Jaws of Life

Outside JEB column about the functional morphology of the jaws of various African cichlids

The Hagfish Gland Thread Cell: A Fiber-Producing Cell Involved in Predator Defense

Fibers are ubiquitous in biology, and include tensile materials produced by specialized glands (such as silks), extracellular fibrils that reinforce exoskeletons and connective tissues (such as chitin and collagen), as well as intracellular filaments that make up the metazoan cytoskeleton (such as F-actin, microtubules, and intermediate filaments). Hagfish gland thread cells are unique in that they produce a high aspect ratio fiber from cytoskeletal building blocks within the confines of their cytoplasm. These threads are elaborately coiled into structures that readily unravel when they are ejected into seawater from the slime glands. In this review we summarize what is currently known about the structure and function of gland thread cells and we speculate about the mechanism that these cells use to produce a mechanically robust fiber that is almost one hundred thousand times longer than it is wide. We propose that a key feature of this mechanism involves the unidirectional rotation of the cell’s nucleus, which would serve to twist disorganized filaments into a coherent thread and impart a torsional stress on the thread that would both facilitate coiling and drive energetic unravelling in seawater

The Visceral \u3cem\u3eRetia Mirabilia\u3c/em\u3e of Tuna and Sharks: An Annotated Translation and Discussion of the Eschricht and Müller 1835 Paper and Related Papers

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 relia mirabilia in a fish. In this work, Eschricht & Müller meticulously elucidate the pattern of blood flow to, within, and from the viscera. In addition they describe and speculate about the function of such peculiar anatomical structures such as: the visceral relia mirabilia, the radiating liver vessels and the unusually large visceral nerves seen in this species. We have annotated the translation in order to connect the findings of Eschricht & Müller with our current understanding of warm fishes. Eschricht & Müller published a supplement to the tuna article in which they describe the 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 & Müller were aware of Davy\u27s measurements and discuss them briefly in their paper on tuna visceral anatomy. We also include plates from the 1923 paper by Kishinouye and sorne color photographs of the visceral relia from our dissections. The last two sections of this volume are facsimiles of the two texts by Eschricht & Müller as they appeared in their original form

The Best Predictions in Experimental Biology are Critical and Persuasive

A powerful way to evaluate scientific explanations (hypotheses) is to test the predictions that they make. In this way, predictions serve as an important bridge between abstract hypotheses and concrete experiments. Experimental biologists, however, generally receive little guidance on how to generate quality predictions. Here, we identify two important components of good predictions – criticality and persuasiveness – which relate to the ability of a prediction (and the experiment it implies) to disprove a hypothesis or to convince a skeptic that the hypothesis has merit. Using a detailed example, we demonstrate how striving for predictions that are both critical and persuasive can speed scientific progress by leading us to more powerful experiments. Finally, we provide a quality control checklist to assist students and researchers as they navigate the hypothetico-deductive method from puzzling observations to experimental tests

Cellular Mechanisms of Slime Gland Refilling in Pacific Hagfish (\u3cem\u3eEptatretus stoutii\u3c/em\u3e)

Hagfishes use their defensive slime to ward off gill-breathing predators. Slime gland refilling is a surprisingly slow process, and previous research has shown that the composition of the slime exudate changes significantly during refilling, which likely has consequences for the functionality of the slime. This study set out to expand our understanding of slime gland refilling by examining the cellular processes involved in refilling of the glands, as well as determining where in the gland the main slime cells – the gland thread cells and gland mucous cells – arise. Slime glands were electro-stimulated to exhaust their slime stores, left to refill for set periods of time, and harvested for histological and immunohistochemical examination. Whole slime glands, gland thread cell morphometrics and slime cell proportions were examined over the refilling cycle. Slime glands decreased significantly in size after exhaustion, but steadily increased in size over refilling. Gland thread cells were the limiting factor in slime gland refilling, taking longer to replenish and mature than gland mucous cells. Newly produced gland thread cells underwent most of their growth near the edge of the gland, and larger cells were found farthest from the edge of the gland. Immunohistochemical analysis also revealed proliferating cells only within the epithelial lining of the slime gland, suggesting that new slime cells originate from undifferentiated cells lining the gland. Our results provide an in-depth look at the cellular dynamics of slime gland refilling in Pacific hagfish, and provide a model for how slime glands refill at the cellular level

Emptying and Refilling of Slime Glands in Atlantic (\u3cem\u3eMyxine glutinosa\u3c/em\u3e) and Pacific (\u3cem\u3eEptatretus stoutii\u3c/em\u3e) Hagfishes

