The hagfishes are best known for their ability to produce vast amounts of cohesive slime. Hagfish slime is unique in that it is reinforced with fibers, which consist of a homologue of epidermal keratin intermediate filaments. These threads greatly facilitate localization and hydration of the mucin component of the slime. We conducted stress-strain tests on wet and dry isolated hagfish slime threads, and found that they are exceptionally strong, extensible, and tough, and exhibit low resilience. Our results suggest that the threads lend strength and toughness to the slime, which could be used to literally tie up predators or prey, and/or blunt attacks by predators. The mechanical properties of hagfish slime threads rival those of another exceptional class of biopolymers, the spider silks.
Many poikilotherms are known to adjust the membrane composition of their cells in response to a temperature change so that membrane fluidity, and therefore, function is conserved. Such compensatory changes in membrane composition are considered "homeoviscous adaptations." In this study, we examined a heterothermic tissue, the visceral rete mirabile of the bluefin tuna, for evidence of homeoviscous adaptation. We measured the proportions of phospholipid fatty acids and phospholipid head groups as a function of position along the rete thermal gradient, which has been estimated to be about 10 C. We found no effect of position along the rete on the composition of either phospholipid fatty acids or head groups. Our results were unexpected in light of our previous demonstration of compensation of metabolic enzyme activity in the same tissue. The lack of evidence for a homeoviscous response may be due to the fluctuating nature of the thermal gradient along the visceral retia, i.e., membranes may be adapted to a eurythermal existence rather than being fine-tuned to a particular temperature.
The size, power, and speed of tunas and other pelagic fishes (e.g. swordfish, marlins, sailfish) have made it a challenge to study their biology. These species are most often composed of large populations with broad geographic ranges and individuals are capable of traversing ocean basins in weeks or months. Data on dispersal patterns are hard to obtain because of the limited resolution of analytical tools available for studying pelagic fish. What is currently needed are technologies that can augment conventional tagging data sets to better define the geographic ranges and potential overlap of stocks. Archival, satellite and molecular technologies offer the fisheries research community the new techniques required to resolve the movement patterns and stock structure of highly migratory species.
We describe in this paper our use of archival tags on Atlantic bluefin tuna (Thunnus thynnus thynnus). Archival tags provide a record of T. thynnus ambient and internal body temperature, pressure, and light. From light intensity, augmented with data on sea surface temperature, it is possible to estimate latitude and longitude. In recent years, archival tags have dramatically increased the understanding of the biology of several species of fish. Use of the tags has the potential to address major questions concerning stock structure hypotheses of Atlantic bluefin tunas. We have developed the handling and surgical procedures necessary for internal placement of archival tags in medium and giant bluefin up to 234 kg. Additional studies to examine the survivorship and healing rate of archival tagged fish are being conducted using captive tuna populations in Monterey, California and acoustic and satellite technologies on wild fish.
We measured enzyme activities along a heterothermic tissue, the visceral retia mirabilia of the bluefin tuna, to test current theories of enzyme temperature adaptation. The heterothermic tissue model is ideal for the study of fundamental temperature adaptation because it eliminates confounding effects of whole-animal acclimation. Enzymes were measured at six positions along the rete at four temperatures (15, 20, 25, and 30 degrees C). Five enzymes (aspartate aminotransferase, citrate synthase, glucose-6-phosphate dehydrogenase, glutamate dehydrogenase, and pyruvate kinase) exhibited a significant positive compensatory effect, with activity at the cold end of the rete 1.2 to 3.1 times higher than at the warm end. Two enzymes (alanine aminotransferase and lactate dehydrogenase) exhibited no significant compensation. Based on the activation energies of enzymes along the rete, differences in activity were due to differences in enzyme concentration, and not isozymes or enzyme modification. Analysis of the compensatory responses of the enzymes in light of their thermal sensitivities leads us to conclude that the pentose phosphate shunt is especially enhanced at the cold end of the rete.
The focus of this volume is an annotated translation of the classic work by J. Müller and D.F. Eschricht on the visceral anatomy of the bluefin tuna, Thunnus thynnus, published in 1835. This text, with its outstanding figures, is to this day the definitive work on the anatomy of the bluefin viscera and especially on the circulation to and from the viscera. In addition, the text is historically important in that it represents the first comprehensive description of visceral retia mirabilia in a fish. In this work, Eschricht and Müller meticulously elucidate the pattern of blood flow to, within, and from the viscera. In addition they describe and speculate about the function of the large visceral nerves seen in this species. We have annotated the translation in order to connect the findings of Eschricht and Müller with our current understanding of warm fishes. Eschricht and Müller published a supplement to the tuna article in which they describe visceral anatomy of the common thresher shark, Alopias vulpinus. We provide an annotated translation of this text as well. The main point of the supplement is that the vascular arrangement of the thresher viscera is completely analogous to that in T. thynnus, and distinct from that found in the other warm sharks, such as Lamna nasus, implying that endothermy has evolved independently at least twice within elasmobranchs. Finally, to round out the historical aspect of this volume, we include two papers and their abstracts by John Davy, who is credited with the first body temperature measurements of warm fishes. Eschricht and Müller were aware of Davy's measurements and discuss them briefly in their paper on tuna visceral anatomy. We also include plates from the 1923 paper by Kishinouye and some colour photographs of the visceral retia from our dissections. The last two sections of this volume are facsimiles of the two texts by Eschricht and Müller as they appeared in their original form.