The high mortality of cephalopod early stages is the main bottleneck to grow them from paralarvae to adults in culture conditions, probably because the inadequacy of the diet that results in malnutrition. Since visual analysis of digestive tract contents of paralarvae provides little evidence of diet composition, the use of molecular tools, particularly next generation sequencing (NGS) platforms, offers an alternative to understand prey preferences and nutrient requirements of wild paralarvae. In this work, we aimed to determine the diet of paralarvae of the loliginid squid Alloteuthis media and to enhance the knowledge of the diet of recently hatched Octopus vulgaris paralarvae collected in different areas and seasons in an upwelling area (NW Spain). DNA from the dissected digestive glands of 32 A. media and 64 O. vulgaris paralarvae was amplified with universal primers for the mitochondrial gene COI, and specific primers targeting the mitochondrial gene 16S gene of arthropods and the mitochondrial gene 16S of Chordata. Following high-throughput DNA sequencing with the MiSeq run (Illumina), up to 4,124,464 reads were obtained and 234,090 reads of prey were successfully identified in 96.87 and 81.25% of octopus and squid paralarvae, respectively. Overall, we identified 122 Molecular Taxonomic Units (MOTUs) belonging to several taxa of decapods, copepods, euphausiids, amphipods, echinoderms, molluscs, and hydroids. Redundancy analysis (RDA) showed seasonal and spatial variability in the diet of O. vulgaris and spatial variability in A. media diet. General Additive Models (GAM) of the most frequently detected prey families of O. vulgaris revealed seasonal variability of the presence of copepods (family Paracalanidae) and ophiuroids (family Euryalidae), spatial variability in presence of crabs (family Pilumnidae) and preference in small individual octopus paralarvae for cladocerans (family Sididae) and ophiuroids. No statistically significant variation in the occurrences of the most frequently identified families was revealed in A. media. Overall, these results provide new clues about dietary preferences of wild cephalopod paralarvae, thus opening up new scenarios for research on trophic ecology and digestive physiology under controlled conditions.
14have revised the manuscript critically for important intellectual content and have approved the final version to 15 be published. The authors declare that the research was conducted in the absence of any relationships that 16 could be construed as a potential conflict of interest. We also declare that the manuscript has not been 17 published previously or split up into several parts, neither the data have been fabricated or manipulated. 18 19 Acknowledgments 20 We are indebted to the captain, crew and technicians of R/V 'Mytilus' (IIM, CSIC Vigo) for their assistance 21 in collecting the zooplankton samples. We are grateful to Mariana Cueto for assisting us with laboratory 22 42 discrimination of genus were hatching ring length and head width, while tentacle length helped to 43 differentiate A. media from A. subulata. These discriminant functions should be tested with more paralarvae 44 from different origins and seasons to account for body shape plasticity, but suggest a promising result to 45 facilitate loliginid paralarvae identification for future research. 46 KEY WORDS 47 Squid; early stages; morphometries; population genetics 48 49 157
Sepiolid paralarvae are poorly studied, at least in part, because of the difficulty of accurate identification using morphological analysis. To unravel the biodiversity of sepiolid paralarvae collected in the Ría de Vigo during the upwelling season (2012)(2013)(2014), and to overcome the difficulties of traditional identification, sepiolid paralarvae were identified by amplifying the barcoding gene cytochrome c oxidase subunit I (COI). In addition, morphometric analysis (Generalised Lineal Models, GLM and Discriminant Analysis, DA) was used to identify morphometric patterns useful for paralarval species identification. Genetic barcoding successfully identified 34 Sepiola pfefferi, 31 Rondeletiola minor, 30 Sepiola tridens, 4 Sepiola atlantica, 2 Sepietta neglecta and 1 Sepiola ligulata. COI analysis also allowed us to infer that the paralarvae of the three most abundant species belonged to the same populations independently of the year sampled. GLM suggested that total length (statistically different among the three species) and tentacle length (statistically larger in S. pfefferi from the other two species) were good variables to distinguish among species. DA succeeded in separating S. pfefferi from S. tridens, but R. minor overlapped along the first axes with both species, decreasing the accurate classification rate to 67%.
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