The link between true larvae and parasitic forms within Isopoda – insights from the fossil record

Schädel, Mario (2022): The link between true larvae and parasitic forms within Isopoda – insights from the fossil record. Dissertation, LMU München: Fakultät für Biologie

Abstract

Isopoda is a species-rich ingroup of Eucrustacea (crustaceans and insects), whose representatives live in a variety of habitats from the deep sea to arid terrestrial landscapes. Isopoda is very diverse regarding the life styles which are present in its species. There are herbivorous and detritivorous species as well as predators and scavengers. Parasitism, as an interaction between animals where one animal exploits resources from the other to it’s disadvantage, is far from being a fringe phenomenon inside Isopoda – a large proportion of species in Isopoda are either parasites or micro-predators (also referred to as temporary parasites). Parasitic forms (in the wider sense, including micro-predators) of Isopoda can be found in a few ingroups, which are generally thought to be closely related or to form a monophyletic group. Among the parasitic forms there are many species whose development includes a strong ecological and morphological differentiation between the immatures and the adults (larval development). Despite the ecological importance in modern ecosystems, the fossil record of parasitic forms of Isopoda is rather sparse. The aim of this study was to recognise and thoroughly document potentially parasitic forms of Isopoda in the fossil record, using modern imaging techniques. By interpreting the systematic positions of the extinct species, the fossil forms could be compared with closely related extant forms for which there are observations of their behaviour in their natural environment. The goal was also to recognise potentially immature forms, which could provide insights into the evolution of developmental patterns within Isopoda, especially with respect to the parasitic forms in which there seems to be a stronger tendency for differentiation between adults and their offspring. Fossils have the potential to yield combinations of characters that are not present in extant species and are thereby important to reconstruct the evolution of characters. Fossils of such value were explicitly searched for. Furthermore, the fossils inspected in the studies of this dissertation should be used to provide a temporal context to the evolution of parasitism and larval development within Isopoda. Two well-preserved fossils of presumably non-parasitic forms within the group Cymothoida (in which there are also parasitic forms) from fossilised mid-Cretaceous resin were studied (study I). One of them was interpreted as an immature, which resembles the other, larger, specimen, which is assumed to be of a later developmental stage, in most aspects of the body morphology – except for the absence of a well developed leg on the posterior-most walking leg, which absence in immatures is an apomorphy of the group Mancoidea, which comprises Isopoda. This represents, together with a recently published fossil of the same site, the oldest record of an immature specimen in Isopoda. Multiple minute fossils of the group Epicaridea (parasites of crustaceans) from two different mid- and Late Cretaceous amber localities (studies I and III) were studied. They represent the oldest body fossils of the group Epicaridea, which has a rich record of fossil traces which its representatives left on their host (growth responses by the host) while feeding on them. Based on the available morphological features, the fossils were identified as either larvae (of the cryptoniscium stage) or paedomorphic adult males. Their presence in the fossil record suggests that the complex life cycle that is found in extant species of Epicaridea was already present in the Cretaceous. An assemblage of multiple strongly compressed fossils from the Eocene of the Czech Republic was documented (study IV). The specimens were identified as being either close relatives to or representatives of group Cymothoidae (mostly parasites of fishes in the extant fauna). This marks the first and therefore oldest reliable record of this lineage in the fossil record. The assemblage contains specimens of different body sizes. Together with differences in the overall body shape this indicates the presence of immature stages. Fossils of Urda, an extinct, potentially non-monophyletic group with a unique combination of characters, were analysed (study V). The fossils are interpreted as the closest so far known relatives of the extant group Gnathiidae (temporary parasites of fishes), with which representatives they share a number of apomorphic characters; a convincing apomorphy for Urda could not be found. The fossils, for which there is no indication that they represent remains of immatures, are very similar in many aspects to immature forms of Gnathiidae, in contrast to which they, however, lack the paedomorphic absence of legs on one segment of the trunk. The occurrence of some fossils of Urda on fossils of fishes suggests a syn-vivo interaction, such as parasitism or commensalism. Fossils of Urda provide important information about the character evolution towards modern, fish-parasites of the group Gnathiidae. The evolution of larvae within Isopoda seems to be deeply interlinked with the evolution of parasitism. The fossil record yields specimens with a larval development that date back to the mid-Cretaceous. These specimens simultaneously represent the oldest fossils which can be identified as belonging to extant groups in which all species have a parasitic life style. Close relatives of extant parasites date back even further, to the Lower Jurassic. Overall, despite still being patchy, the fossil record of Isopoda provides unique insights into the evolution of parasitic forms as well as into the differentiation between adults and immature forms.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Fakultät für Biologie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 29. September 2022
• 1. Bericht­erstatter:in: Haug, Joachim T.