Abstract
Cephalopods are primarily active predators throughout life. Flying squids (family Ommastrephidae) represents the most widely distributed and ecologically important family of cephalopods. While the diets of adult flying squids have been extensively studied, the first feeding diet of early paralarvae remains a mystery. The morphology of this ontogenetic stage notably differs from other cephalopod paralarvae, suggesting a different feeding strategy. Here, a combination of Laser Capture Microdissection (LCM) and DNA metabarcoding of wild-collected paralarvae gut contents for eukaryotic 18S v9 and prokaryotic 16S rRNA was applied, covering almost every life domain. The gut contents were mainly composed by fungus, plants, algae and animals of marine and terrestrial origin, as well as eukaryotic and prokaryotic microorganisms commonly found in fecal pellets and particulate organic matter. This assemblage of gut contents is consistent with a diet based on detritus. The ontogenetic shift of diet from detritivore suspension feeding to active predation represents a unique life strategy among cephalopods and allows ommastrephid squids to take advantage of an almost ubiquitous and accessible food resource during their early stages. LCM was successfully applied for the first time to tiny, wild-collected marine organisms, proving its utility in combination with DNA metabarcoding for dietary studies.
Highlights
Cephalopods are active carnivorous predators, with only a few exceptions: Nautilus spp. are mainly scavengers and opportunistic predators (e.g.,1,2), the vampire squid Vampyroteuthis infernalis is a detritivore[3], and the mesopelagic Ram’s horn squid Spirula spirula feeds mainly on detritus and zooplankton[4]
Hatched paralarvae are provided with numerous filamentous buccal papillae around the mouth[17] (Fig. 1d), which become less abundant as the paralarvae grow until they totally disappear[17] (Fig. 1e), coinciding with the split of the proboscis into raptorial tentacles[16] (Fig. 1b,e)
Whereas the adult individual E3 did not provide any read, the late T. sagittatus paralarvae E5 to E7 provided 4,814–208,655 identical sequences that were discarded by the software because they did not match any sequences in the database
Summary
Cephalopods are active carnivorous predators, with only a few exceptions: Nautilus spp. are mainly scavengers and opportunistic predators (e.g.,1,2), the vampire squid Vampyroteuthis infernalis is a detritivore[3], and the mesopelagic Ram’s horn squid Spirula spirula feeds mainly on detritus and zooplankton[4]. Hatched paralarvae are provided with numerous filamentous buccal papillae around the mouth[17] (Fig. 1d), which become less abundant as the paralarvae grow until they totally disappear[17] (Fig. 1e), coinciding with the split of the proboscis into raptorial tentacles[16] (Fig. 1b,e) The function of these papillae is unknown. Studies on wild caught ommastrephid paralarvae did not provide recognizable prey[20,21] until the proboscis began to split, the filamentous buccal papillae disappeared and the remains of crustaceans and cephalopods appeared in their stomach contents[20,24]. The high number of reads that NGS platforms produce allows the detection of DNA traces or underrepresented prey, highly improving the understanding of the diet of the focal species[28] Despite these advantages, co-amplification of the target species (self-contamination, hereafter) is an important problem. Piñol et al.[27] stressed the fact that such blocking molecules are not necessary given the huge number of sequence reads obtained by NGS platforms, which are sufficient to study the diet of focal species even if its DNA co-amplifies
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