Drivers and consequences of molluscan body size in space and time

Chattopadhyay, Debarati (2019) Drivers and consequences of molluscan body size in space and time. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

This work is an effort to understand the nature of body-size variation of shallow marine molluscan fauna as a response to global and regional changes in physical environment; it also tries to understand the effect of body-size in shaping the dynamics of a community. General rules regarding body-size variation are established primarily for terrestrial organisms; the effect of various environmental factors (such as temperature, salinity, productivity, oxygen concentration) on marine ectothermal body size remains poorly documented. Molluscs are one of the most abundant organisms in the shallow marine habitat. Their abundance along with their durable shells makes them a perfect model organism to study the long term variation in body size over time and space. The broad objectives of this study are the following: I. Evaluating the long-term effect of temperature fluctuation on body size of molluscs using Cenozoic global record. II. Evaluating the effect of spatial variation in the environmental parameters on molluscan body size using recent bivalves along the western coast of India. III. Evaluating the effect of body size on prey- predator dynamics using Miocene microbivalve assemblage of Kerala, India. These objectives are achieved through a thorough the evaluation of both recent and fossil record of molluscs by evaluating the body size with respect to the (i) Cenozoic temperature record (ii) regional environmental variation, and (iii) nature of the biotic interaction. In all of these studies, we also tried to evaluate the dependence of the response on ecological character of taxa. Body-size variation of molluscs, although documented at different scales, is still poorly understood because of the lack of knowledge regarding the drivers. Consequently, it limits our ability to predict the future pattern of body-size variation of marine molluscs in wide spatial and temporal scale and appreciate its true effect on overall community structure. This thesis tries to fill the existing knowledge gap by providing a detailed account of molluscan body-size variation in space and time, evaluation of potential drivers and assessment of the effect of such body-size variation. This thesis is organized into five chapters. In the 1st chapter, I introduced the general rules regarding body-size changes, emphasizing both temporal and spatial patterns. I also discussed the nature of taxon-specific body-size variation for ectotherms and endotherms and the contributing environmental factors. A detailed look at the outcome of previous studies highlighted the gap area in our existing knowledge about the predictability of body-size variation for marine ectotherms. The following chapters are designed to bridge this gap in knowledge by evaluating molluscan body-size change across various spacio-temporal scales. In the 2nd chapter, I tried to evaluate the effect of temperature in shaping the body-size variation by testing a few predictions based on recent observations. I tested the following hypothesis using the global record of temperature molluscan occurrence over last 65My. (i) Marine molluscs should follow a negative Temperature- size rule (TSR). (ii) Families of smaller body size and narrow latitudinal range should show higher magnitude of change in body size. (iii) The body-size response to temperature of species with tropical affinity should be greater in comparison to that of temperate species. (iv) The body-size response to temperature of species that are infaunal should be much more pronounced than that of epifaunal species. To address these questions, we compiled the global data on body size of marine molluscs of Class Gastropoda and Bivalvia from published literature through paleobiology database. Global temperature over the Cenozoic was calculated from the curve published by Zachos et al. (2001). Our molluscan species record does not show any signature of TSR for any taxonomic, regional, or ecological category during the Cenozoic. We did not find any evidence supporting heightened response in groups with limited latitudinal spread or with large body size. The body-size response do not show any significant pattern with temperature neither for different zones (tropical and temperate), nor for different ecological habitats. Hence, refutes the predicted variation due to difference in their thermal-specialization. These observations show the limited validity of “universal rules” in explaining the climate-induced morphological response of marine communities in deep time. These results underscore the complex nature of climate-induced morphological change among marine species and the need for detailed regional study to evaluate the controlling factors in the absence of a universal rule governing marine ecosystems. In the 3rd chapter, I tried to evaluate the effect of reginal environmental parameters on body size of recent marine bivalves in tropical setting i.e. west Indian coasts by addressing the following questions: i. Does the pattern of body-size variation of tropical bivalves follow Bergmann’s rule within small latitudinal spread (8–22° N)? ii. Does the variation in physical