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Blood is Not Thicker than Water

It is a universal trait amongst all sharks that there is no "parental care". This means that mothers and fathers do not rear their offspring and the newly born sharks must fend for themselves. During this phase of their lives, the juveniles live in aggregations with other members of their litter and other young sharks, but do the sharks stay in touch with their litter-mates and other family members as they grow-up? Do they even know who they are related to? And why have they evolved this reproductive strategy?


Blacktip reef sharks live in relatively shallow habitats, often around coral reefs (Image Credit: Yann Hubert / Shutterstock)


Many Sharks are Very Social

Every shark reproduces a completely unique way; some lay eggs, others give birth to live young (to learn more you can check out Don't put All Your Eggs in One Basket), but what is shared accross all species, is that sharks do not rear their young. This does not mean they are completely careless with their offspring! The female will lay her eggs or give birth (depending on the species) in a "nursery habitat" - a particular area which can supply lots of food and also has very few predators. This gives the youngsters the best chance of survival, until they are big and strong enough to head out to join the adult population (Heupel et al, 2007; Mourier & Planes, 2020).

Sharks may grow up independently of their close relatives this does not mean that do not have relationships with other members of their species. On the contrary, many sharks live in very structured social groups and have even have friends! Young sharks live in groups in their nurseries and as adults living in a group means the sharks can hunt as a team and reduces the risk of predation (to learn more, check out You've Got a Friend in Me) (Mourier & Planes, 2020). But scientists have long-wondered... Can sharks recognise members of their own family?


To try to answer this question a team of scientists In French Polynesia, attempted to understand the group structure of blacktip reef sharks (Carcharhinus melanopterus). Blacktip reefers (not to be confused with blacktip sharks (Carcharhinus limbatus)) are medium sized sharks, which live in shallow waters around coral reefs in live the Pacific and Indian oceans. Their nursery areas are closer to shore, in very shallow water (Mourier & Planes, 2020).


Like all sharks, blacktip reef sharks, have no parental care and instead grow up in nursery habitats with other youngsters (Image Credit: Ali Abdul Rahman / Unsplash)


How do You Study Shark Social Groups?

Blacktip reef sharks are especially good for sociability studies, because they live in relatively large groups and because they are non-migratory, they spend their whole lives in one area. This means we can watch them for extended periods of time (Mourier & Planes, 2020).


Blacktip reef sharks can be found in abundance around Moorea Island in French Polynesia (Image Credit: Enzo Eymenier / Wikimedia Commons)

It is also easy to tell the blacktip sharks apart because each individual has unique pigmentation patterns on their fins. Much in the same way that every human being has unique colours in their eyes. This allows us to track an individual shark throughout their life (Mourier & Planes, 2020).


The Polynesian research group watched their blacktip reef sharks around Moorea Island and noted which sharks were often close to each other (less than 10 m apart) and they also took DNA samples from the sharks, to determine which individuals were actually related (Mourier & Planes, 2020).



Groups of Blacktip Reef Sharks Have complex Social Structures

The scientists in Polynesia found that blacktip reef sharks around Moorea Island live within groups that have extremely fine-scaled social structures (Mourier & Planes, 2020).


The extended community of 105 different blacktip reef sharks (each individual shown as a spot on the image), could be divided into five distinct groups (shown as different colours in the image) of different sharks, which associated with each other regularly (shown as lines on the image). These different gangs spent time in different areas within the study site, with only a little overlap at the edges of their ranges (Mourier & Planes, 2020).


These relationships were not based on kinship! In fact, the genetic analysis suggested that there was a low probability that closely related sharks would ever interact. And what was especially fascinating, was that this social network was dynamic - the patten changed during mating season. At this time, the five groups merged into three larger communities and each shark interacted with a higher number of individuals (Mourier & Planes, 2020). To learn more check out The Social Network.




Blacktip Reef Sharks can Recognise their Family Members

The scientists suggested that blacktip reefers' complex social structures probably developed because the sharks do not raise their young. The experts concluded that the young sharks may have some affinity for their litter mates during their time in the nursery habitat, but as they grew up and expanded their home range, they formed associations with individuals which were not genetically related, creating an organised social network of different communities (Mourier & Planes, 2020).

Group living can increase reproductive output, reduce predation risk and increase foraging success (Image Credit: Ed Ralph / WikimediaCommons)

The community structure changed during the mating season - with the groups expanding and intermingling - so that the sharks could meet new potential mates. This would have increased their chances of finding a great partner to breed with, whilst also reducing the likelihood that they accidentally reproduced with a genetic relative (known as "inbreeding") (Mourier & Planes, 2020).


So this behaviour is not just about having nice friends to hang around with, it it vitally important for keeping the population healthy and thriving.



References

Heupel MR, Carlson JK & Simpfendorfer CA (2007). Shark nursery areas: Concepts, definition, characterization and assumptions. Marine Ecology Press Series, 337: 287–297. Access online.


Mourier J & Planes S (2020). Kinship does not predict the structure of a shark social network. bioRxiv. Access online.


Mukharror DA, Susiloningtyas D, Handayani T & Pridina N (2019). Blacktip reefshark (Carcharhinus melanopterus) individual’s identification in Morotai waters using its fin’s natural markings. AIP Conference Proceedings 2202, 020085. Access online.



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