The present study provides a comprehensive survey of the planktonic hydrozoan fauna from the Paranaguá Estuarine System (PES; southern Brazil, 25°S 48°W), a subtropical estuary considered a Natural World Heritage site by UNESCO. Extensive collections were performed throughout the estuary in five sampling campaigns during the summer and winter periods of 2012 and 2013 and summer of 2014, totalling 185 samples. About 49,000 organisms were analysed which together with the few previous records resulted in a total of 36 hydromedusae and three siphonophore species. We highlight the presence of Cnidostoma fallax in high abundance (>19,000 individuals captured; ~40% of all planktonic hydrozoans). The high abundance of this species throughout three consecutive summers suggests a change in the local assemblage structure, since previous reports in the 1980s, 1990s and 2000s had shown Liriope tetraphylla recurrently as the dominant species which now ranks fourth. It is difficult to ascertain the causes of such changes due to the paucity of previous studies on Brazilian estuaries, but one possibility is that C. fallax has been recently introduced. In any case, the high dominance of this hydromedusa was not expected and this scenario would potentially cause changes in the local food web since C. fallax and L. tetraphylla are quite different morphologically and ecologically and thus probably play different trophic roles. Future studies are necessary to check the long-term permanency of C. fallax and to assess its biology and ecology and the impact of this assemblage change on the ecosystem.

We describe and illustrate a new genus and species of sea anemone from the intertidal in the Gulf of California (Mexico). Tenactis gen. nov. is characterized by mesenteries decamerously arranged, a single pair of directives attached to a single and strong siphonoglyph, small conchula, endodermal marginal sphincter muscle, pedal disc with basilar muscles, cinclides in pedal disc and proximal column, column with verrucae and pseudoacrorhagi, and a cnidom with p-mastigophores A2 and p-mastigophores B1 and B2a. Tenactis riosmenai gen. et sp. nov. is characterized by having a column not divisible into regions with longitudinal rows of verrucae distally and pseudoacrorhagi forming spherical structures at the tip, only 10 pairs of perfect mesenteries, diffuse endodermal marginal sphincter muscle and p-mastigophores A2 and p-mastigophores B1 and B2a in the filaments. Because of the decamerous arrangement of the mesenteries Tenactis gen. nov. most closely resembles some members of Actiniidae, Haloclavidae, Minyadidae, Oractiidae and Limnactinidae (Actinioidea), Halcampidae (Metridioidea) and Halcuriidae (Actinernoidea). However, the combination of having a decamerous arrangement of mesenteries, single siphonoglyph, conchula and cinclides indicate this new genus belongs to Haloclavidae. Additionally, the presence of a diffuse endodermal marginal sphincter muscle, basilar muscles, a single pair of directives, column with longitudinal rows of verrucae distally and pseudoacrorhagi, p-mastigophores A2, and p-mastigophores B1 and B2a in the filaments distinguish Tenactis riosmenai gen. et sp. nov. within Haloclavidae. This is the first record of the family Haloclavidae for the Gulf of California.

Mechanized dredging impact on discards was assessed along the northern Alboran Sea (W Mediterranean Sea). Data from 101 dredging operations were analysed for contrasting spatial and seasonal variability of damage, with the use of a three-level damage scale. 4.5% of discarded individuals displayed intermediate damage, whereas 11.3% displayed severe damage. Echinoderms displayed the highest level of damage (~75% of total collected individuals) and Echinocardium cf. mediterraneum was the most susceptible discarded species (85% with severe damage), followed by bivalves (7.3%) and crustaceans (3.3%). The target Chamelea gallina showed a low proportion of damaged individuals, probably due to their thick protective shell, which promotes the survival of discarded undersized target individuals. Spatial differences in damage level on discards were linked to some gear characteristics, to the higher amount of gravels and to longer tow durations, whereas damage was generally higher in cold months and partly related to higher quantities of hard shelled molluscs, in both cases increasing the abrasion and damage to retained organisms. Data suggest that dredges with a lower number of narrower iron teeth and towed for a shorter time could decrease the damage rate in discards of this fishery. A spatial management plan based on the type of grounds would be useful in order to improve efficiency of these fisheries and minimize their impact to soft bottoms with different commercial catches and biological communities.

