Foregut Morphology and Feeding Strategies in the Syncarid Malacostracan Anaspides Tasmaniae: Correlating Structure and Function

Abstract

The foregut in malacostracan crustaceans is an ectodermally derived structure that shows morphological variation among taxa. Foregut morphology was studied in a syncarid, Anaspides tasmaniae, with the dual aims of describing the structure and of elucidating similarities and differences between A. tasmaniae and other malacostracans. The feeding habits of A. tasmaniae were also studied to correlate structure and function. The feeding strategy adopted by A. tasmaniae, predominantly detritivory, but preying occasionally on large food items, may be influenced by constraints of the morphology of the foregut. Scanning electron microscopy and conventional histological techniques revealed that the foregut of A. tasmaniae lacks ossicles and that internal skeletal elements are fashioned from invaginations of the external wall. These internal elements, which direct food particles and act as sieves, are densely setose. There is no evidence of calcification nor of the ossicles that characterize many decapod foreguts. The musculature is simple and acts principally to squeeze the entire foregut rather than to cause discrete movements of individual skeletal elements. The foregut of A. tasmaniae is not easily compared to the highly modified foreguts of decapods, but it shows many similarities to the foreguts of nondecapod taxa.The foregut of malacostracan crustaceans has been investigated anatomically in many taxa, but only recently have authors begun to correlate structure, function, and evolutionary constraints. Comparison of foregut structure and operation in related taxa can provide insights into evolution, and may be useful in taxonomy, in a similar way as comparison of external features. Here, feeding behavior and foregut morphology of a representative of a little-studied group of malacostracans, the Syncarida, is described and compared with those of other malacostracans.Anaspides tasmaniae Thompson, 1893, is the largest representative of the Syncarida. The syncarids are potentially a pivotal group in studying malacostracan evolution, because of their many undifferentiated or "primitive" traits, such as their lack of a carapace and their nearly complete segmentation. Despite their potential in evolutionary and comparative investigations, they are a poorly studied group, because many syncarids are interstitial or cave dwellers and the largest members that live above ground are found only in Tasmania, Australia. Moreover, since this animal, A. tasmaniae, is found only in alpine and subalpine mountain streams and tarns, it is relatively inaccessible even in its natural environment.There have been numerous studies of the morphology of the foregut in malacostracans. Two distinct foregut types are recognized, the "isopod type" and the "decapod type." These share basic similarities but differ in many respects (Ullrich et al., 1991 ). Workers in the field have tended to segregate themselves into two groups based on the type of foregut studied. This has led to two relatively separate bodies of literature concerning the foreg...

Phylogenetic analysis of the Malacostraca (Crustacea)

“…In several recent analyses the foreguts of different taxa were described and comparatively discussed (e.g. Wallis and Macmillan 1998). However, no analysis discussed in detail all structures through all malacostracans.…”

“…In the Anaspidacea only one inferomedianum is present. According to Wallis and Macmillan (1998) it represents the anterior inferomedianum, and there is no separate inferomedianum arising in the pylorus. There is also no atrium in the Anaspidacea.…”

The Tasmanian Mountain Shrimps, Anaspides Thomson, 1894 (Crustacea, Syncarida, Anaspididae)

“…The phylogenetic significance of Anaspides for Mala costraca has prompted numerous morphological, ultrastructural and physiological studies, all under the name A. tasmaniae. Those applicable to A. tasmaniae sensu stricto are as follows: embryology (Hickman, 1937); giant lateral neurone (Silvey & Wilson, 1979); organ of Bellonci (Kauri & Lake, 1972); locomotory function (Macmillan et al, 1981); spermogenesis (Jespersen, 1983); foregut morphology (Wallis & Macmillan, 1998); and ommatidial structure (Richter, 1999); and cuticular sclerites (Kutschera et al, 2015). Manton's (1930) study of habits and feeding are based on A. tasmaniae and A. swaini.…”

DNA evidence for morphological and cryptic Cenozoic speciations in the Anaspididae, ‘living fossils’ from the Triassic

“…The idea has been further entrenched by the interpretation of the unusual morpholgical characters seen in the Anaspididae, such as the lack of a carapace and the leaf‐like epipodic gills, as plesiomorphies ( Dahl, 1983). As a result, the Anaspidaceae and Syncarida have generally been regarded as a basal or ‘primitive’ lineage within the Malacostraca ( Williams, 1980; Dahl, 1983; Wallis & MacMillan, 1998).…”