Effectiveness of small size against drilling predation: Insights from lower Miocene faunal assemblage of Quilon Limestone, India

Abstract

The nature of drilling predation, although well documented for molluscan fossils, is understudied for micromolluscs (<5mm). Studying predation in micromolluscs is especially critical in evaluating the adaptive significance of micromorphy against predation and assessing the importance of predator-prey size relationship (PPSR). This study documents drilling predation event in microbivalves from early Miocene (Burdigalian) fossil assemblage of Quilon limestone from Kerala, India. Our sample of ~2000 valves represent nine families with an average drilling frequency (DF) of 0.06 and an incomplete drilling frequency (IDF) of 0.26. The characteristic drillhole morphology and occurrence of five genera of modern drilling gastropods (Naticid: Natica, Tanea and Polinices; Muricid: Triplex and Dermomurex) from the same locality reveals the predator identity. Predation in the studied assemblage is found to be highly selective in terms of prey taxa, size, mobility and site selection. Six out of nine families show evidence of predation indicating taxon selectivity.Poor correlation between DF and abundance further supports this view. Failed attacks are strongly correlated with morphological features such as surface ornamentation (Lucinidae), presence of conchiolin layers (Corbulidae). Drilling occurs primarily on medium size class and prey outside this size range show lower rate of attack. This indicates the existence of an "inverse size refugia" for extremely small prey along with the classical size refugia existing for large prey. Mobility is found to be a deterrent to drilling predation and it also increases failure.Microbenthos of Quilon limestone shows a lower predation intensity in comparison to the Miocene macrobenthos worldwide including coeval formation of the Kutch Basin. The interaction in microbenthos is more strongly size-dependent in contrast to the Kutch fauna.Reduced predation intensity in microfauna and existence of "inverse size refugia" support the .

Micromorphy Offers Effective Defence Against Predation: Insights From the Cost-Benefit Analyses of Micro Gastropod Predation Record

“…Low values of drilling frequency in microbivalves in comparison to coeval global averages have been used to establish the effectiveness of micromorphy against drilling predation (Chattopadhyay et al, 2020). Our study confirms this finding for both drilling and durophagous predation.…”

“…It is also important to note that none of the individuals below 0.35mm are drilled. This also confirmed the "negative size refuge" exists in microgastropods similar to microbivalve prey (Chattopadhyay et al, 2020). The cost-benefit analysis also confirms that the micromorphy may act as an effective defense strategy by making the smaller sizes less preferred.…”

“…Smaller brachiopods demonstrated the lower intensity of shell-breaking predation (Harper et al, 2009). A fossilized assemblage of micro bivalves also revealed a lower intensity of drilling predation in the smaller size class supporting the existence of a negative size refugia (Chattopadhyay et al, 2020). Such predation resistance among extremely small invertebrates points to a complex relationship between size and predation intensity.…”

“…The field locality is situated on the cliffs along the shores of Ashtamudi lake, near Padapakkara village, Kerala, India (N 98 08° 58'36", E 076° 38'08") (Figure 1). The collection protocol has been described in detail in Chattopadhyay et al (2020).…”

“…Consequently, the predator targets the medium-sized prey to maximize their energy gain (Boggs et al, 1984;Kelley, 1988;Kitchell et al, 1981;Chattopadhyay et al, 2020;Kitchell et al, 1981;Pahari et al, 2014). The low DF in the smallest size class in our sample supports this and suggests that the smaller size class is less likely to be attacked and it provides refuge from predation.…”