Summer diet of Weddell Seals (Leptonychotes weddelli) in the eastern and southern Weddell Sea,Antarctica

Summary Stomach and intestine samples from 21 adult Weddell seals were used to study the diet of these seals from the eastern and southern Weddell Sea coast from January to February 1983 and 1985. Fish occurred in all seals, squid in five, octopods in three and Euphausia crystallorophias in one seal. Pleuragramma antarcticum was the predominant fish in the diet, constituting 61.1% of otoliths in 1983 samples and 93.8% in 1985. Aethotaxis mitopteryx, Dissostichus mawsoni, unidentified Trematomus spp. and channichthyids were also recorded. Size and wet weight of P. antarcticum were calculated from uneroded otoliths, found in 6 seal stomachs with liquid food pulp, collected during early morning hours in 1985. Size distribution of P. antarcticum from individual seals was reasonably constant, ranging between 5.0 and 22.0 cm SL; adult fish from about 14.0 to 19.0 cm SL predominated. P. antarcticum in seals from the southern area had a larger median size (16.5 cm SL), than those from further east (15.5 cm SL). Calculated wet weights of all P. antarcticum from individual seal stomachs ranged between 4.7 and 16.9 kg the mean was 12.8 kg. Comparisons with net-hauls from the southern Gould Bay suggest that Weddell seals feed mainly in deeper water layers (>400 m) where adult P. antarcticum occur at higher densities.     

Carnivora: the primary structure of Weddell Seal (Leptonychotes weddelli, Pinnipedia) hemoglobin

The hemoglobin of Weddell Seal (Leptonychotes weddelli, Pinnipedia) comprises two components with identical beta-chains. The alpha-chains differ in positions 15 (Gly/Asp) and 57 (Ala/Thr). We present the primary structure of the chains which have been separated by reversed-phase high-performance liquid chromatography. The sequences have been determined by automatic Edman-degradation with the film-technique or the gas-phase method, using the native chains and the tryptic peptides of the oxidized chains. Compared to the corresponding human chains we found 22 substitutions in the alpha-chains and 14 in the beta-chains. In the alpha-chains exchanges involve one heme- and three alpha 1/beta 1-contacts. In the beta-chains one heme contact, one alpha 1/beta 1- and one alpha 1/beta 2-contacts are substituted. The sequences are compared to those of other Pinnipedia and Arctoidea hemoglobins.     

Morphological distinction between adult Contracaecum radiaturn and Contracaecum osculatum (Nematoda, Anisakidae) from the Weddell seal (Leptonychotes weddelli)

Morphological characters of the anisakid ncmatodcs Contracaecum radialurn and C. osculutum from the Weddcll Sea are reviewed. Both species occur in stomachs of scals in high southern latitudes. Thc investigation aims at providing some rcliable criteria for quantitative routine light microscopy. Characters allowing a safe identification of the two spccics arc: spiculcs and tail structures in males; lips, intcrlabia and collar in both sexes. Oesophageal shape and intcrlabial striation cannot be recommended for idcntification purposes.     

A theoretical analysis of diving performance in the Weddell seal (Leptonychotes weddelli)

Marine mammals are constrained in their foraging behaviour because, as obligate air breathers, they must undertake regular trips to the water surface to satisfy the need for respiratory gas exchange. Maximum underwater endurance time is determined by O2 supply and demand, but this does not necessarily imply that O2 is the main factor regulating individual dive and surface times. This study presents a theoretical analysis of diving performance that emphasizes a key role for CO2 in the proximate control of diving behaviour. Computer simulations, based on a mathematical model of the mammalian cardiorespiratory control system, are used to investigate the influence of swimming to depth and other energetic stresses (feeding, thermogenesis, sleep) on predicted diving behaviour in an average adult Weddell seal. The plausibility of the proposed model is supported by the study, which replicated published observations of natural diving behaviour in this species. It is suggested that diving behaviour is tuned to oscillations in respiratory drive and that behavioural and physiological factors can alter the dynamic characteristics of the system to achieve a highly adaptable reciprocal interaction that blurs the boundary between physiology and behaviour.     

Individual variation in pup vocalizations and absence of behavioral signs of maternal vocal recognition in Weddell seals (Leptonychotes weddellii)

Individually stereotyped vocalizations often play an important role in relocation of offspring in gregarious breeders. In phocids, mothers often alternate between foraging at sea and attending their pup. Pup calls are individually distinctive in various phocid species. However, experimental evidence for maternal recognition is rare. In this study, we recorded Weddell seal (Leptonychotes weddellii) pup vocalizations at two whelping patches in Atka Bay, Antarctica, and explored individual vocal variation based on eight vocal parameters. Overall, 58% of calls were correctly classified according to individual. For males (n= 12) and females (n= 9), respectively, nine and seven individuals were correctly identified based on vocal parameters. To investigate whether mothers respond differently to calls of familiar vs. unfamiliar pups, we conducted playback experiments with 21 mothers. Maternal responses did not differ between playbacks of own, familiar, and unfamiliar pup calls. We suggest that Weddell seal pup calls may need to contain only a critical amount of individually distinct information because mothers and pups use a combination of sensory modalities for identification. However, it cannot be excluded that pup developmental factors and differing environmental factors between colonies affect pup acoustic behavior and the role of acoustic cues in the relocation process.     

