The Population and Foraging Ecology of the Hydrurga leptonyx (Leopard Seal).

The Leopard seal inhabits the Antarctic continent region, and is one of six species of Antarctic seals (Antarctica Guide). It is the second largest of these species, after the Southern Elephant Seal, which is the largest species of seal on earth, but the Hydrurga leptonyx isn’t far behind, as the average body length is around 3.5 metres, and weighing approximately 550 kg (Oceanwide Expeditions).
There are two families of seals in this habitat, the Phocids (earless seals) and the Otariids (eared seals), with the earless often being called the ‘true seals’. The Hydrurga leptonyx is in the Phocidae family, of the order Pinnipedia. It is easy to distinguish Leopard seals from their relatives due to their distinctive ‘reptilian-like’ look (Antarctica Guide), with a long sinewy neck, and side facing eyes. This image is linked to their lifestyle as the most voracious hunter of the Antarctic Seals, being the only species that hunts other seals, along with a complex and varied source of other prey.
The Antarctic seals are perfectly adapted to their habitat, with thick, stratified (A.I. Guerrero, 2016), layers of blubber to keep their body temperature high, while being perfectly streamlined so they can move at great speed and ease underwater. Due to these adaptations, despite the Antarctic being home to only 6 of the 35 species of seal, these species make up the majority of seal abundance worldwide. Furthermore, the Leopard seal has been placed in the category of Least Concern on the ICUN List of Species, with scientific studies estimating the circumpolar population at around 300,000 individuals (E. Gurarie, 2017).
This successful and widespread carnivorous species has an undoubtable affect on the environment and ecosystem it inhabits, which is why this essay will be focussing on firstly the population ecology of the Leopard seal, but then moving onto their foraging ecology, to determine how this population interacts with its environment, the impact it has on the ecosystem, and its general behaviour and lifestyle.

Leopard seal distribution and abundance has been investigated in many studies, and focused on is what can be called the ‘metapopulation’ of leopard seals, as the individuals are not located in the same area throughout the year, but these groups of populations can still interbreed. This is supported by the information that these seals often stray far from the Antarctic pack ice on which they breed. This species breeds on the outer fringes of the ice sheet, and on the whole, remain in this region when the pack ice retreats during austral spring, when the ozone is at its greatest depletion (Chemistry Dictionary). However, there have been records of Leopard seal sightings as far as the Pitcairn Island (B. Stewart, 2014), which is 3,500 miles from the nearest Antarctic ice (Google Maps). Seasonal movements have been correlated to changes in sea ice distribution among other factors (B. Stewart, 2014), such as prey movements. The seals move to more northerly land masses such as Australia, South Africa, and even Chile (J. Acevedo, 2017) during the autumn and winter months as the ice expands, and there are even records that these wandering individuals have formed a year-round population in these locations, and the first records of pups being born in South America occurred this year (J. Acevedo, 2017).
Despite Leopard seals being relatively numerous, their sparse and solitary distribution in the heavy pack ice in an unforgiving environment means that, access to study these animals is limited, especially due to the large proportion of their time that is spent cruising, partially underwater (C.Kuhn, 2006). This means that research on their population abundance and distribution is difficult to attain, however their impact on their ecosystem and environment is clearly seen in experiments studying other organisms in the Antarctic biotic sphere. For the reasons stated previously, the study of the Hydrurga leptonyx’s foraging ecology sheds more light on the importance of these animals.