Fertilization success will determine the rate at which a population can expand and is especially important when considering small, establishing or enduring communities. Introduced species frequently fail to establish reproductively functional populations due to strong Allee effects associated with low densities. The native European oyster, Ostrea edulis broods its fertilized eggs in the pallial cavity for a period of 8–10 days before releasing the larvae. It is considered a partial broadcast spawner and was used as a model species to assess the importance of Allee effects such as inter-individual distance on reproductive success. Distances between individual oysters within test plots in areas of known oyster density were used in conjunction with standardized brood size (n larvae g−1 total wet weight) to assess fertilization success. A significant, positive relationship was observed between brood size and oyster density. Oysters with a nearest neighbour ≤1.5 m were found to brood significantly more larvae than individuals with nearest neighbours ≥1.5 m. Therefore, high density sites need to be maintained to ensure the recovery and enhancement of this OSPAR Convention recognized species in decline.

Although Aplysia depilans is a widely known European species and the type species of the genus, the distinctions that separate it from other related congeners are not entirely clear. Aimed at improving this scenario, a detailed morphological description is performed herein, showing some unprecedented and useful features. Based on our anatomical examination, this study has found the opaline gland to be dark in colour, as well as a metapodium with an indistinctly delimited pedal sucker, a crop divided into two equal chambers of similar volume by a circular muscle, gizzard plates arranged in groups, and a peculiar penial sheath structure. All of these characters appear to be significantly different from the other species, based on a review of the anatomical literature and several examined samples of ongoing studies. Our central objective is to provide new data on the anatomy of the type species of Aplysiidae; and, based on topological evidence, to propose a new terminology and possible homologies to some important characters, such as parapodia lobation, cephalic tentacles, crop, penial sheath, gizzard plates and visceral ganglion in anaspideans, for future phylogenetic analysis. The subgenus Subaplysia Medina, Collins & Walsh, 2005 is regarded as a junior synonym of subgenus Aplysia Linnaeus, 1767.

The amphipod Gmelinoides fasciatus from Lake Baikal is an example of a species being introduced into a number of water bodies in Russia, including the water system of North-west Russia, to expand food reserves of commercial fish. The interest in this crustacean has been attributed to its successful adaptation and expanding habitat area in the region. In order to assess the role of genetic mechanisms in adaptation to new conditions the frequency of chromosomal aberrations (ChA) at anaphase and telophase stages of mitosis in G. fasciatus embryos from six local populations of Lake Baikal and two invasive populations of the Gulf of Finland, originating from natural habitats were studied in two sequential years. The average level of ChA varied slightly between 0.95% in the samples collected in Lake Baikal and 2.9% in those from the Gulf of Finland. A significant increase in the frequency of ChA was found at two sites in Lake Baikal in 2015 and at two locations in the Gulf of Finland in 2016. First information on G. fasciatus chromosome number and constitution acquired by means of molecular cytogenetic techniques is presented and discussed. The results enable us to suggest this amphipod as a sensitive model to study possible mechanisms of biological adaptation and at the same time as a natural bioindicator of the environmental state.

Symbiotic relationships are a common phenomenon among marine invertebrates, forming both obligatory and facultative dependencies with their host. Here, we investigate and compare the population structure of two crustacean species associated with both shallow and mesophotic ecosystems: an obligate symbiont barnacle (Ceratoconcha domingensis), of the coral Agaricia lamarcki and a meiobenthic, free-living harpacticoid copepod (Laophontella armata). Molecular analyses of the Cytochrome Oxidase Subunit I (COI) gene revealed no population structure between mesophotic and shallow barnacle populations within south-west Puerto Rico (ΦST = 0.0079, P = 0.33). The absence of population structure was expected due to the pelagic naupliar larvae of the barnacles and the connectivity patterns exhibited by the coral itself within the same region. Laophontella armata exhibited significant structure based on the mitochondrial COI gene between the mesophotic reef ecosystem of El Seco, Puerto Rico and mangrove sediments of Curaçao (ΦST = 0.2804, P = 0.0). The El Seco and Curaçao copepods shared three COI haplotypes despite the obligatory benthic development of harpacticoid copepods and the geographic distance between the two locations. Three other COI haplotypes from El Seco exhibited higher than expected (up to 7%) intra-species variability, potentially representing three new cryptic species of harpacticoid copepods or rare, deeply divergent lineages of L. armata. This result is evidence for the urgent need of a deeper investigation into the meiofauna diversity associated with mesophotic coral ecosystems (MCEs), arguably the most diverse metazoan component of MCEs.