Hagfishes are known for their unique defensive slime, which they use toward off gill-breathing predators. Although much is known about the slime cells (gland thread cells and gland mucous cells), little is known about how long slime gland refilling takes, or how slime composition changes with refilling or repeated stimulation of the same gland. Slime glands can be individually electrostimulated to release slime, and this technique was used to measure slime gland refilling times for Atlantic and Pacific hagfish. The amount of exudate produced, the composition of the exudate and the morphometrics of slime cells were analyzed during refilling, and as a function of stimulation number when full glands were stimulated in rapid succession. Complete refilling of slime glands for both species took 3–4 weeks, with Pacific hagfish achieving faster absolute rates of exudate recovery than Atlantic hagfish. We found significant changes in the composition of the exudate and in the morphometrics of slime cells from Pacific hagfish during refilling. Over successive stimulations of full Pacific hagfish glands, multiple boluses of exudate were released, with exudate composition, but not thread cell morphometrics, changing significantly. Finally, histological examination of slime glands revealed slime cells retained in glands after exhaustion. Discrepancies in the volume of cells released suggest that mechanisms other than contraction of the gland musculature alone may be involved in exudate ejection. Our results provide a first look at the process and timing of slime gland refilling in hagfishes, and raise new questions about how refilling is achieved at the cellular level

A Test of Biochemical Symmorphosis in a Heterothermic Tissue: Bluefin Tuna White Muscle

To test predictions of biochemical symmorphosis, we measured the activity of seven consecutive glycolytic enzymes at three positions along the heterothermic white muscle of the bluefin tuna. Biochemical symmorphosis predicts that adjustments in sequential enzyme concentrations along a thermal gradient should occur as a function of the thermal sensitivity of the enzymes to ensure that no one enzyme in the pathway is in excess at any point along the gradient. We found no evidence for adjustments in enzyme quantity or quality along the thermal gradient, as well as no evidence for the prediction that the more temperature-sensitive enzymes would exhibit more dramatic compensation. Conservation of glycolytic flux in the cold exterior and warm interior muscle may be achieved by the near insensitivity of glyceraldehyde- 3-phosphate dehydrogenase to temperature in this tissue. This may have the added benefit of moderating flux during seasonal or transient changes in the thermal gradient. According to the strictest application of biochemical symmorphosis, such a mechanism represents adequate, yet suboptimal desig

The effects of actomyosin disruptors on the mechanical integrity of the avian crystalline lens

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited, used for non-commercial purposes, and is not altered or transformed. Original Publication: Won, G.-J., Fudge, D. S., & Choh, V. (2015, January 27). The effects of actomyosin disruptors on the mechanical integrity of the avian crystalline lens. Molecular Vision, 21, 98-XX. http://www.molvis.org/molvis/v21/98/Actin and myosin within the crystalline lens maintain the structural integrity of lens fiber cells and form a hexagonal lattice cradling the posterior surface of the lens. The actomyosin network was pharmacologically disrupted to examine the effects on lenticular biomechanics and optical quality.This project was generously funded by the Natural Sciences and Engineering Research Council of Canada (V.C. and D.S.F.) and the Canadian Optometric Education Trust Fund (G.W.)

Evolution of a Remarkable Intracellular Polymer and Extreme Cell Allometry in Hagfishes

The size of animal cells rarely scales with body size, likely due to biophysical and physiological constraints.1,2 In hagfishes, gland thread cells (GTCs) each produce a silk-like proteinaceous fiber called a slime thread.3,4 The slime threads impart strength to a hagfish’s defensive slime and thus are potentially subject to selection on their function outside of the body.5, 6, 7, 8 Body size is of fundamental importance in predator-prey interactions, which led us to hypothesize that larger hagfishes produce longer and stronger slime threads than smaller ones.9 Here, by sampling a range of sizes of hagfish from 19 species, we systematically examined the scaling of GTC and slime-thread dimensions with body size within both phylogenetic and ontogenetic contexts. We found that the length of GTCs varied between 40 and 250 μm and scaled positively with body size, exhibiting an allometric exponent greater than those in other animal cells. Correspondingly, larger hagfishes produce longer and thicker slime threads and thus are equipped to defend against larger predators. With diameter and length varying 4-fold (0.7–4 μm and 5–22 cm, respectively) over a body-size range of 10–128 cm, the slime threads characterize the largest intracellular polymers known in biology. Our results suggest selection for stronger defensive slime in larger hagfishes has driven the evolution of extreme size and allometry of GTCs

Functional Plasticity in Lamellar Autotomy by Larval Damselflies in Response to Predatory Larval Dragonfly Cues

Adaptive autotomy is the self-amputation of an appendage in response to external stimuli that benefits survival. Variation in the ease of appendage removal among populations suggests that autotomy performance is under selection, evolves, or is phenotypically plastic, although the latter has never been experimentally tested. We model an autotomy threshold that optimally balances how the benefits of surviving predator attack versus the costs of losing an appendage vary with predator presence. We test for functional plasticity in autotomy threshold in the caudal lamellae of Enallagma damselfly larvae by experimentally manipulating non-lethal cues from predatory dragonfly larvae. Predator cues lead to functional plastic responses in the form of smaller lamellar joints that required lower peak breaking force. This is the first experimental demonstration of functional plasticity in autotomy to cues from a grasping predator, a novel form of indirect predator effects on prey, realized through plasticity in morphological traits that govern the autotomy threshold. This supports the model of optimized autotomy performance and provides a novel explanation for variation in performance among populations under different predator conditions. Plastic autotomy responses that mitigate costs in the face of variation in mortality risks might be a form of inducible defense