Taxon of the Week #2: Trachinoidei

I'm continuing the fishy theme for the Taxon of the Week. The Trachinoidei are what are known as the weevers and allies - mostly elongate, often dorsoventrally flattened, regularly benthic fish. In keeping with the benthic habitat (most species bury themselves in the sand for camouflage), the eyes are generally placed on top of the skull with the mouths pointing upwards. This suborder includes the stargazers and sand lances.

To be honest, I'm having a hard time finding what, beyond a vague overall similarity, are the actual uniting characters of this group. Bond (1996) emphasises the jugular pelvic fins (placed under the throat), but this feature is not unique to trachinoids - the Blennioidei (blennies), Gobiesocidae (clingfishes) and Callionymoidei (dragonets) all also have jugular pelvics. Pietsch & Zabetian (1990) gave two characters - presence of a pelvic spur and pectoral radials being small, short or wide - uniting a collection of families that they referred to as the core of, but not necessarily delimiting, the Trachinoidei. Not surprisingly, all the molecular studies that have addressed the issue appear to recover the trachinoids as polyphyletic (Stankovic et al., 2005), and authors have disagreed significantly about which families should be included in the suborder. Being a trachinoid, it seems, is not so much a reality as a state of mind.

Rather than give an entire list of families, I'll simply direct you to Mooi and Johnson's page on the subject. A number of trachinoids have poisonous spines, and the name "weever" for members of the Trachinidae is supposedly derived from the Anglo-Saxon word for "viper". Uranoscopus (stargazers) also have electric organs behind the eyes - Bond (1996) credits them with producing 50 volts.

Cheimarrichthys fosteri (the torrentfish) is a freshwater fish unique to fast-running streams in New Zealand that is sometimes placed in its own family (the page I've linked to includes it in a family with other marine species - note that the 'blue cod' mentioned is no relation to the Atlantic cod).

My definite favourite among the families assigned to this suborder, however, has to be the Chiasmodontidae (black swallowers), another addition to the mesopelagic freakshow. Black swallowers get their name from their ability to distend their stomach to three times their own size and so swallow fish larger than themselves whole (memories of snork-eater-eaters...). Doubtless this is a very handy adaptation in the fairly sparse environment of the mesopelagic, where meals may be few and far between.

REFERENCES

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

Pietsch, T. W., & C. B. Zabetian. 1990. Osteology and interrelationships of the sand lances (Teleostei: Ammodytidae). Copeia 1990 (1): 78-100.

Stankovic, A., K. Spalik, P. Golik, A. V. Balushkin, P. Borsuk, M. Koper, S. Rakusa-Suszczewski & P. Weglenski. 2005. Polyphyly of Scorpaeniformes and Perciformes: new evidence from the study of notothenioid's mitochondrial and nuclear rDNA sequence data. Journal of Ichthyology 45 (Suppl. 1): S171-S182.