environment between NW and SW region (separated by 15° N) along the west Indian coast control the body-size variation of bivalves? iii. Does the body size variation depend on the substrate relationship of bivalves? To conduct this study time-averaged physical samples were collected along west Indian coast. Ecological information of each families were extracted from paleobiology database. Environmental data such as temperature, salinity, productivity were collected from Sarkar et al., 2017. Oxygen concentration data for each latitudinal bin was calculated using www.nodc.noaa.gov/OC5/SELECT/dbsearch/dbsearch.html. A significant positive size- latitude relationship exists along west Indian coastline. Our correlation value suggests that tropical bivalves do not follow Bergmann’s rule within 8–22° N latitude. However, we found overall size of SW is significantly smaller in comparison to NW region of western coast, but the difference is primarily dependent on their ecological character. Only infaunal bivalves show a significant body-size reduction in SW compared to NW, not the epifaunals ones because epifauna can tackle environmental fluctuation more efficiently than infauna. Our study highlights the role of individual ecology in shaping the body-size variation in tropical region. Once change in body size take place due to change in regional environmental condition, it might have an important effect on community structure that is highly size structured. Because predation is one of the major drivers of evolution and change in size distribution can impact the nature of predation, it is crucial to evaluate the effect of body-size in molluscan predator-prey dynamic. In the 4th chapter, I addressed the following questions by evaluating the predation record in an extremely small size-class (<5mm) of molluscs: i. What is the nature of prey-predator dynamics (in terms of prey selectivity, size selectivity, site selectivity) in the extreme size class? ii. Does the prey-predator dynamics comparable to the macro-molluscan assemblage of coeval formations of the same biogeographic province? To understand the prey predator relationship in extremely small size class bulk sample was collected from Quilon Limestone bed of Kerala representing early Miocene (Burdigalian) age. We also compared our data with previously published predation data from the bivalve fauna of coeval Chhasra formation of Kutch, India (Chattopadhyay and Dutta, 2013). The average body size of the bivalve specimens recovered from this assemblage is extremely small, ranging between 0.57- 4.76 mm. Our sample of ~2000 valves representing nine families with six families carrying record of predatory drilling, shows an average drilling frequency (DF) of 0.06 and an incomplete drilling frequency (IDF) is 0.25. The majority of the complete drillholes represent wall drilling (82%). The drillhole morphology is similar to those made by naticid and muricid gastropods. Prior report of three genera of naticid (Natica, Tanea and Polinices) and two genera of muricid (Triplex and Dermomurex) from the same locality further supports our finding. A strong positive correlation exists between overall prey and predator size. Drilling is primarily observed in Anomidae, Corbulidae, Cardiidae, Lucinidae and Glycymerididae families and no evidence of drilling in Veneridae, Tellinidae, and Arcidae. A significant difference in body size is observed for drilled ones with individual undrilled families, indicating a possible size refugia for both upper and smaller size class. Lucinidae showed highest incomplete drilling frequency may be because of their surface ornamentation and presence of conchiolin. Within ecological groups, mobile preys are more successful to evade from predatory attack over immobile ones. In comparison to the predation in macrobenthos of the same biogeographic province of coeval formation, microbenthos shows a lower level of predation intensity and rate of failure. Apart from size difference, this difference in predation intensity may have been affected by the protective seagrass environment of Kerala. The interactions in microbenthos seems to be more strongly size-dependent compared to those among the macrobenthos that are often characterized by a lack of prey-predator size relationship. Such change in drilling pattern highlights the role of body size dependent predation pattern among marine invertebrates. In the 5th chapter, I summarized the final conclusions and the broad implications of the work. This present work is first of its kind to document the long-term effect of environmental parameters on marine molluscan body size using a combination of global and regional records of fossil and recent fauna. This work demonstrates that the body-size variation of marine molluscs does not follow a general rules in deep time; it is primarily governed by regional conditions such as temperature and salinity. This study also shows, for the first time, that the role of various ecological groups of molluscs in shaping their body-size variation in times of changing environmental condition. This study also demonstrates the influence of body size on community structure through changes in nature of biotic interaction, which plays a significant role in the natural selection of a group.

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