These top tier carnivores are especially significant to the ecosystem because they are so widespread and well adapted to feeding on prey at a range of trophic levels; they are unusual in this way, as they are equally well adapted to prey at the top and near the bottom of the food web (D. Hocking, 2013).
Due to the difficulty in studying these creatures, the best quantitative estimates of their diet comes from methods such as stomach content and faecal examination, using stable carbon and nitrogen isotope analysis, as well as some direct observational information (C. Southwell, 2012).
Evidence from these methods shows that Krill, more specifically the Antarctic keystone species E. superba (J. Cornejo-Donoso, 2008), was the most numerous and frequent source of prey in Leopard seal’s diet, followed by fish and penguins in importance of mass (R. Casaux, 2009), but also occasionally supplemented with seal pups from other Antarctic species.
It has been argued that the quantity of Krill in the Leopards diet may have been overestimated, because they may be indirectly consuming a great deal of it by also preying upon krill-feeding penguins (R. Casaux, 2009). Despite this, even considering a possible overestimation within the research methods, krill is still a hugely important factor in the diet of Leopard seals. This is shown in an experiment that focused on examining the fatty acid composition of their blubber, where the fatty acid ‘DHA’, was found at a higher concentration in Leopards than the other Antarctic seals, and this substance is characteristic of the Krill species E. superba (A.I. Guerrero, 2016).

These opportunistic foragers (S. Kienle, 2016) have a series of morphological adaptations suited to being successful hunters of a wide range of prey.
Firstly, there are four known feeding methods of phocid seals: grip and tear, suction, filter, and pierce feeding (S. Kienle, 2016). It has been observed that Hydrurga leptonyx are able to use a mixture of three of these, grip and tear, filter and suction (P. Hocking, 2013). This versatility gives this species a great advantage, because if one of their food sources has a dip in population, the seals will be able to survive off their alternative sources, and also it minimises the chances of intraspecific competition for food. This is significant because a substantial degree of prey overlap has been recorded between Leopards and the Antarctic fur seal, as both penguins and E.superba make up the majority in their diets. However, as both species are versatile hunters, and their prey is relatively abundant, the competition among predators in the Antarctic remains low.

Their physical adaptations that allow three feeding methods are as follows. The grip and tear technique is utilised for their larger amniote vertebrate prey items, birds and mammals. This is enabled by their long mobile necks, which allows them to perform a ‘head strike’ action (D.P. Hocking, 2013), with which they can grab the penguin or seal pup at great speed and force, and then use the mobility to thrash the prey side to side, which in turn tears the flesh and allows the Leopard seal to feed. Additionally, their necks are a useful feature when feeding on other prey items, as they can strike into the middle of a shoal of either small fish or krill, and withdraw again repeatedly, very quickly.
The suction feeding of the leopard seal has been observed in captivity, and has been found to be used to capture small fish (D.P. Hocking, 2013). The seals have adapted four stages of feeding for this method, beginning with the jaw opening in stages, followed by gular depression, where the suction is generated using the tongue, which draws in the prey item to their mouths, and completed with the closing of the jaw and tongue movement that causes water expulsion, leaving the extracted prey in their mouth. This is perfect for their medium sized prey, as it allows great precision to target specific fish.

Figure 1 – The Hydrurga leptonyx’s dentition (D. Hocking, 2013)

The dentition of the Hydrurga leptonyx is vital to the final feeding method, filter feeding. As although they have long sharp canines, these are used in the grip and tear method, but do not aid in the capture of krill. Instead, the seals use their specifically shaped tricuspid post-canines, which are evenly spaced along the jaw, to serve a sieving function. As displayed in Figure 1, the slits in the post-canines are perfectly adapted to separate the small prey items in the seal’s mouth, from the sea water that they will have otherwise also have ingested. Using a similar technique to the suction feeding method, the tongue and hyoid apparatus, which involves the pharynx and larynx (Science Direct), create a force which pushes out the unwanted water via the sides of the mouth, leaving the krill trapped with the cage of teeth.

The behaviour of Leopard seals appears to be intrinsically linked to their foraging ecology, which is why it has been questioned that they do not seem to take advantage of the lifestyle of their main prey: E. superba.

