Today on Sex is Weird: I’m going to tell you about how one animal avoids the hassles of finding a mate. You see, for many animals, finding a mate is simply a matter of selecting the best candidate out of many. Other animals (and many plants for that matter) just release their gametes and leave the rest to chance. Still, for some, even locating a mate can be hard. In this case, animals will tend to stick together for the remainder of their years. Then there’s Krøyer’s deep-sea angler fish, Ceratias holboelli. In the vast and desolate expanse of the deep ocean, they take mating for life to a whole new level.
First, a bit of basic biology. Ceratias holboelli lives in every ocean at depths from 400m to 2000m. It is the largest of the deep-sea angler fish with females usually measuring 77cm up to 120cm. Free-living males, on the other hand, top out at 1.3cm. Females have appendages on their heads that they use to lure their prey, whereas males have really big eyes and sharp, exaggerated teeth (Binohlan 2011). Sexual dimorphism (physical differences between females and males) is hardly rare, so what makes this species so special? The answer lies in the way that the males use their adaptations.
Lots of deep sea predators have immense eyes and big pointy teeth, but C. holboelli males don’t even eat. No, these animals are specifically adapted to find and hold onto a mate. Once they spot a female, they latch on with their teeth and hold on tight. Enzymes are released that cause the skin and circulatory systems of the two individuals to merge, and the males essentially live as “parasitic gonads” on the females. I suppose calling them parasites is not entirely accurate since they do provide a benefit to the females (and they are more than just gonads). They release sperm, and save the females the energy of having to search for a mate. Additionally, the fused males still breathe through their own gills, have their own heart beats, and filter waste through their own kidneys; they simply rely on the females for nutrition via the fused circulatory systems (Gould 1983).
So when mates are scarce, one very effective way to keep one is to bite down, fuse circulatory systems, and never let go. Sex is weird.
Binohlan, C. B. 2011. Ceratias holboelli.in R. Froese and D. Pauly, editors. www.fishbase.org,World Wide Web electronic publication.
Gould, S. J. 1983. Hen's teeth and horse's toes. WW Norton & Company