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Identification of the two sympatric species, Alloteuthis media and Alloteuthis subulata , has long relied on a set of identifying morphometric parameters and descriptive guidelines. To resolve taxonomic status of Alloteuthis in the Eastern Adriatic, we used morphological and molecular approach on a dataset collected during MEDITS expeditions sampling the entire Eastern Adriatic over consecutive summers. Phylogenetic analyses inferred from mitochondrial DNA cytochrome oxidase I (COI) gene sequences confirmed presence of both species in the Eastern Adriatic, with A. subulata occurring only in its central and southern parts. Analyses of genetic diversity showed that A. subulata samples in the Eastern Adriatic shared a single haplotype while A. media showed high haplotype diversity. Comparison of Eastern Adriatic A. media samples and populations from other regions showed statistically significant genetic differentiation between the Atlantic haplotypes and each of the Adriatic, Aegean, and Ionian populations. Conversely, A. subulata had low genetic diversity with only two haplotypes present across samples collected globally. There was no single morphometric character with strong enough power to discriminate between species, however, when morphological traits were looked as a composite metric rather than in isolation, the majority of individuals were correctly classified into one of three groups ( A. media males or females and A. subulata ).

Accurate species classification is essential to understand complete life cycles of cephalopods. Identifying freshly caught or fixed loliginid paralarvae to species level with the traditional taxonomic guides is challenging. Therefore, the aim of this work was to identify genetically loliginid paralarvae captured in NW Spain (a region where at least three loliginid species are known to coexist) during 2012, 2013, and 2014, and to seek a means to distinguish the species from each other based on their morphometry. First, the barcoding region (COI gene) was amplified to identify each paralarva, and to obtain population molecular diversity indices and genetic structure for the different species. Afterwards, discriminant analysis (DA) was used to evaluate the performance of the selected morphometric measurements to distinguish among the species previously identified. Molecular analyses revealed three loliginid species (Alloteuthis media, A. subulata, and Loligo vulgaris), with different patterns of molecular diversity. DAs based on body morphometrics correctly categorised 75% of paralarvae to genus (Loligo and Alloteuthis) and 72% of Alloteuthis individuals to species level (A. media and A. subulata). When statolith measurements were included in the morphometric analysis, successful classification increased to 94 and 82%, respectively. The most useful variables for the discrimination of genus were hatching ring length and head width, while tentacle length helped to differentiate A. media from A. subulata. These discriminant functions should be tested with more paralarvae from different origins and seasons to account for body shape plasticity, but suggest a promising result to facilitate loliginid paralarvae identification for future research.

We morphologically analyzed 79 cephalopod specimens from the North and Baltic Seas belonging to 13 separate species. Another 29 specimens showed morphological features of either Alloteuthis mediaor Alloteuthis subulata or were found to be in between. Reliable identification features to distinguish between A. media and A. subulata are currently not available. The analysis of the DNA barcoding region of the COI gene revealed intraspecific distances (uncorrected p) ranging from 0 to 2.13 % (average 0.1 %) and interspecific distances between 3.31 and 22 % (average 15.52 %). All species formed monophyletic clusters in a neighbor-joining analysis and were supported by bootstrap values of ≥99 %. All COI haplotypes belonging to the 29 Alloteuthis specimens were grouped in one cluster. Neither COI nor 18S rDNA sequences helped to distinguish between the different Alloteuthis morphotypes. For species identification purposes, we recommend the use of COI, as it showed higher bootstrap support of species clusters and less amplification and sequencing failure compared to 18S. Our data strongly support the assumption that the genus Alloteuthis is only represented by a single species, at least in the North Sea. It remained unclear whether this species is A. subulata or A. media. All COI sequences including important metadata were uploaded to the Barcode of Life Data Systems and can be used as reference library for the molecular identification of more than 50 % of the cephalopod fauna known from the North and Baltic Seas.

