Rediscovery of an internal organ in heart urchins (Echinoidea: Spatangoida): morphology and evolution of the intestinal caecum

A thorough understanding of the sea urchin (Echinodermata: Echinoidea) digestive tract anatomy is a prerequisite for the correct interpretation of physiological and biomechanical analyses focusing on the gut architecture of this ecologically important group of marine invertebrates. A number of studies have addressed the general arrangement of the sea urchin digestive tract, but accessory structures such as siphons and caeca have received less attention. Two studies carried out to analyze the gut physiology of various marine invertebrates briefly mentioned the presence of a previously undescribed pouch in the posterior digestive tract of the heart urchin (Echinoidea: Spatangoida) species Brisaster latifrons. Dissections, histological, and magnetic resonance imaging data, as well as three-dimensional reconstructions corroborate these findings. The novel structure—here termed the intestinal caecum—is suspended by a thin mesentery within a coil formed by the posteriormost part of the intestine. The kidney-shaped organ constitutes a derivative of the intestine, to which it is laterally connected through a narrow canal. In contrast to the sediment-packed main digestive tract, the intestinal caecum is filled with liquid and a flocculent mass. The organ’s histology is characterized by a thin connective tissue layer with only a small number of hemal lacunae and muscle fibers, as well as an inner simple columnar epithelium that contains numerous dark-brown vacuoles. The intestinal caecum is found exclusively among members of the Schizasteridae (Spatangoida: Paleopneustina). Specifically, the organ is present in selected species of the genera Abatus, Brisaster, and Tripylaster, but not in the other seven schizasterid genera analyzed. The intestinal caecum is not homologous to the sometimes equally named accessory structure present in the posterior digestive tract of other spatangoid taxa such as Echinocardium or Heterobrissus. Consequently, the previously introduced term recto-intestinal caecum is here applied for this latter organ. No correlation could be found between the absence or presence of the intestinal caecum and any known biological or morphological characteristics of schizasterid heart urchins. The distribution of the organ among schizasterids supports a close relationship of the genera Brisaster, Tripylaster, and selected species of Abatus.