An immunohistochemical study of structure and development of the nervous system in the brine shrimp Artemia salina Linnaeus, 1758 (Branchiopoda, Anostraca) with remarks on the evolution of the arthropod brain

Abstract

Brain morphology is an important character in the discussion of arthropod relationships. While a large body of literature is available on the brains of Hexapoda and Malacostraca, the structure of the brain has been rarely studied in representatives of the Entomostraca. This account examines the morphology and development of the nervous system in the brine shrimp Artemia salina Linnaeus, 1758 (Crustacea, Branchiopoda, Anostraca) by classical histology and immunohistochemistry against synaptic proteins (synapsins), and the neurotransmitters serotonin and histamine. The results indicate that the shape of the developing larval brain in A. salina (a circumstomodeal ring of neuropil) closely resembles that in malacostracan embryos. Furthermore, the organization of the central complex as well as the tritocerebral innervation pattern of the labrum is homologous in this species and in Malacostraca. Nevertheless, differences exist in the layout of the deutocerebrum, specifically in the absence of olfactory glomeruli in A. salina while the glomerular organization of the olfactory lobe is a character in the ground pattern of Malacostraca. These findings are compared to the brain structure in other Euarthropoda and possible phylogenetic implications are discussed.

Introduction

In the new debate on arthropod phylogeny, studies on the morphology, development, and structural evolution of the nervous system have contributed important arguments. Among the features that recently have been in the focus of interest are the compound eyes and optic ganglia (Melzer et al., 1997, Nilsson and Osorio, 1997, Strausfeld, 1998, Paulus, 2000), development of the visual system (Hafner and Tokarski, 1998, Harzsch et al., 1999a, Melzer et al., 2000, Hafner and Tokarski, 2001, Harzsch and Walossek, 2001a, Harzsch and Wildt, 2001, Wildt and Harzsch, submitted), morphology of the brain (Breidbach, 1995, Strausfeld et al., 1995, Strausfeld, 1998, Strausfeld et al., 1998, Strausfeld and Hildebrand, 1999, Utting et al., 2000), protocerebral neuroendocrine centers (Wägele, 1993), motoneurons (Wiens and Wolf, 1993, Kutsch and Breidbach, 1994, Kutsch and Heckmann, 1995), development of the ventral nerve cord (Whitington, 1996, Whitington and Bacon, 1997, Gerberding and Scholtz, 1999, Gerberding and Scholtz, 2001, Harzsch, 2001), individually identifiable neurons in the ventral nerve cord (Dircksen, 1998, Harzsch and Waloszek, 2001b), and neuronal gene expression (Duman-Scheel and Patel, 1999). Entomostracan crustaceans (sensu Walossek, 1999, Walossek, 2002) play a key role in this discussion on insect–crustacean relationships. However, compared to the wealth of literature on structure (reviews Sandeman et al., 1992, Sandeman et al., 1993, Sandeman and Scholtz, 1995) and development (review Harzsch, 2002) of the central nervous system (CNS) in representatives of malacostracan crustaceans our knowledge on the cellular organization of the CNS in Entomostraca is scarce so that neurodevelopmental data on representatives of this taxon are urgently wanted.

Beginning in the late 19th century, the structure of the nervous system in the brine shrimp Artemia salina Linnaeus, 1758 (Crustacea, Branchiopoda, Anostraca) and closely related anostracan crustaceans has been analysed by classical histological methods (Claus, 1886, Hanström, 1924, Hanström, 1928, Warren, 1930, Hentschel, 1963, Benesch, 1969, Elofsson and Lake, 1971). Furthermore, data on the structure and development of the compound eyes and optic neuropils (Hentschel, 1963, Elofsson and Dahl, 1970, Elofsson and Odselius, 1975, Nässel et al., 1978, Elofsson and Hagberg, 1986, Harzsch and Wildt, 2001, Wildt and Harzsch, submitted) and the median light sensitive organ, the nauplius eye (Elofsson, 1966, Rasmussen, 1971, Anadón and Anadón, 1980) at the cellular level are available (summarized by Criel (1991)). The localization of neurotransmitters and neurohormones in the central nervous system has been studied histochemically (biogenic amines: Elofsson and Klemm, 1971, Aramant and Elofsson, 1976; acetylcholinesterase: Raineri and Falugi, 1983) and immunohistochemically (crustacean hyperglycaemic hormone: Zhang et al., 1997; serotonin: Harzsch and Waloszek, 2001b).

The present account focuses on structure and development of the brain in A. salina. The morphology of the adult central nervous system was studied in serial sections of plastic embedded specimens. Furthermore, the developmental expression of synaptic proteins (synapsins; Klagges et al., 1996, Harzsch et al., 1997, Harzsch et al., 1999b) and the localization of neurons that express the neurotransmitters serotonin and histamine was analysed immunohistochemically. The structure of the brain in this species is compared to that of malacostracan crustaceans and possible phylogenetic implications are discussed.