Common Name: Horseshoe Shrimp
Scientific Name: Hutchinsoniella macracantha
Species: H. macracantha
External morphological features of Cephalocarida have long been interpreted as plesiomorphic with regard to those of other crustaceans. Based on transmission electron microscopy and light microscopy, however, the brain in the cephalocarid Hutchinsoniella macracantha has been shown to contain a number of structures that are more difficult to interpret in an evolutionary context. These include the multi-lobed complex, a unique cluster of neuropils associated with the olfactory lobes. To establish a well-founded comparison of phylogenetically relevant, neuroanatomical data from Cephalocarida to other arthropods, we investigated the brain in H. macracantha using immunolabeling (acetylated α-tubulin, serotonin, RFamide, histamine) and nuclear counter stains of whole mounts and vibratome sections analyzing specimens with confocal laser scanning microscopy and computer-aided 3D-reconstruction. Other 3D-reconstructions were based on serial 1 μm semi-thin sections. The multi-lobed complex features a pedunculus and shows detailed homologies with the mushroom bodies of certain Insecta and Lithobiomorpha (Chilopoda), suggesting that the hemiellipsoid bodies in Remipedia and Malacostraca have derived from a cephalocarid-like pattern. Like the corresponding tracts in Insecta, the olfactory globular tracts linking the multi-lobed complex to the olfactory lobes are ipsilateral, probably constituting the plesiomorphic pattern from which the decussating tracts in Remipedia and Malacostraca have evolved. The olfactory lobes in H. macracantha are uniquely organized into vertical stacks of olfactory glomeruli whose exact shape could not be identified. Similarly to Malacostraca and Insecta, the olfactory glomeruli in H. macracantha are innervated by serotonin-like, RFamide-like, and histamine-like immunoreactive interneurons. This suggests homology of the olfactory lobes across Tetraconata, despite the different morphological organization. Although H. macracantha lacks elongated, unpaired midline neuropils known from the protocerebrum of other Arthropoda, the possible rudiment of a central-body-like neuropil that receives decussating fibers from anterior somata was revealed by the serotonin-like immunoreactive pattern.
The central nervous system of the cephalocarid Hutchinsoniella macracantha is well developed, although missing several structures which are characteristic of the general crustacean plan. There are no signs of eyes either in the adult or in the larva. The organ of Bellonci, central body, protocerebral bridge, and paracentral lobes are absent. The mushroom bodies occur in the central nervous system in a form different from that found in malacostracan crustaceans and are unexpectedly well developed. They are connected to the olfactory lobes, which in this species are displaced ventrally into the clypeus (anterior part of the so-called labrum), extremely large, and of hitherto unseen construction. The central nervous system of Hutchinsoniella does not conform to a paradigm of overall primitiveness and must have evolved separately in the cephalocarid line for a long period.
Hutchinsoniella Macracantha is a member of the sub phylum crustaceans group, but is also in the subclass of Cephalocarida. Cephalocarida is made up of about 12 benthic shrimp like species, one of which is the Huchtinsoniella Macracantha. Hutchinsoniella Macracantha was discovered in 1955, off Long island, in muddy sand by Howard L Sanders. This is one of the latest subclasses to be added as it was found that none of these creatures actually fitted into any of the other crustacean subgroups. They are similar to three other subgroups, branchiopoda, malacostraca, and copepods. However they are ruled out from joining them by certain factors. They are ruled of joining the branchiopoda group due to them having the wrong type of thoracic appendages. The number of body segments that it has rules it out from being in the malacostraca and the copepods group. They haven't got any known significance to humans apart from for intellectual reasons.
Author: Nick G.
Photo Credit: http://linnaeus.zoology.gla.ac.uk/~rpage/wiki/index.php/Hutchinsoniella_macracantha