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Background The presence of Thysanoteuthis rhombus Troschel, 1857 in the eastern Pacific Ocean was estimated based on the geographic range of the species constructed from scattered locations of adults, juveniles, and egg masses. Goals This note describes morphometric characteristics of the southernmost registry of the diamond squid T. rhombus in Mexican waters of the Gulf of Tehuantepec (16°N-95°W). Methods In January 2015 we found a stranded adult female weighing a total of 12.5 kg, with a total length of 92 cm, and a mantle length of 45.5 cm. Results This was a mature female, with oocytes inside the oviducts that averaged 1.2 mm in diameter, and seven attached spermatophores on the buccal membrane of the female, as evidence of mating. Conclusions This late registry in the Gulf of Tehuantepec could simply be due to their relative scarcity in the eastern Pacific Ocean.

Lolliguncula argus is a squid species endemic to the Tropical Eastern Pacific and caught incidentally by the artisanal fishery around Puerto Angel, Oaxaca (Mexico). Due to the low abundance of Argus brief squid, basic aspects of the species’ reproductive biology have not been adequately studied. Therefore, in this study, we assess size at maturity (L50), gonad maturation, ovarian development and spawning pattern by means of both histological and oocyte size–frequency analyses. Our results are based on 581 squid specimens: 534 females (11.9–82.4 mm dorsal mantle length, DML) and 47 males (16.0–68.2 mm DML) caught by artisanal fishery from May 2017 to April 2018. The L50 was 58.0 mm DML for females and 55.4 mm DML for males. The ovulation pattern in L. argus is asynchronous, with multiple-batch spawning in a relatively short period of time (intermittent spawning). Lolliguncula argus breed in the coastal waters off Puerto Angel, in the western margin of the Gulf of Tehuantepec, and exhibit gregarious behaviour during spawning events, which is associated with the regional oceanographic conditions. Based on these results, we determine that the opportunistic reproductive strategy of L. argus occurs in response to suitable regional environmental conditions.

Carcharhinus falciformis is an abundant oceanic species, which occurs in equatorial and tropical zones, with an important catch in the Atlantic and Pacific coasts of Mexico. Samples were taken from December 2000 to December 2007 in four landing sites of the artisanal fleet on the coast of Oaxaca. During the period of study 1236 specimens (602 females and 634 males) of C. falciformis were registered. Total length (TL) ranged from 49 to 217 cm for females (mean = 111.3 cm) and from 59 to 265 cm for males (mean = 111.7 cm). The sex ratio of females to males was 1:1 ( 2 0.05 = 0.78, P > 0.05). The present data suggest a size at first sexual maturity of about 184.8 cm TL for females and 178.5 cm TL for males. The catches were composed mainly of young. In the 52 gravid females examined, the average number of embryos per female was seven; with a range of 3-14 embryos. Mean TL of embryos ranged from 10 to 66 cm with evidence of seasonal changes in the size structure. Results obtained showed that C. falciformis gives birth most of the year, with the highest proportion of births during the rainy season (May to October).

Cephalopod fauna from the Gulf of Tehuantepec was collected by direct sampling and from the analysis of top depredators stomach contents, such as sailfish, dolphinfish, sharks, and tunas. Direct sampling was conducted by scuba diving and from shrimp trawls. In total 1,661 cephalopods belonging to 8 families and 15 species were identified from the stomach contents of sailfish, tuna, skipjack, dolphinfish and silky shark, giving a clear image of the cephalopods fauna in the study area. Families more representative were Ommastrephidae (4 species) and Octopodidae (3 species). In the reef corridor Puerto Ángel-Huatulco 4 species of octopus were collected and 3 more species from adjacent shallow coastal lagoons. Four species without previous records were found: Euaxoctopus panamensis, Octopus fitchi, O. chierchiae y O. digueti. Taking into account all information sources for the studied area, the up-to-date list of cephalopods of Gulf of Tehuantepec consists of 24 species. Only Octopus hubbsorum has interest for fisheries whereas Dosidicus gigas, Ommastrephes bartramii, Sthenoteuthis oualaniensis, O. bimaculatus, Lolliguncula panamensis, and L. diomedeae are considered potential unexploited fishery resources and often caught as bycatch.