In order to confirm the genetic relationship between the Yellow Sea and East China Sea populations of mantis shrimps Oratosquilla oratoria, fragments of mitochondrial DNA COI gene samples were analysed. A total of 212 individuals from nine localities in the East China Sea and Yellow Sea were collected and 108 haplotypes were detected. Neighbour-joining analysis revealed a complete genetic break between the Yellow Sea and East China Sea, which was consistent with the previous mtDNA 16S rRNA. Pleistocene isolation and the current physical barrier were responsible for the complete genetic break between the East China Sea and Yellow Sea. Furthermore, local adaptation in the COI gene may also be contributed to by the genetic differentiation between the populations of the Yellow Sea and East China. The different Ka/Ks ratios between the two clades may reflect different selection pressures and local adaptation on the fragment of COI gene.

The aim of the present study is to test the resource economic monopolization hypothesis and the hypothesis of monogamy using the shrimp Alpheus estuariensis as a model. The shrimps were collected in two areas in the Vaza-Barriz estuary, north-east Brazil, from August to November 2016. The average abundance of refuges was obtained through 30 random replicates. The shrimp presented a random distribution in both areas. Males and females found together showed a weak relation between their sizes, with males being larger than females. In addition, the cheliped of males grows proportionally more than that of females. The great abundance of refuges present in the environment, added to the aforementioned results, do not support the idea of refuge-guarding behaviour or monogamy. These results, which are in disagreement with those already found for some shrimps of the same family, genus, and even species, reinforce the idea that Alpheidae can be used as a model in the study of how environmental conditions are capable of shaping the social behaviour of a species.

During the period from April to September for the years 2014–2016, 998 swordfishes caught by the Moroccan artisanal longline fishery in the Strait of Gibraltar were sampled to study the reproduction of this species in this mixing area between the Mediterranean Sea and the North Atlantic. The results showed that the sex ratio is slightly in favour of males for sizes smaller than 130 cm LJFL (Lower jaw-fork length), whereas females are more numerous in sizes larger than 140 cm LJFL. Fifty per cent of females were estimated to be mature at 170 cm LJFL, while for males, the size at first maturity was estimated to be 95 cm LJFL. The swordfish spawn from June to September, probably in the Mediterranean Sea. The findings of this study suggest that the reproductive characteristics of swordfish caught in the Strait of Gibraltar are similar to those of the Mediterranean swordfish, and a high mixing rate between the Mediterranean and the North Atlantic stocks occurs in the study area.

In the southern Gulf of Mexico, the spotted eagle ray (Aetobatus narinari) is the second most frequently caught batoid in small-scale fisheries off Campeche. Ecological aspects of this ray are unknown in this region, hampering the understanding of the relationship between its distribution and prey availability in the fishing area. In order to study the feeding habits of this batoid and characterize its potential prey in the study area, stomachs and intestines of 154 specimens (68 females and 86 males) were analysed. The results indicated that A. narinari near Campeche is a specialist and selective predator that feeds mainly on gastropods (92.7% IRI), with no significant differences in the diet found between sexes, size groups, or between stomach and intestine contents. In addition, the results indicated that the most important prey species in the diet were among the most common benthic species in three of the four sampling transects positioned in or adjacent to fishing areas for rays. These most important prey species were Strombus pugilis (53.33% IRI) and Americoliva reticularis (25.6% IRI). Other prey species included Lobatus costatus (5.6% IRI) and Petrochirus diogenes (3.6% IRI). This study suggests that this widely distributed ray species feeds in Campeche's coastal waters and that the study of its potential prey increases the understanding of ecological aspects of the species, which emphasizes the added importance of monitoring fishery impacts on prey species (e.g. the conch fishery off Campeche) to help support integrated assessment and management of fisheries.

