Mating in the dark- how deep sea creatures do it!
Living at the bottom of the ocean is no easy task. Water pressure here feels like an elephant is crushing you, it is pitch black and food is scarce. Reproducing in this vast, unfriendly darkness requires creativity. Deep sea species have evolved remarkable and unique reproductive strategies. Many of these are highly tailored to the environments in which species inhabit.
Below are the reproductive secrets of 3 species.
1. Octopus mothers with attachment issues
The Deep Sea Octopus, Graneledone boreopacifica
G. boreopacifica octopuses are the longest brooding animals on the planet. They are also thought to be the longest living.
Most cephalopod’s (i.e. octopuses and squids) only live for 1 to 2 years. Female octopuses generally brood over eggs for 1 to 3 months. Brooding involves guarding and cleaning eggs, often in the absence of feeding. Octopuses are semelparous, meaning they have only one breeding opportunity in their lifetime, upon the hatching of eggs the mother will die.
Living at depths of 1400ms and in the dark, calls for a specialised brooding strategy . In 2007, deep-sea marine surveys led by researcher Bruce Robison off the coast of Monterey US, found a female G. boreopacifica octopus clinging to a rocky ledge. She was brooding over an egg clutch of around 160 in number.
Over the following 4 ½ years, researchers undertook another 18 surveys to observe her progress. During each survey, the female octopus was seen cleaning and fiercely protecting her eggs on the same rocky ledge. Her translucent eggs revealed octopuses developing inside. Over the years, researchers observed that her body was wasting and slowly loosing colour, a sure indication this female was not leaving her eggs to feed. By late 2011, only egg casing remained.
Surviving 4 ½ years in the absence of food is an extraordinary evolutionary ability. Never leaving her eggs seems to allow her offspring to be fully developed on hatch, and gives them a greater potential for survival in what can be a perilous environment at such depths.
2. Bi-curious squids – a hit and miss love affair
Deep-sea squid, Octopotheuthis deletron
These small ruler-length squids live their whole life in solitary darkness 800 meters below the surface.
Mating has not been witnessed in this species, but these squids have telltale signs of how they get the act done. Male O.deletron have modified penis-like arms. It is assumed they use these arms to glue sticky packets of sperm onto females.
The sticky packets are called spermatangia, and as the name suggests, they are tangle-like structure.
Research by Dr. Hendrik-Jan Hoving’s team used remote operated cameras (ROV) to follow these squids in the dark hoping to learn more of these squids’ mating behaviours. They were surprised to find that an equal number of males and females had spermatangia strewn across their bodies, and ranging between 15-150 packets each.
Like other Cephalopods, these squids are semelparous and have a ‘live fast die young strategy’. This might explain their indiscriminate mating and seemly waste of sperm . Semelparity is also common to other deep-sea creatures. It is hard enough to find anyone down in the vast darkness, let alone a mate! Semelparous males often allocate a lot of energy towards sperm production to maximize mating success.
For these small cephalopods, whacking packets of sperm willy-nilly seems more cost effective than investing energy towards more specialised sex-specific behaviours like courtship displays and sex discrimination behaviours (pheromones/sex hormone smells).
3. Latching on for life – sexual parasitic habits of anglerfish
Deep-sea anglerfish from the Ceratidae family are bizarre looking creatures. They can be identified by bioluminescent lanterns protruding from their heads to attract prey and by the frightful sharp teeth bulging from their jaws. These fish are found between 1000-3000 meters below the surface in the complete absence of light.
Male deep-sea anglerfish are primitive and tiny (often 60 times smaller) compared to females. They lack light emitting lures and fangs. However, what they lack in size they make up in their sense of smell! Males possess olfactory organs that allow them to track the smell (pheromones) emitted by females in the darkness.
As male anglers mature, they undergo a metamorphosis and loose both their jaw teeth and digestive system. In their place they develop pincher-like dentures for grasping. This may seem like a backwards step in evolution, but male anglers are in fact highly specialised parasites! They begin searching for a female to hitch a ride and feed on for life!
On encountering a female, male anglers latch onto her and pierce through the epidermal layer of her flesh. Her tissue begins to fuse with his and he slowly gains access to her blood stream which provides him with nourishment. When the time is right, the male’s testes ripen and he becomes a ready source of sperm for the female. A single female can carry multiple male parasites on her body for life.
Sexual parasitism is well suited for life in the vast darkness of the deep sea. Hitching a ride for life gets rid of the need to search for your mate in the dark for ever! What is more remarkable, anglerfish appear to be the only deep-sea creatures to do this!
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