During two sampling trips carried out over the continental platform of Gulf of Tehuantepec, a total of 101 Lolliguncula panamensis Berry, 1911 were caught. Thirty mature specimens (19 females and 11 males) were found to have unusually short arms. Dorsal mantle length, arm's length, and body weight were measured from the fresh specimens. Evidence of regeneration was observed at different points along arms; these may have been the result of partial autonomies. In the injured arms, the surface of the arm's tip was wholly covered with skin. Fracture planes were found in our histological sections; the autotomized arms exhibited constricted muscle fibers in the longitudinal sections indicative of wound closing. The arms of the specimens observed were very similar to the regenerating arms of other squids. This study represents the first to report arms regeneration and hectocotylus in this squid wild-caught. These results suggest that Lolliguncula panamensis exhibit partial autotomy, and the ability to regenerate the arms. Additionally, each of the eight arms can do so, presumably during defensive interaction or mating.

The buoyancy properties of anthropogenic microparticles (AM) have contributed to their wide dispersion across rivers and oceans, as they tend to float and can be transported by wind and marine currents. The aim of this study was to evaluate the abundance and distribution of AM in relation to zooplankton abundance in three coastal localities from the southern Mexican Pacific. Thirty-nine zooplankton samples were collected along eleven transects perpendicular to the coast of Oaxaca; in three localities: Puerto Escondido, Huatulco, and Salina Cruz, from April 30 to May 12, 2017- Sixty-eight percent of samples showed evidence of AM (mainly microfibers), with abundances varying by locations between 0.004 to 8.72 microparticles per 1000 m 3 . The relationship between zooplankton biomass and AM abundance was not statistically significant ( r 2  = 0.0104), likely due to the dispersion of AM towards the open ocean by offshore winds that reduce coastal concentrations.

The squid Lolliguncula (Lolliguncula) argus is a rare species, endemic to the Eastern Tropical Pacific Ocean, by their morphological characteristics could be easily confused with other species of the family Lolliguinidae. The distribution and meristic measures remains unclear in spite of recent work in the north part of the distribution. Eleven males and 28 females collected in artisanal fisheries and in the shrimp fishery of the Gulf of Tehuantepec were examined. These records expand species geographic distribution in the southern Mexican Pacific and morphometric and biological data are presented.

This updated checklist of chondrichthyans from the Gulf of California (GC) with notes on zoogeography, conservation status, and commercial value includes 2 subclasses, 14 orders, 31 families, 55 genera, and 106 species. Euselachii (Batomorphi 44.3% and Selachii 51.8%) is the most diverse subclass (13 orders, 29 families, 53 genera, and 102 species). The order Carcharhiniformes (4 families, 16 genera, and 36 species) and Myliobatiformes (6 families, 11 genera, and 24 species) represent 56.6% of the total cartilaginous fish diversity in the GC. Based on the analysis of biogeographic affinities, most species include their ranges of distribution into the “Sinus-Californian” (55.7%), Mexican (41.5%), and Panamic (36.8%) provinces; 29 species are circumtropical, 7 amphipacific, 5 amphiamerican, and 4 endemic. Based on the IUCN Red List, 1 species is “Critically Endangered,” 2 are “Endangered,” 14 are “Vulnerable,” 23 are “Near Threatened,” 26 are of “Least Concern,” 3 “Not Evaluated,” and 36 are “Data Deficient.” Mexican laws fully protect 9 species classified as “Threatened,” at “Risk of Extinction,” or under “Special Protection,” whereas the rest are protected from fishing at least during 3 months of the year. Thirty-seven species (35%) are of commercial value; another 30 (28%) rare species of low commercial importance have been reported in the catches of artisanal and semi-industrial fisheries during the last 2 decades. Sørensen's coefficient of similarity and cluster analysis between major faunal divisions in the GC suggest the existence of 2 large chondrichthyan assemblages for this region ([NGC-CGC] + [SGC]). This study increases to 106 the species richness of Chondrichthyes known in the GC and serves as the baseline for formulating and implementing of conservation programs for this group of cartilaginous fish.