As apex predators, sharks are known to play an important role in marine food webs. Detailed information on their diet and trophic level is however needed to make clear inferences about their role in the ecosystem. A total of 335 stomachs of smooth hammerhead sharks, Sphyrna zygaena, were obtained from commercial fishing vessels operating in the Ecuadorian Pacific between January and December 2004. A total of 53 prey items were found in the stomachs. According to the Index of Relative Importance (%IRI), cephalopods were the main prey (Dosidicus gigas, Sthenoteuthis oualaniensis, Ancistrocheirus lesueurii and Lolliguncula [Loliolopsis] diomedeae). Sphyrna zygaena was thus confirmed to be a teutophagous species. The estimated trophic level of S. zygaena was between 4.6 and 5.1 (mean ± SD: 4.7 ± 0.16; males: 4.7; females: 4.8). Levin's index (BA) was low (overall: 0.07; males: 0.08; females: 0.09), indicating a narrow trophic niche. We found that sharks <150 cm in total length consumed prey of coastal origin, whereas sharks ≥150 cm foraged in oceanic waters and near the continental shelf. The analyses indicate that S. zygaena is a specialized predator consuming mainly squids.

Basking sharks Cetorhinus maximus tend to aggregate in summer at favoured locations along Britain and Ireland's west coast. Sharks have been described approaching and close-following one another, often to one side. This has been interpreted as putative pre-mating behaviour. At aggregation sites around the Inner Hebrides we used boat-based observation and in-water and overhead drone video-photography to document behaviour and to determine the sex of individuals. It was confirmed that a shark will frequently move purposely towards another from a distance and swim to maintain a position either directly behind or closely to one side of a conspecific for short periods. Contrary to expectation, we found no relationship between the sex of a shark or its size and close-following. This suggests that following behaviours are not mainly related to courtship. Further, abrasions on the nose suspected to be related to male behaviour were found to occur on both sexes, although abrasions on pectoral fins, similarly suggestive of mating-related behaviour, were predominantly on females. Breaching by basking sharks has also been proposed as a means of attracting the opposite sex. We observed breaching by solitary sharks but commonly by sharks within aggregations, and at other times by more than one shark on the same day at the same time; but there was not any clear evidence to indicate that breaching is primarily related to mating. More likely individuals show close following chiefly for feeding-related hydrodynamic advantage. It remains plausible however that mature sharks make use of feeding aggregations to initiate pre-courtship behaviour.

The stomach contents of thresher sharks (Alopias spp.; 162–385 cm total length) were collected from five landing points in northern Peru during 2015. A total of 128 thresher sharks were sampled, with 38 individuals identified as Alopias pelagicus and 90 aggregated to the genus level Alopias sp. to prevent any misidentification. The diet comprised 13 and 10 prey taxa for Alopias sp. and A. pelagicus, respectively. Humboldt squid Dosidicus gigas dominated the diet of both groups, with only minor differences in the diet in relation to size classes and location. The diet of Alopias shows a high degree of specialization and the average trophic positions were high for both groups, A. pelagicus (4.4 ± 0.13) and Alopias sp. (4.5 ± 0.14).

The coastal waters of east Lewis from the Butt of Lewis to Loch Erisort are a proposed Marine Protected Area (MPA) for Risso's dolphins (Grampus griseus). A total of 100.4 h (2006.4 km) of active search effort (Beaufort sea states ≤3) was collected during 72 dedicated boat surveys between 2010 and 2017 (primarily in August and September) in the southern part of the MPA and south to the Shiant Isles. Forty Risso's dolphin sightings and 24.1 h of encounter effort were recorded, predominantly along the southern and eastern Eye Peninsula in 20–40 m water depths and at distances <1 km from shore. Group size ranged from one to 50 animals (mean = 11.8 dolphins) and calves occurred in 37.5% of sightings. A total of 2404 shore-based scans (Beaufort sea states ≤3) carried out from Tiumpan Head between September 2011 and December 2017 resulted in 271 (11.3%) ‘dolphin-present’ scans. Dolphins were present year-round, with a seasonal increase between May and October. ‘Calf-present’ scans only occurred between April and October. Photo-identification images from 28 boat surveys produced a minimum population size of 117 animals. There was evidence of high inter- and intra-annual site fidelity, with individual dolphins photographically captured in up to six of the eight survey years, and between two and seven capture dates being recorded for over 45% of individuals within most years. The combined datasets support the importance of east Lewis for Risso's dolphins, and recommendations are made for ongoing monitoring of dolphin occurrence throughout the wider MPA.