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Epipodites or Epipods

Epipodites are structures at the outer edge of the limb base of eucrustaceans. In a strict sense, as outlined below, there are no other euarthropods (and likewise arthropods) that have such structures, and even within the Eucrustacea it remains uncertain if these are homologous.
Unlikely is the homology of "gills" of various arthropods and even wider, of the different animals. Such structures evolved convergently.

Yet, various authors have regrettably mismatched function and morphology and limbs/limb parts and structures on them, so having confused many people by no longer being able to tell, which one is what and where.

The story is fairly easy, however, but requires some explanation (see particularly Maas et al. 2009).

1. Function

There are two functions of epipods: respiration and osmoregulation. Both functions are closely related to the epithelium morphology, and it is quite easy to understand that epithelia for respiration and osmoregulation can occur widespread in animals and everywhere, so not being necessarily originating from the same structure and ancestor. For a long time the two functions could not be distinguished clearly because of the methods: light miscroscopy and staining techniques. Classical silver staining indeed stains epithelia of both functions. It was the transmission electron microscopy which allowed to recognize the differences:

• Respiratory epithelium is flat and allows passive trespassing of oxygene.
• Osmoregulatory epithelium is also flat but its cells are modified at their bases by having a folded surface and basal membrane, many mitochondria in the cells (providing energy) and active ion pumps. This is why it is also called ion-transport epithelium.

Such epithelia are, as said, widespread and not restricted to anything like limbs or structures. Its abundant occurrence on gill structures has to be seen in another way: animals developed structures specialised for respiratory function, and humans call this a gill in the case of outer body outgrowths, usually oxygen is extracted from water. In the case of lungs, oxygen is usually directly taken out of the air, infolding of the body surface inside the body, and indeed do scientists name such organs lungs not only in tetrapods but also in spiders or land-living woodlice. Terminology is a real problem for us all. Not to completely disintegrate, we focus here on arthropods.

2. Structures in arthropods serving for these two functions

Since much written about it is simply incorrect, we start using the phylogenetic system as a helper to see what has been developed originally (ground pattern or stem species) and what emerged from it within the diverging evolutionary lineages. We also have to recognize that the arthropod epidermis (outer epithelium = ectoderm) develops a cuticle composed of two major layers, the epicuticle, which is hydrophobous, and the procuticle, which contains e.g. chitin, an amino polysugar. The cuticle may become rather rigid or thick, but in either way, it must serve several functions: protection against penetration of stuff but likewise transport into the body, water resistance to uphold internal blood concentration, sensing the surroundings, expelling gland products, and so on.

And NOTE: all these necessities existed from scratch and everything was developed to work in the sea! Not on land. Astonishingly, most articles on arthropod cuticles are on land animals like beetles. So wax and water-loss protection as well as being an armature against enemies were NOT the principle duties of the cuticle, not least because we know for long that the first arthropods were soft "worms on legs" = lobopodians. The Recent velvet worms (onychophorans) depict much of this early phase!

• Arthropoda s. l.: cuticle soft and flexible, animals worm-like and very flexible. Animals in the cm range. Osmoregulation, if present, through the entire body surface, so is with respiration. Excretion by segmental metanephridia with sacculi, respiration enhanced by a long tubular heart with segmental ostia within a uniform body cavity, a primary body cavity. Coelom existed early during development, but disappears (see paper by G. Mayer) – recent example of this phase: Onychophora
• Arthropoda s. str.: Three taxa of the Lower Cambrian Chengjiang fauna, Fuxianhuia, Chengjiangocaris and Shankouia, tell us a little on morphology and structural design in the ground pattern of this group:
  • The dorsal cuticle was stiffened but subdivided in the long axis of the animal into tergites connected by membranous cuticle (head shield is an enlarged single tergite, see Waloszek et al. 2005). Each tergite was subdivided abaxially into a central raised area and two outwardly pointing tergopleurae, which were free marginally to allow flexure of the animal in dorso-ventral aspect. At the starting points of each pleurotergites there were kind of joints, so-called pivot joints, to allow flexure but hold the tergites together.
  • The ventral surface was soft and continuous. Hence the segmental subdivision was not as elaborate as dorsally. Each segment bore a pair of limbs, but the region between them might not have been strongly sclerotized initially – so-called sternites may have evolved significantly later! Accordingly the entire ventral surface may have, in the main, served for respiration and osmoregulation.
  • Limbs: The trunk limbs behind the single cephalic limb, the antennula, were composed of a multi-annulated corm and a flap-shaped outer ramus. Its rigidity suggests that it was not a kind of epipod but should be called exopod. It may have served for locomotion and/or steering though it also functioned in a respiratory way as all surfaces naturally do. This does not validate however to call it a gill.
    
