Thysanoteuthis rhombus

 The Diamond Squid is a large nektonic species of cephalopod that can be found in tropical and subtropical Waters world wide. It can reach 130 cm long and 30kg, and although it possesses big fins and a hidrodynamic shape, it is considered a largely inactive migrant that uses the warm currents to perform their migrations. Curiously, it is a common knowledge of worldwide fishermen that capture squids that this species swims in pairs, usually a couple of the same age and size; this anecdotal knowledge has been described on the short documentary “Diamonds of the Depths” by Jovana Milanko (2012), which explores the local artesanal fishery of the Dominican Republic and features recordings of this species alive. In the paper “Notes on the occurrence and biology of the oceanic squid, Thysanoteuthis rhombus Troschel, in Japan” from 1966, Saburo Nishimura describes the same pairing behavior to be reported from japanese waters, and they go so far as to describe in detail an observed behavior of flying from 1929, and then note how the observations made by them did not correlate this report nor the local fishermen confirm the existence of this “leap” mentioned almost a century ago. T. rhombus is described as sluggish and “waddling” in this paper, although capable of contracting the muscles and swimming away in case of danger, demonstrated well on Milanko’s documentary where the fishermen spend up to na hour fighting the squid on the line. The mentioned pairing behavior is understood as an adaptation for the low density worldwide population, the hypothesis being that a male and female swimming in pairs is advantageous in garanteeing reproduction.

    A hypothesis about it’s sluggish movement, where T. rhombus will swim slowly ondulating it’s big fins, is that it may be a tradeoff for it’s quick growth and metabolism. The Diamond Squid completes it’s life cycle in about a year, so as a species that grows to it’s size it needs to have a quick metabolism, but as it usually hunts between 600-130m deep, prey might be scarce so it’s not interesting to keep actively swimming and spending unnecessary energy. T. rhombus’s egg masses resemble pyrosomes, and float on the surface of the sea being carried by the currents, and it’s larval forms are quite beautiful. Maybe I’ll draw it someday?

                Finally, it’s relation with humans is historically based on sightings of this animal beached, but more recently the Diamong Squid has been explored as a comercial food source based on it’s size and availability. The problem is that this fishery started without much data and still today data is lacking in many places where this species is captured, so there are catch declines reported by fishermen in some places (one of the examples is in Milanko’s documentary), and deficiency of data being widespread means little action is being done to prevent overexploitation of the species – how can you keep the captures to a sustainable limit if you don’t even know what’s that limit? We may be pushing this species to extinction, and we don’t even know it!

Nishimura’s paper: core.ac.uk/download/pdf/393014…
Milanko’s documentary: www.youtube.com/watch?v=xAG0qe…