Variations in underwater vocalizations of Weddell seals (Leptonychotes weddelli) at the Vestfold Hills as a measure of breeding population discreteness

Weddell seal vocalizations from Davis Station showed similarities to those from McMurdo Sound and Palmer Peninsula, but none were identical. One vocalization, DD1, was unique to Davis Station. At all sites trills, or territorial defense calls, were the most common and had more types than other calls. Chugs, an aggressive sound, were common at all sites. Weddell seal calls from Davis Station showed similarities to sounds from McMurdo by sharing the use of prefixes and suffixes. Sounds from Davis Station shared the use of both ascending and descending trills and whistles with calls from Palmer Peninsula.Commonalities in underwater vocalizations may indicate the degree of mixing between breeding populations, in which case, Weddell seals in the Davis area probably are from a breeding population distinct from those at either McMurdo Sound or Palmer Peninsula.     

Sex‐related differences in the postmolt distribution of Weddell seals (Leptonychotes weddellii) in the southern Weddell Sea

The population of Weddell seals (Leptonychotes weddellii) in the southern Weddell Sea is in a unique position on the continental shelf edge, with vast shelf waters to the south, and deep Southern Ocean to the north. We describe sex‐related differences in the winter distribution of this population, from data collected by 20 conductivity‐temperature‐depth satellite relay data loggers deployed in February 2011 at the end of the annual molt. The regional daily speed was calculated, and a state‐space model was used to estimate behavioral states to positions along individuals’ tracks. GLMMs estimated that males and smaller individuals, diving in shallower water, traveled less far per day of deployment (males 14.6 ± 2.26 km/d, females 18.9 ± 2.42 km/d), and males were estimated to dive in shallower water (males 604 ± 382 m, females 1,875 ± 1,458 m). Males and smaller individuals were also estimated to be more resident; males spent an average 83.4% ± 7.7% of their time in a resident behavioral state, compared to females at 74.1% ± 7.1%. This evidence that male and female Weddell seals in the southern Weddell Sea are adopting different strategies has not been shown elsewhere along their circumpolar distribution.     

Chemical restraint of Weddell seals (Leptonychotes weddellii) with a combination of tiletamine and zolazepam.

A 1:1 combination by weight of tiletamine hydrochloride and zolazepam hydrochloride was administered to 30 adult Weddell seals (Leptonychotes weddellii) in doses varying from 100 to 300 mg. Full immobilization was achieved in 16 seals, moderate sedation in seven and light sedation in seven. Three animals died; two were fully immobilized and one was moderately sedated prior to death. The drug combination was considered satisfactory, although its usefulness was limited by the lack of chemical antagonists when complications were encountered in immobilized animals.     

A gross and microscopic study of the respiratory anatomy of the Antarctic Weddell seal, Leptonychotes weddelli.

Four species of Phocidae, or true seals, inhabit the waters surrounding the Antarctic continent. These animals are thought to have different diving capabilities. The Weddell seal, Leptonychotes weddelli, is known to be capable of attaining depths up to 600 meters. The respiratiory system of the Weddell seal shows the usual adaptations to an aquatic environment characteristic of other marine. These include lungs that undergo compression acollapse at depths greater than 70 meters; hyaline cartilage in the tracheo-bronchial tree as far as the terminal bronchioles; and large amounts of smooth muscle surrounding the distal-most bronchioles. The collapsible lungs provide a mechanism by which air is forced from the alveoli adjacent to the pulmonary capillary beds thereby preventing the absorption of nitrogen gas into the bloodsteam. The presence of hyaline cartilage throughout most of the tracheo-bronchial tree increases the effective dead air space that accommodates most of the air forced from the collapsed lungs. The smooth muscle surrounding the respiratory bronchioles prevents their collapse while under the pressures of a deep dive. Collapse of the respiratory bronchioles not supported by cartilage would trap air in the lung alveoli during a dive. In addition, large- sac-like "diverticulae" are found in the submucosa throughout the tracheo-bronchial tree. These diverticulae, which open directly into the lumen of the tree, appear to be modified glands whose cells, in most cases, do not appear to be specialized for secretory function. They are most numerous in the more distal bronchi and terminal bronchioles where they are situated on both the luminal and adventitial sides of the hyaline cartilage supporting the walls of the air passages. Diverticulae are not found in the respiratory bronchioles or in the respiratory portion of the lungs.     