    
• Euarthropoda: Regarding respiration or osmoregulation there seem to be no further morphological structures evolved in the Euarthropoda that serve exactly these functions. The head enlarged the limb base was now a rigid element called basipod, on which the inner ramus, endopod, and the outer ramus, exopod, inserted. The exopod itself did not change compared to the Arthropoda s. str. ground pattern.
  
  
• Crustacea: As mentioned above it the Crustacea the only arthropod group, in which epipodites occur that may serve especially for respiration and/or osmoregulation.
  • For ostracodes it could be shown that all older authors were wrong in saying that the inner layer is for respiration (Keyser 1990. Morphological changes and function of the inner lamella layer of podocopid Ostracoda. In Whatley, R. & Maybury, C. (eds.): Ostracoda and Global Events, 401–410. British Micropaleontological Society Publications, Chapman and Hall, London).
  • For branchiopods and their epipodites it is clear only for anostracans, having two epipodites, that their distal lobe-shaped one serves for respiration, while the proximal one serves for osmoregulation. Phyllopoda have maximally one baseball-club-shaped epipodite for respiration, unclear if also for osmoregulation.
  • In 1993, Waloszek said that he did not find clear evidence of epipodites in the Cambrian branchiopod Rehbachiella kinnekullensis, but fragments of very large specimens might show such structures. We have to re-investigate this. Bredocaris, Skara, Dala, and Walossekia have no structures that can be named epipods. The exception is the newly described Yicaris dianensis from China. It has three epipodites on the outer basis of the maxilla and all trunk limbs. The maxillula has only two fine setae in the same position.
  • There are no clear "gill" structures in copepods and the other maxillopods, cephalocarids and remipedes.
  • In Malacostraca the situation is difficult to say at present because nebaliid phyllocarids have a blade-like thoracopodal epipodite that is two-lobed. There seem not to be investigations with regard to the two functions.
  • Stomatopods have fancy "gills" at the outer side of their pleopods – no other crustacean has them there.
  • Within Pancarida/Peracarida the plesiomorphic condition is having fine epipodites on many of the thoracopods, but these become reduced in favour of a respiratory surface underneath the shield in the other taxa (e.g. mysids) or even on the exopods in woodlice (developed into lung analogues in terrestrial forms). Since epipodites should have become the oostegites for brood care, it is unclear to us how this goes together in terms of homology of structures.
  • Those Syncarida with epipodites have blade-like ones – Anaspides tasmaniae – or club-shaped ones, as in bathynellids.
  • Euphausiaceans have, at their outer basipod rim, a lobate epipodite with marginal filaments. It is mostly named gill, but its homology with decapod "gills" is at best unclear.
  • Decapoda have several types of gills, all of them being rather complicated. The reason is their elaborate circulatory system that also runs into the gill stems and branches. From crabs it is known that the proximal gills of the more posterior pereiopods serve for osmoregulation.
  • Homology between decapod gills (particularly those on the outer body sides = pleurobranchia) and the lobate epipodites with their simple form and epithelia must be treated as uncertain, not least because no phylogeny proposed is without contradictions, and all cluster different groups anyway, so a basal design – ground-pattern state – cannot be reconstructed at present.
• For tracheates – or all separate if you wish (chilopods, diplopods, insects) – it is clear that they did not have gills or epipodites initially – unless one wishes to follow the arguments by some authors that wings should have been gills originally. Again, of what shape and where originally? No word about this in those papers dealing with gills and wings.
• Chelicerata and the rest of euarthropods, including fossil taxa such as trilobites and naraoiids, had, most likely, no "gills" or epipodites originally. Their rami are (just) the endopod and the exopod stemming from the basipod mediodistally and on its outer slanting margin. To us it is very unclear if some had lamellar instead of round marginal spines. It may well be just preservational to have different appearances. Spines are hollow, so in a compressed state they would be flat.
  • Even Euchelicerata like Offacolus kingi from the Silurian Herefordshire lagerstätte had only exopods, on prosomal legs and on opisthosomal ones!
  • Leanchoilia/Alalcomenaeus we studied ourselves. It had just fine spines. So to us the situation in limulids and possibly all Euchelicerata, only opisthosomal limbs had structures for respiration/osmoregulation. And these were/are located at the posterior side of the exopods in form of a large number of fine blades (only exopod of a prosomal leg in Limulus). Such elements have not yet been shown for any of the fossils in these surroundings, so their origin remains obscure. Yet, it has been suggested long ago that these blades transform into a meshwork of trabeculi forming the gill lungs and lungs subsequently. This should have happened in parallel with a backward orientation of the according limbs and their association with the body cuticle. No intermediate state could be discovered so far to validate this nice, but speculative hypothesis.

