Abstract

The "American" horseshoe crab (Limulus polyphemus) is a marine invertebrate that lives along the western Atlantic coast of North America from Maine south to the Yucatan peninsula, and serves as a multiple-use resource, e.g., as a fertilizer, a livestock feed supplement, whelk and eel bait, food source for migrating shorebirds, a research animal model, an inhabitant of touch pools in public aquaria, and a source of Limulus Amoebocyte Lysate which is capable of detecting minute quantities of endotoxin produced by gram (-) bacteria.

Survey and contrast (Omnipaque®, Gastrografin® and barium sulphate) radiographic studies of the cardiovascular and gastrointestinal systems of the horseshoe crab were undertaken using conventional and fluoroscopic techniques. Survey films easily identified the three main body sections of the exoskeleton: the frontal prosoma (cephalothorax); the hindbody opisthosoma (abdomen), and the posterior telson (tail). The book gills and appendages (a single pair of modified chelicera, six pairs of segmented legs and brachial appendages) were also easily distinguished, but the internal organs could not be resolved.

Contrast angiography immediately opacified the large tubular cardiac sinus, with rapid opacification of the remainder of the arterial system within three seconds. The single frontal artery, pair of anterior arteries, and four pairs of lateral arteries were readily observable. Real-time fluoroscopic imaging demonstrated rhythmical contractions of the cardiac sinus of approximately one beat per 1.5 seconds. By the second heartbeat, contrast media could be seen at the periphery of the appendages, and opacification of the gills occurred by the third heartbeat. The venous circulatory phase however was poorly visualized.

The gastrointestinal system was well visualized with contrast imaging. The horseshoe crab's rudimentary mouth (camerostome) leads to a short esophagus that extends cranially into a small proventriculus for storing food items, and then into a gizzard-like ventriculus for grinding food items. A well-developed pyloric valve protrudes into the lumen of the intestine that is further divided into a midgut, hindgut and anus. Real-time fluoroscopic images demonstrated gastrointestinal peristaltic waves as slow rhythmical contractions from proximal to distal. Following contrast administration of the iodine-based contrast media (Omnipaque®), the multi-branched digestive diverticulae that interdigitated into the hepatopancreas gave this region of the body a mildly increased opacity (or "blush") at 24, 48 and 72 hours.

The three contrast agents used for the gastrointestinal studies varied minimally in their radiographic quality and transit times. Angiography and gastrointestinal contrast studies were easily performed in this species, and no adverse effects from the radiographic procedures were observed in any of the horseshoe crabs.