The Influence of Morphological and Microstructural Characteristics to Land Snail Degradation in Forest Environment
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Charles University, Faculty of Science Department of Invertebrate Zoology Doctoral study programme Summary of the Doctoral thesis The Influence of Morphological and Microstructural Characteristics to Land Snail Degradation in Forest Environment Vliv morfometrických a mikrostrukturálních charakteristik na rozklad ulit plžů v lesních ekosystémech Mgr. Dagmar Říhová Supervisor: doc. RNDr. Lucie Juřičková, PhD. Praha, 2018 Abstract The decomposition of land snail shell is a complex process involving a number of factors and influences, including the characteristics of conchs themselves. In particular, it is the shell size with which the progress and the rate of degradation are tightly bound. Post-mortem changes begin with the loss of the original colour and, in the case of transparent species, by the opacification of the shell wall. Subsequently, the periostracum disruption and dissolution of calcium layers occur. However, this sequence may be reversed for some small species (e.g. Columella aspera, Nesovitrea hammonis). Animals mechanically destroy empty shells, humic acids from the substrate cause their artificial dyeing. Fungal mycelium or colonies of Streptomyces grow on the surface of the conchs. The plant roots are also involved in shell decomposition. While degradation of large shells starts with periostracum disruption and subsequent ostracal dissolution, periostracum of small shells persists even after dissolution of ostracal layers. The phenomenon is caused by high resistance of the periostraca of small species. In the case of large shells, periostracum has primarily a “building” function during shell formation, and sometimes it is flaking off already during the snail's life. For small species, it is an important part of the conch and increases its durability, which is also reflected in a different course of shell decomposition. The shell size affects highly the rate of decomposition: small shells disappear very quickly, depending on the habitat type in order of months. On the contrary, large shells could persist for several years. The habitat influence is also very important. Acid and humid sites promote rapid decomposition; in dry and basic woods shells are kept relatively unchanged for several years. A unique and site-specific combination of the above-mentioned damage types, so called taphonomic signature, is created. Although the decomposition of the shells is a complex process, empty shells can provide valuable information and must not be overlooked. Key Words Molluscan shell, calcium carbonate, periostracum, taphonomic signature, forest, SEM 2 Abstrakt Rozklad schránek suchozemských plžů je komplexní proces, na kterém se podílí množství činitelů a vlivů, včetně vlastností schránek samotných. Důležitá je především velikost ulity, se kterou je svázán průběh i rychlost rozkladu. Posmrtné změny začínají ztrátou původního zbarvení a u průhledných druhů zakalováním stěny schránky. Následně dochází k narušení periostraka a rozpouštění vápenatých vrstev. Tato posloupnost však v případě některých malých druhů (např. Columella aspera, Nesovitrea hammonis) může být obrácena. Živočichové mechanicky ničí prázdné schránky, huminové kyseliny ze substrátu způsobují jejich obarvování. Na povrchu schránek vyrůstají houbová mycelia či kolonie bakterií r. Streptomyces. Na rozkladu se rovněž podílí kořeny rostlin. Zatímco velké schránky se rozkládají způsobem periostrakum nejprve, periostrakum malých ulit vytrvává i po rozpouštění ostraka. Jev je způsoben vysokou odolností periostraka malých druhů. V případě velkých schránek má periostrakum především stavební funkci při tvorbě schránky a již za života plže někdy oprýskává. Pro malé druhy představuje důležitou součást schránky, která zvyšuje její odolnost, což se rovněž projevuje odlišným průběhem rozkladu. Velikost schránky ovlivňuje především rychlost rozkladu: malé schránky mizí velmi rychle, v závislosti na typu lokality i v řádu měsíců. Velké schránky naopak vytrvávají dlouho, po dobu několika let. Lokalita, na které k rozkladu dochází, je rovněž velmi důležitá. Místa značně kyselá a vlhká podporují rychlý rozklad, v lesích suchých a bazických se schránky uchovávají po několik let i v podobě relativně nezměněné. Různé lokality na schránkách zanechávají tzv. tafonomický podpis: unikátní směs výše zmíněných typů poškození. Přestože je rozklad schránek komplexní záležitostí, mohou prázdné schránky poskytnout cenné informace a nesmí být přehlíženy. Klíčová slova Schránka, uhličitan vápenatý, periostrakum, tafonomický podpis, les, SEM 3 Introduction We are familiar with almost all aspects of pulmonate life: we know many about their physiology, ethology, reproduction, ecology or evolution. We