Conclusion 1: As the exopods of stem chelicerates (e.g. Leanchoilia) and stem euarthropods (Shankouia etc.) are leaf-shaped and possibly even fairly soft, it may well be that they were not only associated to locomotory function but also nice for respiration – BUT who knows. A hypothesis, but even if so this was the area of the exopod flap, NOT the marginal setae or spines.

Conclusion 2: it MAY be possible that trilobite exopods served for respiration – and also even larger and softer exopods, such as those of naraoiids. Yet this has been refuted convincingly by Jan Bergström recently (Bergström 2009).

In any way, "gills" of any kind are structures associated to limbs but not the limbs themselves. It is simply bad to term a limb a gill. Another mistake is to mismatch exopods with epipodites: Limbs having epipodites may also have exopods of course. So far so good, and now to function: Any epidermal part can be made flat-celled enough to serve for "gill" function, if it is the inner side of a shield or a limb portion – or the only animal, as in tiny ones. Cuticle, in this respect, seems not to be a problem.

BUT no one knows so far for many of the arthropods and particularly for fossils, until there is good evidence, direct or indirect, for it. It is not OK just to transfer function to such a part and even further to a whole limb, as certain authors do for limulid limbs etc. It is even worse, as Størmer did, to adopt a terminology to this and to go on mismatching all limb parts on this basis – the way how trilobites "received" their gills.

So, whatever may be possible: It is impossible to make any clear statements for fossils, just assumptions. We did this for the euarthropod Agnostus pisiformis (unclear if closer to crustaceans than to trilobites, as traditionally assumed) and its banana-shaped structures on the outer distal edges of the endopodal podomeres, but it would have been the only euarthropod to bear "gills" on an endopod. Yet, it is remarkable that the endopodal bananas grow out from spine or setal sockets at the outer distal edges of each endopod podomere (Müller & Walossek 1987). Also the epipodites of Yicaris carry a small setae and seem to have developed from their sockets too (Zhang et al. 2007). Another striking thing is that Agnostus has fine soft setae at the outer limb bases. The same is true for the stem crustacean Henningsmoenicaris. At least for the epipods of Yicaris there would exist a nice precursor structure in the same position.

For the different species described from Orsten sites see also: