Unusual symbiotic relationship in nature

Krishna: The word 'strangest' is just a  perception. I prefer to call them 'unthinkable' so that I could give several examples: 

1. Scientists discovered the world's only known example of a vertebrate cell hosting the cells of a completely different species in an act of symbiosis between a salamander(Ambystoma maculatum) and a species of algae )Oophila amblystomatis) (1). While similar relationships can be found in animals without a backbone, such as coral and molluscs, this unusual discovery posed a bunch of questions about how the partnership is even possible in a vertebrate, and who it's benefiting. 

Pitcher plants are carnivores that use nectar at the rim of their tube-like structure to attract prey such as insects and small vertebrates. A slippery substance at the rim causes these animals to fall into the digestive juices contained in the plant's equivalent of a stomach.

While you might think it would be prudent for animals to avoid these plants where possible, some bats voluntarily clamber inside them.

Woolly bats are known to roost in Nepenthes hemsleyana, a tropical pitcher plant found in Borneo.

While the bat gets a hidey-hole to rest in, the plant benefits by catching the guano (faeces) that the little mammal produces. This provides the plant with the nutrients it needs to survive.

A similar relationship occurs between tree shrews and another Bornean pitcher plant, Nepenthes lowii. The shrews climb onto the pitcher's rim to feed on the nectar. In return, with the plant's hollow body acting a bit like a toilet bowl, the shrews drop their nutritional faeces into the plant's stomach.

3. Oxpeckers and large mammals(4)

There are two species of oxpecker: the red-billed oxpecker (Buphagus erythrorhynchus) and yellow-billed oxpecker (Buphagus africanus). Both regularly spend time clinging to large grazing mammals such as wildebeest, rhinos and zebras.

The birds pick at parasites on the mammal's body, including ticks and blood-sucking flies. This may help keep the mammal's parasite load under control, and the birds get an easy meal.

Like a number of other species, oxpeckers will raise the alarm and warn their hosts of impending danger. People have observed that the birds will help hosts such as rhinos (which are short-sighted) evade humans.

However, mammals and oxpeckers may not be a perfect example of mutualism, as the birds can harm their hosts. The birds remove parasites and seem to prefer hosts with large numbers of them, but they will also dig into wounds. While the mammals appear relatively tolerant of this behaviour, it's not beneficial to them.

4. Honeyguides and humans 

The eggs, larvae and beeswax contained in bee nests are a key food source for greater honeyguides (Indicator indicator). One of the ways these birds gain easy access to a nutritious meal is by leading other honey-coveting species to the nest and allowing them to do the hard work of breaking into it.

The human-honeyguide relationship is the best-documented of these partnerships. The wild honeyguides recruit people with a demanding call, indicating that they have found a bee nest. The honey-hunting humans reply with calls passed down through generations and follow the bird.

When they reach the nest, the humans subdue the bees, such as with smoke, break into the nest and help themselves to the sugar-rich honey contained within. The Hadza people of Tanzania are one group known to work with honeyguides. It has been estimated that up to 10% their diet is acquired with the help of the birds.

With the bees dispatched and the humans satisfied, the honeyguides are left to dine on the beeswax, eggs and larvae left behind.

5. NILE CROCODILE AND EGYPTIAN PLOVER and betweenSHARKS AND PILOT FISH(3)

The Nile crocodile is well-known for being hyper-aggressive whenever an uninvited visitor steps into their territory. However, there is one creature that the scaly reptilians will not only tolerate, but practically welcome into their aquatic domain. Incredibly, the Egyptian Plover aka “Crocodile Bird” will fly into the crocodile’s open mouth and feed upon the decomposing meat stuck between their teeth. The plover gets a not so scrumptious meal while the crocodile gets a little free dental work! Similar to the mutualistic relationship between Nile crocodiles and Egyptian plovers, many species of shark have established an unlikely alliance with pilot fish. While the pilot fish helps to rid the shark of parasites and clean away fragments of food caught between their teeth, it benefits from protection against other predators. The companionship between these two species is said to be so strong that there are even tales of distressed pilot fish following trawler vessels months after catching “their” shark.

6. HERMIT CRABS AND SEA ANEMONES and between COLOMBIAN LESSERBLACK TARANTULA AND DOTTED HUMMING FROG

Another unlikely marine pairing is that of hermit crabs and sea anemones. By poking the anemone with its pincers and holding it in place, the crab encourages it to attach to its shell. While the anemone bags itself a free ride across the seabed upon the hermit crab's back, they effectively serve as bodyguards, providing shelter and using their barbed tentacles to actively fend off hungry hermit predators. It might seem odd to think that a creature as sinister-looking as a tarantula could form a symbiotic relationship with an animal it could easily kill and eat. But that is exactly the case in this odd partnership between the Colombian lesserblack tarantula and dotted humming frog of South America. In fact, the large creepy crawly even allows the tiny frog to share its burrow! While the plucky little amphibian enjoys protection from fierce predators, the spider benefits from the frog devouring carnivorous ants that attack and eat the tarantula’s eggs.

7. DRONGOS AND MEERKATS(3)

This incredible partnership between bird and mammal was made famous in the BBC wildlife documentary “Africa” hosted by the legendary David Attenborough. The drongo serves as a lookout for hunting meerkats, giving a warning cry whenever a predator is within the vicinity. Upon hearing the alarm, the spirited little mammals scuttle back to their burrows, often dropping any prey they have captured in order to speed up their escape. Most of the calls made by the drongo are genuine. However, the mischievous drongo has learned that by raising false alarms, it can swoop down and pick up a free meal dropped by a fleeing meerkat. A master of mimicry, the drongo has even been witnessed making warning calls made by meerkats!

2. https://www.nhm.ac.uk/discover/mutualism-examples-of-species-that-w...

3. https://wildlifewoods.net/blog/2018/8/23/6-surprising-symbiotic-rel...