The Hammerhead Hoverfish

Order: Spathospondyliformes

Family: Thyreoichthyidae

Subfamily: Thyreoichthyinae

Genus: Thyreoichthys

Spathospondyliform pelagys are highly diverse, all encompassing an endless amount of diversity. They all share their preference for median paired-fin swimming and have often reduced their tail muscles and use their tail rhachis only for display. A unique clade of spathospondyliformes, the diverse thyreoicthyids (shield-fish) are known for their thick coverings of armour made of a material similar in composition to a mix of dentine and enamel (like teeth) and are noticeably more sedentary than other spathospondyliforms. By far the strangest of the thyreoichthyids is the namesake of the family, the thyreoichthyines, a subfamily with 50 species of pelagys dubbed hoverfish, all of which are extremely small, and are the strangest of the pelagys. 

The hammerhead hoverfish, so called because its eyes are on the ends of a laterally extended t-shaped struckture that seems to widen its range of vision (hammerhead sharks have a similar structure known as a cephalofoil) and gives it depth perception and binocular vision.

Now, the hoverfish are completely unrecognizable as pelagys, because they just look so unusual. As thyreoichthyids, they possess large plates of armour on their cephalon, comprising of the carapace, the dorsal part of the exoskeleton, connected to the plastron on the bottom, which leaves space for the pectoral fins, ventral/dorsal fins and gill-pouch, however disconnected from the armoured tail, which is split into several segments which allow it to flex and turn despite the hard bony plates which serve as protection coating the skin. The tail possesses small tubercules which allow them to 'hook' on objects and improve their general grip. Similar tubercules occur in larger quantities on the carapace, which are now present as large bony prongs which project off the distal extensions of the carapace.

For most of its life, it rests clinging onto a blade of carpgrass, a rock or anything it can wrap its prehensile tail around, sacrificing swimming speed (it is among the slowest of pelagys, moving at 3.5 m/h, on average) for defensiveness, often laying immobile while grasping on a suitable holdfast. When disturbed, they might leave or swim away, but this is rare; hoverfish are terrible swimmers.

The hammerhead hoverfish, despite its silly appearance, has impressive visual acuity, despite being essentially colourblind, they can see polarised light and detect polarity, giving them excellent vision. But what will such a immobile creature have to do with impressive vision?

Signalling and communication between themselves. Pelagys are very visually-inclined organisms, as they have not evolved any apparatus that allows for vocalisation, and thus rely on bright colours or physical contact to communicate with each other. The yellow-coloured caudal rhachis of the hammerhead hoverfish is extremely bright in colour, being a very vibrant hue of yellow, allowing hoverfish to communicate and to settle territorial disputes, but it also is a polarised surface, allowing hoverfish to distinctively tell between individuals of its own species, or even advertise availability to mates.

During the peak of spring, the hoverfish come together and release pheromones into the water, advertising availability while also flashing their bright yellow rhachises to attract the larger females. Soon, they both take part in an elaborate courtship ritual, synchronising their movements, which may take several hours, or even a day in some cases. If the female sees the male as suitable, they head to grasp onto a suitable area, such as a blade of carpgrass, and begin mating. Afterwards, the female releases fertilised eggs in a 'mucus net', and attaches it under her, getting the female to incubate the eggs, holding off eating for the entire incubation period in order to conceal the eggs. Once the eggs hatch, the female resumes normal activities, leaving the planktonic young to fend for themselves.