Fig 2. Ocean habitats

This species of krill moves in a diel migration pattern of decreasing depth at night, in other words, it vertically migrates daily (D. Krause, 2016). Despite this however, Leopard seals have been recorded to remain at relatively static, shallow depths. When depth recorders were attached to individuals, it was found that 90% of the dives measured 30 metres or less (D. Krause, 2016). The other foraging dives into pelagic and benthic zones (Figure 2), are attributed to the hunting of demersal fish, which inhabit the sea floors. However, seeing as Krill make up the majority of Leopard seal diet, and they require an estimated 1kg of krill per day (J. Forcada, 2012), it is curious that they spend no time following them down to the depths, where there would be little to no feeding competition. This has been explained by an experiment investigating diving physiology, where it was discovered that Leopard seals have a low blood oxygen storage capability (C. Kuhn, 2006), and comparative to body size, is far inferior to that of the other Phocid species studied. This proves that their physiological diving performance is inherently weaker, as they would need to replenish the oxygen supply to their muscles too soon, and therefore would not be able to survive the long, deep dives down to the night time habitat of the Krill.

It is clear that this species, Hydrurga leptonyx, plays a huge role in the ecological interactions in the Antarctic, acting as a predator to a wide variety of organisms, helping to control their population, and acting as an evolutionary pressure, but also sometimes being a prey item to Orcas. The Antarctic ecosystem is affected hugely by seasonal changes, such as light availability during winter, which slows the primary production in the trophic system, and ice extension, providing more coastline habitat (J. Cornejo-Donoso, 2008). And despite Leopard seals making extremely long seasonal journeys away during these periods of low productivity in the Antarctic, they are still 3rd from the top of the trophic model of this region, coming below only Sperm whales and Orcas (J. Cornejo-Donoso, 2008).
Their position as tertiary consumers is crucial to the regulation of the ecosystem, as they control overpopulation in lower trophic species. Furthermore, their unusual position as a consumer of prey at many trophic levels means that this species has particular significance for the ecosystem, as their effects are felt throughout the whole food web, from the primary consumer of Krill, to secondary of fish, but also other fellow carnivores, such as penguins and other seals. Leopard seals are also an example of how ‘optimal foraging theory’ is often reversed in marine environments, as instead of focusing their efforts on prey that will maximise energy gain per hunt, these large predators focus on small and plentiful prey, which maximises energy intake in a different way (D. Hocking, 2013). In conclusion, the Hydrurga leptonyx (Leopard Seal), has a great impact on the Antarctic region, due to its unusual foraging ecology, which is enabled by a series of morphological adaptations, but also has effects on other environments, such as Australasia and South Africa, because of the species’ varied population ecology.

References:

  • Antarctica Guide – Antarctic Seals.
    https://www.antarcticaguide.com/antarctica-wildlife-2/antarctica-seals/
  • et al. – Births of leopard seals Hydrurga leptonyx in southern Chile. – Polar Biology, March 2017, Volume 40, Issue 3, pp 713–717
  • R. Casaux et al. – Diet of the Leopard Seal Hydrurga leptonyx at the Danco Coast, Antarctic Peninsula – Polar Biology, February 2009, Volume 32, Issue 2, pp 307–310
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    http://www.iucnredlist.org/details/10340/0
  • S. Kienle et al. – The better to eat you with: the comparative feeding morphology of phocid seals (Pinnipedia, Phocidae) – Journal of Anatomy, Volume 228, Issue 3, March 2016, Pages 396–413
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  • B. Stewart et al. – An extreme wandering leopard seal, Hydrurga leptonyx, at Pitcairn Island, central South Pacific. – Polar Biology, March 2014, Volume 37, Issue 3, pp 423–425
  • Oceanwide Expeditions – Facts about Leopard seals
    https://oceanwide-expeditions.com/blog/danger-beneath-the-water-10-facts-about-leopard-seals
  • Science Direct, the Hyroid Apparatus
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  • C. Southwell et al. – A REVIEW OF DATA ON ABUNDANCE, TRENDS IN ABUNDANCE, HABITAT USE AND DIET OF ICE-BREEDING SEALS IN THE SOUTHERN OCEAN. – CCAMLR Science, Vol. 19, pp 49–74
  • Tree of Life web Project – Cephlapod Ocean Habitats
    http://tolweb.org/accessory/Cephalopod_Ocean_Habitats?acc_id=1998

Published by amyandkatherine

We are two friends of 12 years, trying to start careers in journalism.

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