Hydrographic influences on the summer dive behaviour of Weddell seals (Leptonychotes weddellii) in Atka Bay, Antarctica

In order to gain insights into species-level behavioural responses to the physical environment, it is necessary to obtain information from various populations and at all times of year. We analysed the influences of physical environmental parameters on the mid-summer dive behaviour of Weddell seals (Leptonychotes weddellii) from a little-known population at Atka Bay, Antarctica. Dive depth distributions followed a typical bimodal pattern also exhibited by seals from other populations and seals targeted both shallow water layers of <50 m and depths near the seafloor. Increased stratification of temperature layers within the water column resulted in increased forage efforts by the seals through relatively high numbers of dives to the seafloor, as well as forage effort associated with shallow dives. We interpret these behavioural responses to be due to increased water temperature stratification resulting in the concentration of prey species in particular depth layers.     

Satellite imagery can be used to detect variation in abundance of Weddell seals (Leptonychotes weddellii) in Erebus Bay, Antarctica

The Weddell seal population in Erebus Bay, Antarctica, represents one of the best-studied marine mammal populations in the world, providing an ideal test for the efficacy of satellite imagery to inform about seal abundance and population trends. Using high-resolution (0.6 m) satellite imagery, we compared counts from imagery to ground counts of adult Weddell seals and determined temporal trends in Erebus Bay during November 2004–2006 and 2009, and December 2007. Seals were counted from QuickBird-2 and WorldView-1 images, and these counts were compared with ground counts at overlapping locations within Erebus Bay during the same time. Counts were compared across years and within individual haul-out locations. We counted a total of 1,000 adult Weddell seals from five images across all years (for a total of 21 satellite-to-ground count comparisons), approximately 72% of the total counted on the ground at overlapping locations. We accurately detected an increase in abundance during 2004–2009. There was a strong, positive correlation (r = 0.98, df = 3, P < 0.003) between ground counts and counts derived from the imagery. The correlation between counts at individual haul-out locations was also strong (r = 0.80, df = 19, P < 0.001). Detection rates ranged from 30 to 88%. Overall, our results showed the utility of high-resolution imagery to provide an accurate way to detect the presence and variation in abundance of Weddell seals. Our methods may be applied to other species in polar regions, such as walruses or polar bears, particularly in areas where little is known about population status.     

Histological,histochemical, and ultrastructural investigations on the gastrointestinal system of antarctic seals: Weddell seal (Leptonychotes weddellii) and crabeater seal (Lobodon carcinophagus)

The morphology of the principal sections of the gastrointestinal system of two Antarctic seals with different dietary habits, namely, the Weddell seal (Leptonychotes weddellii) and the crabeater seal (Lobodon carcinophagus), has been investigated. Histologically examined by light microscopy, the tissue layers of the gastrointestinal tract of both seals are almost identical to those observed in most other mammals and no major differences in principle organization could be found between the two seal species. The ultrastructure of the gastric and intestinal epithelial cells has been examined and is also closely comparable to that of these cells in other mammals; however, Paneth cells have not been found in our material. In general, therefore, adaptations of the gastrointestinal tract to the aquatic environment or the diet are not obvious at the morphological levels of organization studied. Histochemical differences are found between the two closely related species; mucins of the surface epithelium in the stomach of Weddell seals are highly sulfated, while those in the crabeater seal are not. Mucous neck cells in Weddell seals contain acid mucosubstances, while those of crabeater seals contain neutral ones. Goblet cells in the small and large intestine in Weddell seals contain both neutral and acid mucosubstances. Both mucin types are detected in the crabeater seal; however, the mucins of the colon in the crabeater seal are more highly sulfated than those in the Weddell seal. The ratio of globet cells to enterocytes in the large intestine of crabeater seals is higher than that in Weddell seals. © 1995 Wiley-Liss, Inc.     

The kidney of Leptonychotes weddelli (Pinnipedia: Phocidae) with some observations on the kidneys of two other southern phocid seals

The structure of the kidney of Leptonychotes weddelli was examined using corrosion casts, India ink injection, and histological methods. Some observations were made on the kidney of the crabeater seal (Lobodon carcinophagus) and the elephant seal (Mirounga leonina). The kidneys in all three species are reniculate, as in many other marine mammalian species. Features that have not been described previously in a phocid seal are a peripyramidal muscle, and venous drainage characterized by a large extrinsic system and a small intrinsic system. Examination of specialized fornices, relative medullary thickness, and the volumes of juxtamedullary relative to peripheral glomeruli (all of which relate to urine concentrating ability) revealed that each reniculus of Leptonychotes is similar to the unilobar kidney of a small mammal that produces only moderately concentrated urine. The high glomerular volume to cortical volume ratio may be related to high glomerular filtration rates after feeding observed in marine mammals. It is concluded that reniculation is more likely to be related to the large size of most marine mammals than to some factor related directly to the marine environment.