In honour of the most romantic time of the year, our “Sea of Love” theme week has featured posts about mating behaviour, sexual selection, and other relationships in the ocean. To kick off a week of excellent articles, let’s visit some of the oddest couples in the deep blue, because sometimes love can hide in the strangest places.
Sometimes you can’t make it on your own, especially in the ocean. Luckily, many underwater inhabitants have formed, or co-evolved with, some interesting relationships to help them cope. Symbiosis (“living together”) describes a close and long-term interaction between different species. These biological exchanges are further defined by how the relationship affects the species involved, such as mutualism (both partners benefit), commensalism (one partner benefits, the other is unaffected), and parasitism (the parasite benefits at the expense of the host).
Clownfish shelter in anemones
The most famous example of marine symbiosis, thanks to the adventures of Marlin, Dory, and Nemo, is between a little orange fish and a squishy, tentacled animal. Clownfishes (Amphiprion spp. and Premnas biaculeatus) are native to the Indian and Pacific oceans, and form obligatory (required for survival) mutualistic relationships with anemone(s). Clownfish benefit by sheltering themselves and their eggs from dangers in the stinging tentacles of the anemone. They also feed on any of the predatory anemone’s leftovers.
The clownfish is protected from the anemone’s stringing nematocysts by a thick, slimy mucus layer. The mucous is quite specialized – when clownfish move into a new home, they have to slowly build up their immunity by lightly brushing against the tentacles.
The anemone benefits as the clownfish removes parasites and provides nutrients from its leftovers and waste. Clownfish, in spite of their name, are also quite territorial and aggressive, and will actively discourage butterflyfish and other anemone-munchers from nibbling on its partner’s limbs. Their constant swimming about and fin-fanning also helps circulate water to the inner tentacles, refreshing oxygen and mineral supplies.
Shrimp room with gobies
Pistol shrimp (Alpheus spp.) and gobies (Amblyeleotris spp.) are best buddies, and room together as part of another mutualistic relationship.
True companions, the strong-armed shrimp digs the hole, and the larger goby protects it. It’s actually pretty adorable. The shrimp relies on the goby’s good eyesight to fend off threats, and repays his friend with a safe place to hide and sleep. The shrimp even checks in with the goby before venturing outside to forage, and will brush the fish with its antennae to stay in contact.
Click here to check out a cute video with some extra info on these BFFs.
Man o’ war fish stalk a venomous colony
The Portuguese man o’ war (Physalia physalis) looks like a jellyfish, but is actually a siphonophore, and is composed of a colony of tiny, specialized individual animals. These strange creatures spend their entire lives drifting in the open ocean, pushed along by a combination of winds, currents, and tides.
They have no means of propulsion outside of natural forces, and sometimes end up in shallow bays and beaches (recently in New Jersey, the UK, and North Carolina), where they have to be carefully removed.
Unlike clownfish, man o’ war fish (Nomeus gronovii) do not build a significant mucous barrier. They are relatively resistant, and have at least one antigen (an important early step in the immune response) in its skin to its host’s toxin.
Instead, the fish rely mostly on their agility to dodge the large, trailing tentacles (dactylozooides) of their host. In this commensal relationship, the fish nibbles on the smaller tentacles and reproductive organs located beneath the siphonophore’s gas bladder. In addition to the open buffet, sheltering in a “floating terror” protects the fish from predators. The siphonophore does not seem to benefit much from its escorts, but their presence of small fish may help attract prey items to be captured in tentacles.
Boxer crabs wear anemones
Anemones must be pretty cool dudes, because they get along with crabs (Lybia tesselata) just as well as clownfish.
In this mutualistic relationship, the crabs add some extra punch to their claws with the anemone’s stingers, while the anemone gets to pick up whatever leftovers float its way from the crab’s mouth (crabs are messy eaters). Sometimes the crabs will even swing their pom-poms back and forth to try and intimidate potential predators. Should the crab lose one of its anemones, it unceremoniously rips the other in two, since anemones can re-generate after bisection.
Cleaner fish do free dental work
Cleaner fishes (a subset of gobies, wrasses, and shrimp) set up shop near coral reefs, and take all sorts of walk-in patients, with which they have a mutualistic relationship. The larger animals (or divers) swim up and allow the cleaners to pick out parasites, dead skin cells, and mucus from their bodies. The cleaner fish benefit from the free, low-risk meal, and from the temporary protection of associating with more intimidating fish. Watch these guys do their job here (skip to the 2 min mark).
What’s interesting about cleaner fish is they usually follow an obvious colour (blue and yellow) or marking (horizontal stripes) scheme, termed “conspicuous colouration.” This signals their job to other fish, and helps them avoid being eaten when they swim into the mouths of their clients. The patients will also communicate, and show their peaceful intentions by changing colour, holding their mouths open, or swimming vertically.
Interspecies interactions & evolution
Many of these very special symbiotic relationships have existed for a long time, and reflect the co-evolution of their participant species. Symbiosis is increasingly being recognized as a major selective force driving evolution, like predation or competition. In fact, the evolution of all eukaryotes (plants, fungi, animals, and protists) is theorized to have resulted from symbiosis between various bacteria, which eventually became important intracellular structures, like mitochondria and chloroplasts.
On a related note, Charles Darwin’s 207th birthday is coming up (February 12th, 2016), and we’ll have a special article out to celebrate, so make sure you check it out.
We’ll also be continuing with awesome “Sea of Love” articles all this week. The poll to pick the theme for March will go live soon, so remember to vote for your favourite!
Brittney is a PhD candidate at McMaster University in Hamilton, ON, Canada, and joined Oceanbites in September 2015. Her research focuses on the physiological mechanisms and evolution of the respiratory and metabolic responses of Fundulus killifish to intermittent (diurnal) patterns of hypoxia.