Multimodal imaging of the sonic organ of Porichthys notatus, the singing midshipman fish

Bioimaging of sense organs and the central nervous system in extant fishes and reptiles in situ: A review

Bioimaging is changing the field of sensory biology, especially for taxa that are lesser-known, rare, and logistically difficult to source. When integrated with traditional neurobiological approaches, developing an archival, digital repository of morphological images can offer the opportunity to improve our understanding of whole neural systems without the issues of surgical intervention and negate the risk of damage and artefactual interpretation. This review focuses on current approaches to bioimaging the peripheral (sense organs) and central (brain) nervous systems in extant fishes (cartilaginous and bony) and non-avian reptiles in situ. Magnetic resonance imaging (MRI), micro-computed tomography (μCT), both super-resolution track density imaging and diffusion tensor-based imaging, and a range of other new technological advances are presented, together with novel approaches in optimizing both contrast and resolution, for developing detailed neuroanatomical atlases and enhancing comparative analyses of museum specimens. For MRI, tissue preparation, including choice of fixative, impacts tissue MR responses, where both resolving power and signal-to-noise ratio improve as field strength increases. Time in fixative, concentration of contrast agent, and duration of immersion in the contrast agent can also significantly affect relaxation times, and thus image quality. For μCT, the use of contrast-enhancing stains (iodine-, non-iodine-, or nanoparticle-based) is critical, where the type of fixative used, and the concentration of stain and duration of staining time often require species-specific optimization. Advanced reconstruction algorithms to reduce noise and artifacts and post-processing techniques, such as deconvolution and filtering, are now being used to improve image quality and resolution.

Sound production biomechanics in three-spined toadfish and potential functional consequences of swim bladder morphology in the Batrachoididaea)

The relationship between sound complexity and the underlying morphology and physiology of the vocal organ anatomy is a fundamental component in the evolution of acoustic communication, particularly for fishes. Among vertebrates, the mammalian larynx and avian syrinx are the best-studied vocal organs, and their ability to produce complex vocalizations has been modeled. The range and complexity of the sounds in mammalian lineages have been attributed, in part, to the bilateral nature of the vocal anatomy. Similarly, we hypothesize that the bipartite swim bladder of some species of toadfish (family Batrachoididae) is responsible for complex nonlinear characters of the multiple call types that they can produce, supported by nerve transection experiments. Here, we develop a low-dimensional coupled-oscillator model of the mechanics underlying sound production by the two halves of the swim bladder of the three-spined toadfish, Batrachomoeus trispinosus. Our model was able to replicate the nonlinear structure of both courtship and agonistic sounds. The results provide essential support for the hypothesis that fishes and tetrapods have converged in an evolutionary innovation for complex acoustic signaling, namely, a relatively simple bipartite mechanism dependent on sonic muscles contracting around a gas filled structure.

Ontogeny of the swim bladder of the Plainfin Midshipman, Porichthys notatus (Percomorphacea: Batrachoidiformes)

The batracoidid Plainfin Midshipmen Porichthys notatus Girard has been extensively studied due to the sound production abilities and specializations of its swim bladder. The present study describes three-dimensional variations of the morphology of the swim bladder and sonic muscles of P. notatus during its post-hatch larval development, with the use of three-dimensional computed tomography. This study also includes descriptions of the relative position of the swim bladder to other visceral organs. The swim bladder, digestive tract, and liver were already present in the smallest examined specimens (5.9 mm; newly hatched larvae) along with the yolk sac. In the smallest specimens, the digestive tract is straight, but from 7.1 mm TL, the digestive tract forms the first intestinal loops, and at 25.5 mm TL, a second intestinal loop. In smallest specimens, the swim bladder is oval, but at 7.1 mm TL, the anterior margin starts invaginating, forming a pair of anterior lobes. The first appearance of the intrinsic sonic muscles in swim bladder occurs at 13.1 mm TL. Additionally, we provide comparisons between the shape of the swim bladder of P. notatus and other species. The shape of the swim bladder of P. notatus and other members of Porichthyinae have an ovoid posterior region with two anterior lobes and differs from the cordiform or semiconected/bilobed the swim bladders observed in the other Batrachoididae.

The Digital Fish Library: using MRI to digitize, database, and document the morphological diversity of fish

Museum fish collections possess a wealth of anatomical and morphological data that are essential for documenting and understanding biodiversity. Obtaining access to specimens for research, however, is not always practical and frequently conflicts with the need to maintain the physical integrity of specimens and the collection as a whole. Non-invasive three-dimensional (3D) digital imaging therefore serves a critical role in facilitating the digitization of these specimens for anatomical and morphological analysis as well as facilitating an efficient method for online storage and sharing of this imaging data. Here we describe the development of the Digital Fish Library (DFL, http://www.digitalfishlibrary.org), an online digital archive of high-resolution, high-contrast, magnetic resonance imaging (MRI) scans of the soft tissue anatomy of an array of fishes preserved in the Marine Vertebrate Collection of Scripps Institution of Oceanography. We have imaged and uploaded MRI data for over 300 marine and freshwater species, developed a data archival and retrieval system with a web-based image analysis and visualization tool, and integrated these into the public DFL website to disseminate data and associated metadata freely over the web. We show that MRI is a rapid and powerful method for accurately depicting the in-situ soft-tissue anatomy of preserved fishes in sufficient detail for large-scale comparative digital morphology. However these 3D volumetric data require a sophisticated computational and archival infrastructure in order to be broadly accessible to researchers and educators.

Opportunities and challenges for digital morphology

Advances in digital data acquisition, analysis, and storage have revolutionized the work in many biological disciplines such as genomics, molecular phylogenetics, and structural biology, but have not yet found satisfactory acceptance in morphology. Improvements in non-invasive imaging and three-dimensional visualization techniques, however, permit high-throughput analyses also of whole biological specimens, including museum material. These developments pave the way towards a digital era in morphology. Using sea urchins (Echinodermata: Echinoidea), we provide examples illustrating the power of these techniques. However, remote visualization, the creation of a specialized database, and the implementation of standardized, world-wide accepted data deposition practices prior to publication are essential to cope with the foreseeable exponential increase in digital morphological data.

Functional colonography of Min mice using dark lumen dynamic contrast-enhanced MRI

Dark lumen MRI colonography detects colonic polyps by minimization of the intestinal lumen signal intensity. Here we validate the use of perfluorinated oil as an intestinal-filling agent for dark lumen MRI studies in mice, enabling the physiological characterization of colonic polyps by dynamic contrast-enhanced MRI. In control and Min (multiple intestinal neoplasia) mice with and without pretreatment with oral dextran sodium sulfate (DSS), polyps as small as 0.94 mm diameter were consistently identified using standard 2D gradient echo imaging (voxel size, 0.23 x 0.16 x 0.5 mm). In serial studies, polyp growth rates were heterogeneous with an average approximately 5% increase in polyp volume per day. In DSS-treated control mice the colon wall contrast agent extravasation rate constant, K(trans), and extravascular extracellular space volume fraction, v(e), values were measured for the first time and found to be 0.10 +/- 0.03 min(-1) and 0.23 +/- 0.09, respectively. In DSS-treated Min mice, polyp K(trans) values (0.09 +/- 0.04 min(-1)) were similar to those in the colon wall but the v(e) values were substantially lower (0.16 +/- 0.03), suggesting increased cellular density. The functional dark-lumen colonography approach described herein provides new opportunities for the noninvasive assessment of gastrointestinal disease pathology and treatment response in mouse models.