invisible to twilight zone predators. (When not illuminated by the headlamps of an underwater submersible, of course.)

Sure!

Daily fish fact #515

Largehead hairtail!

Brought to you by a marine biologist with the one with the elusive name...

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Today’s fish goes by many names, and whenever it comes up in a bottom trawl tow, I can never, for the life of me, remember what it’s called. It’s always at the tip of my tongue. I know it as the Atlantic Cutlassfish (eventually), fishermen call it a beltfish, and AC Pocket Camp calls it the Largehead Hairtail.

We know the deal with common names at this point, right? That they suck? Well, in this fish’s case, so does the scientific name - Trichiurus lepturus. Now, there’s nothing wrong with it necessarily, except that the name may refer to a complex of species, as is sometimes argued, rather than a very widespread, singular species. The Largehead Hairtail is present in most of the temperate and tropical areas of the Pacific, Atlantic, and Indo-Pacific.

So, what is giving scientists the idea that these populations of Largehead Hairtail are actually separate species? Typically, genetics will be the end all, be all thing that says whether an animal is a separate species. Usually. I say that sparingly. Unfortunately, as I spoke about when we talked about the Arowana, and its similar problem, defining the term “species” is already not clear. That just throws a wrench into the whole thing. However, unlike the Arowana, this fish may have more going for it in this argument. Besides genetics, some of the populations, like the Northwest Pacific population near Japan and China, also show morphological (appearance) differences, however slight. We know physical traits come about from genetic information, so there’s that chance it’s actually different. However, with the possibility of hybridization and the definition of species continually eluding us, that’s not a smoking gun. If it ends up being true, though, then Pocket Camp’s hairtail is the Japanese Cutlassfish, T. japonicus. (Yes, it even has its own species name that only some experts and authorities in fish cataloging recognize. Taxonomy is a big deal, okay?)

Anyway. No matter what you call it, it’s clear this thing is a predator. 

By 俊仁 (Toshihito) 小林 (Kobayashi) - Flickr: 海タチウオ, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=19270032

Look at those teeth! Juveniles and adults of this species are predatory, eating invertebrates and other fish, respectively. Though the age classes do separate themselves. Juveniles go about their diel migration, in which they rise to the surface waters at night to feed and then settle back to the seabed in the daytime. Adults do the exact opposite, but, unfortunately, that still doesn’t stop them from cannibalizing younger hairtails sometimes. Such is life when you lay thousands of drifting eggs at a time.