Demersal trawl fisheries generate large quantities of discards which temporarily increase the amount of carrion available to benthic communities and lead to a faster energetic turn-over. This study examines the availability of discarded material to the benthos, assesses consumption times of different items and identifies scavengers attracted to those invertebrates most frequently discarded from Clyde Sea Nephrops trawlers. In field and laboratory trials, heavy-shelled dead whelks (Buccinum undatum, Neptunea antiqua) sank faster than softer-bodied species like cephalopods (Allotheuthis subulata, Rossia macrosoma) or echinoderms (Ophiura ophiura, Asterias rubens), making most discards available to the benthos (at ca. -60 m CD [chart datum]) within minutes after discarding. SCUBA and time-lapse camera observations in the Clyde Sea and Loch Sween indicated bait utilisation times between 24 and 48 h. Fast-moving animals like brachyuran crabs were the first to arrive at discard bait piles whose composition mimicked typical discards from the Clyde Sea Nephrops fishery. Bimonthly deployments of traps baited with invertebrate discards in the north of the Clyde Sea showed that A. rubens, followed by Pagurus bernhardus, Liocarcinus depurator and whelks, were the most abundant megafaunal scavengers. Fine-meshed funnel traps deployed inside those creels yielded up to 2819 amphipods per trap, with Scopelocheirus hopei and Orchomene nanus accounting for most of the catch. Together with whelks, A. rubens and Carcinus maenas, O. nanus showed a clear preference for crustacean bait. By contrast, Pagurus bernhardus was more attracted to A. rubens and, in 1 trial, to O. ophiura bait. Traps deployed in the south of the Clyde Sea yielded generally lower numbers and species diversity in the catch, with Nephrops being the most abundant megafaunal scavenger. It showed a preference for L. depurator and conspecific bait. While the results show that a range of epibenthic species readily utilise invertebrates discarded from Clyde Sea Nephrops trawlers, it is unknown to what extent discards subsidise benthic communities as information on the ecological energetics of the species involved locally is currently lacking.

Apart from one study that reported growth of less than one increment per day in statoliths of the squid Alloteuthis subulata, most studies so far have presumed that one increment was laid down per day in the statoliths of the squid species they examined.

This paper describes the responses of a squid (Alloteuthis subulata) and a fish (Trachurus trachurus) to changes in the angular distribution of light.  An apparatus was made that simulated the angular distribution of daylight in the sea. The apparatus enabled the direction of the brightest light to be changed and the positions of the animals in response to these changes were observed. Squid viewed head-on were observed to roll by a maximum of 20° when the incident light source was at angles between 20° and 90° (where 0° is vertically downwards). When viewed laterally, i.e. in the pitch plane, the squid were observed to position themselves more closely with respect to the angle of the light source, they swam in a near vertical plane when the incident light source was at an angle of 90°. Swimming movements in the roll and pitch plane became more horizontal with positions of the light source between 90 and 180°. Horse mackerel, in contrast, inclined their dorsal surfaces to almost perfectly match the angle of the incident light source, even swimming upside-down when light came from below. These experiments also revealed that squid display a counter-shading chromatophore pattern (‘Flexible Countershading’) in response to light coming from the sides, which involves darkening the side of the body facing the brightest light. The use of chromatophores in this way may explain why the dorsal light reflex in squid is so weak compared to that of fish.

Eleven monthly samples of the myopsid squid Alloteuthis subulata were collected from the English Channel with a small otter trawl in 1985–86. Squid were sexed and assigned a maturity stage. Measurements were made of mantle length and dry weight of gonad and somatic tissue. Maximum length of life for squid from this population is ca 1 yr. Immature squid are recruited into the population twice during the year, in spring and summer. Males and females start to mature at 30 to 40 mm mantle length. The demography of the population is complex, and more difficult to interpret for males. There were apparently 3 spawning groups of females in the year sampled. The length exponent for body weight is very low in both males and females. Males mature earlier than females and male gonad weight is poorly correlated with body weight in squid > 90 mm mantle length. Gonad weight increases logarithmically in relation to total body weight in females but gonad production is not appreciable until mantle length exceeds 100 mm. The gonad accounts for ca 25 % of the body mass of females at the maximum mantle length of 140 mm. This suggests that A. subulata, in common with some other cephalopods, has a low reproductive effort by comparison with the lifetime reproductive effort of species of benthic iteroparous mollusc.

Squid of the genus Alloteuthis from the Aegean Sea and eastern Mediterranean were identified as A. media. It is not possible to distinguish A. media from A. subulata by relative fin length. Both ‘species’ are probably intraspecific forms. Egg size varied from 1·5 to 2·3 mm. Oocyte maturation in the ovary occurs in batches. The potential fecundity is some 1000–4000 eggs, most of these being released during continuous spawning accompanied by female growth.