Microvascular anatomy suggests varying aerobic activity levels in the adipose tissues of diving tetrapods.

Adipose tissue has many important functions including metabolic energy storage, endocrine functions, thermoregulation and structural support. Given these varied functions, the microvascular characteristics within the tissue will have important roles in determining rates/limits of exchange of nutrients, waste, gases and molecular signaling molecules between adipose tissue and blood. Studies on skeletal muscle have suggested that tissues with higher aerobic capacity contain higher microvascular density (MVD) with lower diffusion distances (DD) than less aerobically active tissues. However, little is known about MVD in adipose tissue of most vertebrates; therefore, we measured microvascular characteristics (MVD, DD, diameter and branching) and cell size to explore the comparative aerobic activity in the adipose tissue across diving tetrapods, a group of animals facing additional physiological and metabolic stresses associated with diving. Adipose tissues of 33 animals were examined, including seabirds, sea turtles, pinnipeds, baleen whales and toothed whales. MVD and DD varied significantly (P < 0.001) among the groups, with seabirds generally having high MVD, low DD and small adipocytes. These characteristics suggest that microvessel arrangement in short duration divers (seabirds) reflects rapid lipid turnover, compared to longer duration divers (beaked whales) which have relatively lower MVD and greater DD, perhaps reflecting the requirement for tissue with lower metabolic activity, minimizing energetic costs during diving. Across all groups, predictable scaling patterns in MVD and DD such as those observed in skeletal muscle did not emerge, likely reflecting the fact that unlike skeletal muscle, adipose tissue performs many different functions in marine organisms, often within the same tissue compartment.

Habitat use pattern of the giant parasitic nematode Crassicauda magna within the pygmy sperm whale Kogia breviceps.

The giant (>3 m) parasitic nematode Crassicauda magna infects kogiid whales, although only 3 studies to date have provided detailed descriptions of these worms, all based upon fragmented specimens. These fragments were found within the neck region of kogiids, an unusual anatomic site for this genus of parasites. C. magna is a species-specific parasite among kogiids, infecting only pygmy sperm whales Kogia breviceps, and with a primarily cervico-thoracic distribution. To date, however, the pattern of habitat use within the host and transmission path of this parasite remain unknown. We used detailed dissections (n = 12), histological examination of host tissues (n = 2), and scanning electron microscopy of excised nematodes (n = 7) to enhance our understanding of this host-parasite relationship. Results revealed that a critical habitat for the parasite is an exocrine gland in the whale's ventral cervical region. C. magna male and female tails were found intertwined within the glandular lumen, and eggs were observed within its presumed secretion, illuminating the transmission path out of the host. The cephalic ends of these worms were often meters away (curvilinearly), embedded deeply within epaxial muscle. A single worm's complete, tortuous 312 cm course, from the gland to its termination in the contralateral epaxial muscle, is described for the first time. This study also provides the first scanning electron micrographs of C. magna, which illustrate taxonomically important features of the heads and tails of both male and female worms.

Microvascular characteristics of the acoustic fats: Novel data suggesting taxonomic differences between deep and shallow-diving odontocetes.

Odontocetes have specialized mandibular fats, the extramandibular (EMFB) and intramandibular fat bodies (IMFB), which function as acoustic organs, receiving and channeling sound to the ear during hearing and echolocation. Recent strandings of beaked whales suggest that these fat bodies are susceptible to nitrogen (N2 ) gas embolism and empirical evidence has shown that the N2 solubility of these fat bodies is higher than that of blubber. Since N2 gas will diffuse from blood into tissue at any blood/tissue interface and potentially form gas bubbles upon decompression, it is imperative to understand the extent of microvascularity in these specialized acoustic fats so that risk of embolism formation when diving can be estimated. Microvascular density was determined in the EMFB, IMFB, and blubber from 11 species representing three odontocete families. In all cases, the acoustic tissues had less (typically 1/3 to 1/2) microvasculature than did blubber, suggesting that capillary density in the acoustic tissues may be more constrained than in the blubber. However, even within these constraints there were clear phylogenetic differences. Ziphiid (Mesoplodon and Ziphius, 0.9 ± 0.4% and 0.7 ± 0.3% for EMFB and IMFB, respectively) and Kogiid families (1.2 ± 0.2% and 1.0 ± 0.01% for EMFB and IMFB, respectively) had significantly lower mean microvascular densities in the acoustic fats compared to the Delphinid species (Tursiops, Grampus, Stenella, and Globicephala, 1.3 ± 0.3% and 1.3 ± 0.3% for EMFB and IMFB, respectively). Overall, deep-diving beaked whales had less microvascularity in both mandibular fats and blubber compared to the shallow-diving Delphinids, which might suggest that there are differences in the N2 dynamics associated with diving regime, phylogeny, and tissue type. These novel data should be incorporated into diving physiology models to further understand potential functional disruption of the acoustic tissues due to changes in normal diving behavior.

Locomotor muscle profile of a deep (Kogia breviceps) versus shallow (Tursiops truncatus) diving cetacean.

When a marine mammal dives, breathing and locomotion are mechanically uncoupled, and its locomotor muscle must power swimming when oxygen is limited. The morphology of that muscle provides insight into both its oxygen storage capacity and its rate of oxygen consumption. This study investigated the m. longissimus dorsi, an epaxial swimming muscle, in the long duration, deep-diving pygmy sperm whale (Kogia breviceps) and the short duration, shallow-diving Atlantic bottlenose dolphin (Tursiops truncatus). Muscle myoglobin content, fiber type profile (based upon myosin ATPase and succinate dehydrogenase assays), and fiber size were measured for five adult specimens of each species. In addition, a photometric analysis of sections stained for succinate dehydrogenase was used to create an index of mitochondrial density. The m. longissimus dorsi of K. breviceps displayed significantly a) higher myoglobin content, b) larger proportion of Type I (slow oxidative) fibers by area, c) larger mean fiber diameters, and d) lower indices of mitochondrial density than that of T. truncatus. Thus, this primary swimming muscle of K. breviceps has greater oxygen storage capacity, reduced ATP demand, and likely a reduced rate of oxygen consumption relative to that of T. truncatus. The locomotor muscle of K. breviceps appears able to ration its high onboard oxygen stores, a feature that may allow this species to conduct relatively long duration, deep dives aerobically.

Ultrastructure, histochemistry, and mineralization patterns in the ecdysial suture of the blue crab, Callinectes sapidus.

The ecdysial suture is the region of the arthropod exoskeleton that splits to allow the animal to emerge during ecdysis. We examined the morphology and composition of the intermolt and premolt suture of the blue crab using light microscopy and scanning electron microscopy. The suture could not be identified by routine histological techniques; however 3 of 22 fluorescein isothiocyanate-labeled lectins tested (Lens culinaris agglutinin, Vicia faba agglutinin, and Pisum sativum agglutinin) differentiated the suture, binding more intensely to the suture exocuticle and less intensely to the suture endocuticle. Back-scattered electron (BSE) and secondary electron observations of fracture surfaces of intermolt cuticle showed less mineralized regions in the wedge-shaped suture as did BSE analysis of premolt and intermolt resin-embedded cuticle. The prism regions of the suture exocuticle were not calcified. X-ray microanalysis of both the endocuticle and exocuticle demonstrated that the suture was less calcified than the surrounding cuticle with significantly lower magnesium and phosphorus concentrations, potentially making its mineral more soluble. The presence or absence of a glycoprotein in the organic matrix, the extent and composition of the mineral deposited, and the thickness of the cuticle all likely contribute to the suture being removed by molting fluid, thereby ensuring successful ecdysis.