We present a preliminary taxonomic survey of the metazoan parasites from meso- and bathypelagic fish hosts from the Perdido region, western Gulf of Mexico. In total, 187 individual fish were collected, belonging to five families and 12 species: Cyclothone acclinidens, Cyclothone alba, Cyclothone braueri, Cyclothone pallida, Cyclothone pseudopallida, Cyclothone sp., Manducus maderensis (Gonostomatidae), Notolychnus valdiviae (Myctophidae), Dibranchus atlanticus (Ogcocephalidae), Peristedion greyae (Peristediidae), Sternoptyx pseudobscura (Sternoptychidae), and Chauliodus sloani (Stomiidae). The metazoan parasites collected were the digenean Lethadena profunda from Cyclothone sp., four tetraphillidean larval cestodes from C. acclinidens, Cyclothone sp., C. sloani and P. greyae, a species of Echeneibotrium from D. atlanticus, a species of Anisakis from C. sloani, and the nematode Mooleptus rabuka from C. alba. Four of the parasite species found had already been previously recorded for the northern Gulf of Mexico. Additionally, we report two parasites not recorded before for meso- or bathypelagic fish in the entire Gulf of Mexico: M. rabuka from C. alba and Cyclothone sp.; and the copepods of the family Pennellidae from Cyclothone sp. and P. greyae.

Although parasite community studies are growing in numbers, our understanding of which macro-ecological and evolutionary processes have shaped parasite communities is still based on a narrow range of host–parasite systems. The present study assessed the diversity and endoparasite species composition in New Zealand deep-sea fish (grenadiers, family Macrouridae), and tested the effects of host phylogeny and geography on the structure of endoparasite communities using a distance decay framework. We found that grenadiers from the Chatham Rise harboured a surprisingly high diversity of digeneans, cestodes and nematodes, with different species of grenadiers having different parasite assemblages. Our results demonstrate that community similarity based on the presence/absence of parasites was only affected by the phylogenetic relatedness among grenadier species. In contrast, both phylogenetic distance among grenadiers (measured as the number of base-pair differences of DNA sequences) and geographic distance between sample locations influenced the similarity of parasite communities based on the parasites' prevalence and mean abundance. Our key findings highlight the significant effect of deep-sea host phylogeny in shaping their parasite assemblages, a factor previously neglected in studies of parasite communities in deep-sea systems.

We have estimated the length at maturity and length-weight relationships for five fish species inhabiting the deep-sea from the Andaman and Nicobar Islands off the Indian coast between 295–650 m deep in a trawl survey carried out in March–April 2017. Hauls were carried out by a high-speed Demersal Trawl Crustacean Version trawl net and analysis was performed for a total of 832 specimens. Length at first maturity of the five deep-sea fish species ranged from 14.28–105.73 cm while length at 90% maturity was in the range 17.87–159.83 cm. The length at maturity of the fish are Alepocephalus bicolor (male = 66.09, female = 105.73), Bathyclupea hoskynii (m = 15.14, f = 14.15), Chlorophthalmus corniger (m = 17.54, f = 15.31), Neoepinnula orientalis (m = 20.76, f = 16.76), and Neoscopelus microchir (m = 14.28, f = 15.40). The b value in the length-weight relationship ranged from 0.69–2.60, i.e. Alepocephalus bicolor (m = 1.93, f = 1.62), Bathyclupea hoskynii (m = 3.5, f = 1.66), Chlorophthalmus corniger (m = 2.07, f = 1.56), Neoepinnula orientalis (m = 2.86, f = 2.46) and Neoscopelus microchir (m = 0.89, f = 0.49). Based on these results, the b value showed an allometric relationship with length for all species studied, because these species have a similar morphometry, i.e. a flattened back. Since they are primary or secondary consumers at the bottom of consumer food webs, their roles are as predators of small–medium prey and as prey of top predators of food web chains.

Information about the spatial distribution of bycatch species and their spatial overlap with the target species is essential for fisheries management. The present study used fishery-dependent data (vessel monitoring systems, logbooks and official daily landings) to study the spatial distribution and overlap between black scabbardfish Aphanopus carbo and leafscale gulper shark Centrophorus squamosus taken by the longline fishery operating off mainland Portugal. The geostatistical method kriging was applied to estimate the distribution of the leafscale gulper shark in relation to black scabbardfish and thus assess the impact of the fishery on the leafscale gulper shark. Results indicate that in fishing grounds where the black scabbardfish is more abundant, the relative occurrence of deepwater sharks is reduced. These findings have implications for alternative management measures to be adopted in this particular fishery, particularly where it concerns the minimization of deepwater shark bycatch.

The rare ophichthid eel Asarcenchelys longimanus is reported for the first time from Bahia State, north-eastern Brazil. To date, only two specimens of A. longimanus, the holotype and a paratype, were known. The new finding extends its distribution to about 2400 km southwards along the Brazilian coastline and provides a new maximum size for the species. Comparisons of the morphometric and meristic data between our specimens and those used in the original description are provided.

The occurrence of the merluccid Macruronus novaezelandiae from tropical waters off Bahia, eastern Brazil, is reported for the first time due to the capture of an adult of 712.3 mm standard length in May 2008, from a depth of 400 m. Until then no specimen had been reported north of 32°29′S on the South American Atlantic coast. This new record extends the species' range to about 2500 km northwards along the Brazilian coastline and is the first ever from tropical waters in the world. A comparison of the morphometric characters is provided.

This study gives a first qualitative and quantitative record of the parasitic infracommunities in two deep-sea fish species inhabiting the locality of the Juan Fernández Archipelago, Chile. In December of 2002 a total of 12 individuals of Coelorynchus chilensis and 35 Notacanthus sexspinis was obtained as by-catch of a shrimp Haliporoides diomedeae fishery at around 1000 m depth. The majority of the individuals of both species were infected at least by one parasitic taxon. Forty-five parasites were collected from C. chilensis including larval cestodes, Hepatoxylon trichiuri, Grillotia sp., larval nematodes Anisakis sp., adult digeneans Dinosoma sp., Brachyphallus sp., copepods Protochondia cf. longicauda and myxozoa Myxidium sp. Twenty-two parasitic individuals were collected from N. sexspinis belonging to adult digeneans Lecithochirium sp., and Accacladium sp. (without eggs, immature), larval cestodes Hepatoxylon trichiuri, larval nematodes Anisakis sp. and myxozoa Ceratomyxa sp. Most of the parasites showed a low prevalence and abundance in both host species, except for Myxidium sp., Ceratomyxa sp., Dinosoma sp. and Brachyphallus sp. The parasites Hepatoxylon trichiuri and Anisakis sp. were the only shared taxa. These infracommunities are low in richness, diversity and abundance compared to shallow water fish–parasites communities. Nevertheless the numeric properties of infracommunities are not reliable due to sample size. Given the additional difficulties with reliable sampling of deep-sea fish, these results should not have too much significance attributed to them. The relevance of this study is simply the recognition of parasites that infect the deep-sea fish of the families Macrouridae and Notacanthidae in the southern hemisphere.