Absence of general rules governing molluscan body-size response to climatic fluctuation during Cenozoic

Body size is a key factor in dictating the fate of interaction between an organism and its surrounding environment. A negative temperature-size relationship (TSR) has been suggested as one of the universal responses to climatic warming. It is also predicted that groups with narrow latitudinal range, tropical affinity and higher body size, would show higher sensitivity to climatic fluctuation. Moreover, because of the difference in thermal sensitivity, it is also expected that the response to climatic fluctuation would be different between epifaunal and infaunal groups. To confirm the generality of these relationship among marine families, we compiled the relationship between body-size and global temperature trends over Cenozoic using a database of marine benthic molluscs of class gastropoda and bivalvia resolved to temporal stages. We evaluated the dependence of climate induced body-size response to the existing size and latitudinal spread via correlating the first-difference correlation coefficient of temperature-size (ρ1st (size-temp)) with maximum size and latitudinal spread of family respectively. Cenozoic record of this highly diverse group does not show any signature of TSR for molluscan class or for any other regional, ecological groups during the past 66My long climatic fluctuation. We did not find any evidence supporting heightened response to climatic fluctuation in groups with limited latitudinal spread or with large body-size. The tropical species did not show significant difference in their body-size response in comparison to temperate species. It also shows lack of any difference in response between ecological groups of molluscs with varying substrate relationship and hence, refutes the predicted variation due to difference in thermal specialization. Although a negative correlation between maximum latitudinal spread and ρ1st (size-temp) is observed for infaunal families, it is not statistically significant. Our results highlight the limited validity of “universal rules” in explaining the climate induced morphological response of marine communities in deep time and underscores the complexity in generalizing the biotic outcome of future climatic fluctuation.

higher sensitivity to climatic fluctuation. Moreover, because of the difference in thermal 23 sensitivity, it is also expected that the response to climatic fluctuation would be different ]. Change in body size will, therefore, have consequences for resilience of a group in the 54 event of climate change. It has been recognized that many of the biological predictions 55 about body size response to climatic variation, that have been developed primarily based on 56 data from terrestrial vertebrates, are nonexistent or lack significance when applied for 57 marine organisms [10]. Limitation of such extrapolation regained considerable interest in 58 the present decades, primarily because of its relevance in the biotic response to recent 59 climate crisis of global warming. The overall effect of global warming on ecosystem is 60 appreciated, but general rules to predict the fate of a specific group, as it responds to climate 61 warming, is largely absent. This is especially true for marine ectotherms, though they

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(iv) The body-size response of species that are infaunal should be much more 105 pronounced than epifaunal species.

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In this article, we tested these hypotheses using data on body size and ecological 107 information for 10,388 occurrences of marine benthic molluscs of class gastropoda and measurements (S1 Table). We considered the greatest dimension (length or width) as a Cenozoic. This exercise was conducted for all families and then dividing them into 142 ecological groups (infauna, epifauna). 143 We assigned each species to an ecological group based on their substrate relationship. 144 We divided the species into two groups (tropical and temperate) based on their latitude of 145 occurrence. A specific species is considered to be a tropical species if its occurrences during 146 Cenozoic is restricted between 23.5° N to 23.5° S. A similar rationale was followed to 147 identify temperate species when a species occurrence is strictly restricted between 23.5° to

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The congruence between body size and temperature is observable only for raw data 176 (ρ= -0.01; P= 0.02) (Fig 1) when body size and temperature estimates are both binned by 177 geological stages; however, this relationship is not found in the detrended data, corrected 178 for autocorrelation by first differencing (ρ= 0.18; P= 0.48) (Fig 2A), implying a lack of size-   Table 1. Relationship between body-size and temperature during Cenozoic.

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The significance is set at a value determined by False discovery rate procedure. 198 199 No correlation is observed between maximum body-size and ρ 1st (size-temp) i.e. the 200 coefficient of correlation between 1 st difference of maximum body-size vs temperature for 201 overall data, bivalves or gastropods (Fig 3). Same is true for latitudinal spread and ρ 1st (size-202 temp) (Fig 4)    The significance is set at a value determined by False discovery rate procedure. There is no significant difference in the body size response to temperature between 222 tropical and temperate species (Fig 5, Table 3).  Both epifauna and infauna showed lack of correlation between temperature and body 235 size in the detrended data. There is no significant difference in the body size response to 236 temperature between infaunal and epifaunal species (Fig 6, Table 3). No correlation is 237 observed between maximum body-size and ρ 1st (size-temp) for either epifaunal or infaunal 238 families (Fig 7A-B families (ρ= -0.12; P= 0.36) (Fig 7D, Table 2).

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More importantly, it would be hard to imagine a causal mechanism that would selectively 286 remove a particular size class from a specific time bin, especially within a relatively narrow 287 range of size spectrum (two orders of magnitude).

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The second possibility is that the expectation of a species level TSR for marine  habitats. In the ocean, however, nutrient distribution and productivity is controlled by a  We showed that generalized body size reduction to climatic warming may not have