learn how their shell is arranged (Cameron 1981) and discovered amazing shell shape miscellaneousness (Goodfriend 1986). Yet there is a neglected part of landsnail malacology: we haven't almost any knowledge about persistence and degradation of empty shells. What comes after land snail's death? Is the shell capable of long persistence or does it quickly vanish after the disappearance of a soft body? The information about “behaviour” of empty shells is very important for population dynamics studies or for studies about species diversity and some authors call for information about the rate and the pattern of shell degradation (Schilthuizen 2011). Despite this lack of information, the references to shell degradation, persistence and disappearance are scarce. We know that shells are under some condition capable of accumulation of the soil surface (Emberton et al. 1996, Cameron et al. 2003, Schilthuizen et al. 2003), but quick shell decay was also documented (de Winter & Gittenberger 1998, Müller et al. 2005, Ström et al. 2009). Animals were observed to enhance shell disappearance (Cadée 1999, Appleton & Heeg 1999, Graveland et al. 1994, Mänd et al. 2000, Cadée 2016). Some authors documented the influence of shell properties on shell degradation. Thick and large shells are more durable than small and brittle one (Hotopp 2002, Millar & Waite 1999, Sólymos et al. 2009). Up to our knowledge, the course and the rate of shell degradation itself were so far mentioned in malacological literature only four times: in work of Barrientos (2000), Menez (2002), Pearce (2008) and Cernohorsky et al. (2010). These four papers cover waste range of biotopes extending from tropical rainforest in Costa Rica (Barrientos 2000) through Central European fens (Cernohorsky et al. 2010) and North American temperate forests (Pearce 2008) to Mediterranean (Menez 2002) and refer about number of shell damage characteristics occurring after snail's death. Classification to six types of shell degradation types (mainly caused by predation) was published by Millar & Waite (2004) who worked in temperate canopy forest in England. Not only recent malacologists are interested in shell degradation: from palaeontological literature (e.g. Staff & Powell 1990, Perry & Smithers 2006) we are informed about the influence of habitat types and abiotic conditions on shell persistence in marine environment. Palaeontologists often consider abiotic and biotic factors influencing shell state during fossilisation as a complex forming the so called taphonomic signature. This is defined as “a set of shell alterations typical for one particular site” (Parsons-Hubbard 2005). It has been shown to indicate even small differences in chemical conditions, environmental energy and biologic activity at the burial site (Parsons- Hubbard 2005). The presence of taphonomic signatures can be expected also at the terrestrial sites, since for example soil pH and overall wetness of the site can be expected to interact to influence shell degradation. To our 4 knowledge, no attempt has been made to investigate the existence of taphonomic signatures on land snail shells in a terrestrial environment. Except above mentioned papers by Barrientos (2000), Menez (2002), Millar & Waite (2004), Pearce (2008), Cernohorsky et al. (2010) and our work (see below), the evidence supporting knowledge about landsnail shell degradation is missing. Aims of the study To complete the missing evidence about landsnail shell degradation and persistence, we decide to study shell degradation in Central European forests as well as in laboratory experiments and: (I) find out what happens with an empty shell in temperate forest environment, describe those changes precisely and reveal their possible causes (II) if present, to describe and compare shell degradation patterns (III) to quantify the rate of shell disappearance (IV) to compare the taphonomic signatures of studied forest types (V) to describe microstructural characteristics of shells and their relationship to the overall degradation pattern 5 Material and methods Nine model land snail species were chosen (for details, see Říhová et al. 2018). Six forest sites represent the main types of Central European forests and cover various types of water regime (from dry to seasonally flooded) and soil properties (from acid to alkaline ones). Fresh empty shells were placed in perforated plastic boxes filled with local leaf litter (four boxes to each locality). Five shells of each species (altogether 1080 specimens) were put in each box and the boxes were dig inside leaf litter and topsoil. After 6, 12, 24 and 36 months, boxes were excavated and the content was dried. Shells were picked up and photographed; their condition was checked and measured on the photographs. Subsequent multivariate analyses were performed in CANOCO for Windows 4.56; univariate