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Biology

There’s no place like home

Paper: Feldheim, K.A., et al. Two decades of genetic profiling yields first evidence of natal philopatry and long-term fidelity to parturition sites in sharks. Molecular Ecology (2014) 23:110-117. Doi: 10.1111/mec.12583

Background

Many marine species will travel thousands of miles in their lifetime, but a lot of them always find themselves back at their birthplace in order to reproduce. This phenomenon, known as philopatry, is a fundamental consideration in conservation planning. For instance, a sandy beach known as the host site for sea turtles to lay their eggs would be a horrible place to dig up and build a condominium.

So what exactly makes an animal “remember” where it was born so as to return? One mechanism used to home animals is the imprinting on their birthplace. However, this can pose several challenges as local characteristics of the geomagnetic field change over the years, from small-scale changes of current, to large-scale changes of the earth’s movement. As time elapses, these changes increase and navigational error is expected to occur. This means early-maturing marine species are more likely to accurately return to their birthplace and species that take longer to sexually mature have a harder time homing.

Lemon sharks are an example of a late-maturing species, not reaching sexual maturity until 12 years of age. This not only means that female lemon sharks have a harder time finding their birth site, but it also means there are several logistical difficulties with scientists tracking a mobile lemon shark for 12 years before any hope of return. This paper analyzes the genetic profiles of juvenile and adult lemon sharks in hopes to find evidence of philopatry.

 

Materials/Methods

For seventeen years lemon sharks were captured, examined, and released in North and South Bimini, Bahamas (Fig. 1), the largest nursery area for lemon sharks. Several measurements were taken such as sex, size, and open or closed umbilicus (the umbilicus closes 30 days after birth). A fin clip was taken on all sharks in order to perform genetic analysis, and a passive transponder was placed on every shark as well. Philopatry was detected in two ways. One was to visually spot adult lemon sharks entering or leaving the shallow nursery area and capture them, determining if they had been previously captured as juveniles in the same area through examining their tag and their genetic mark-up.

The other way was through a reconstructed maternal genotype program. This basically just creates a gene map of all the sharks so as to form genetic linkages.Since female lemon sharks are known to give birth approximately 12 years after being born and return every two years thereafter to give birth, a genetic construction was made from all juveniles captured in the later years of the study to compare them to the genotypes of all the female sharks born at Bimini in the early years of the study (any juvenile captured in the nursery was assumed to be locally born) and any matches were considered to be the same shark.

Fig. 1

Fig. 1 A synthesis of the movements of tagged adult female lemon sharks. 1 = a subadult female tagged in Bimini in 2006 that was recaptured by a fishermen at Jupiter, FL in 2008 (National Marine Fisheries Service), 2 = recaptures of individuals tagged as juveniles and recaptured at much larger sizes, 3 = Pop-off satellite tag deployment, 6 weeks after female gave birth in Bimini. The thick white lines, in both the main figure and the upper left inset, show minimum dispersal distance recorded. Shark capture location (Jupiter, Florida) is shown by the black circle, the locations where detections were recorded are shown as white triangles. Inset (top right): Seasonal presence of adult female lemon sharks captured off Bimini (1993–2010). The blue area indicates the months when newborn sharks are also observed. Inset (bottom right): Map of Bimini, the red area highlights nursery habitats on both the North and South Islands.

 

Results and Future Relevance

A total of 249 female lemon sharks were born in Bimini in the first five years of this study. In order to confirm lemon sharks were returning to their birthplace to reproduce, information on these 249 individuals was crucial. Did they survive into maturity? Did they come back to Bimini to reproduce? From recapture information, it was confirmed that 128 of these females lived past the age of two, but unfortunately limited information beyond that was acquired.

It was shown, however, that a total of six lemon sharks were found to provide direct evidence of philopatry. Two returning adult females were recaptured in the North Bimini nursery. Based on the genetic analysis, one shark was detected to have one true offspring and was captured less than 4 km from where its mother had been captured 13 years earlier. The other shark was recaptured in 2008 with readable tags and in 2012 gave birth to four newborns based on the genetic analysis. The other four cases of natal philopatry were discovered upon analyzing the reconstructed genotypes.

In all examples, every female giving birth in the nursery was found to be faithful to one location, whether it be the North side of Bimini or the South side, and no overlap was found (see figure 1 for where these sharks traveled to!). These results are very indicative of female sharks returning to their place of birth in order to reproduce. Continued sampling will enhance the detection of returning lemon sharks and add to our knowledge base of marine animal philopatry. Knowing the locality of nursing behavior will help us not only better understand shark behavior, but will help us in conservation planning. Confidently knowing the lemon sharks will continue to reappear in Bimini will have important implications of long-term sustainability.

References

Stump, K. 2011. Juvenile lemon shark, photograph, viewed 25 February 2014, < http://www.sharkdefenders.com/2011/08/shark-research-in-action-role-of.html>.

 

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