The diamond squid Thysanoteuthis rhombus Troschel, 1857 (Cephalopoda: Thysanoteuthidae) is a large oceanic squid. The adults reach a maximum mantle length of 130 cm and can weigh up to 30 kg (Jereb & Roper, 2010). They are animals with a high growth rate, but low energy consumption (Nigmatullin & Arkhipkin, 1998).

This squid is unique insofar as it is monogamous, living in stable couples throughout most of its life cycle (Nigmatullin et al., 1995), and spawning almost year round in oceanic tropical and subtropical ocean waters (Nigmatullin & Arkhipkin, 1998). This species spawn large (1-2 m) pelagic egg masses, with small pink violet eggs placed around the surface of a gelatinous mucous cylinder (Escánez-Pérez et al., 2012).

Diamond squid have firm meat of very high quality for human food consumption; this has increased interest in the species as a fishery resource (Jereb & Roper, 2010). A significant fishery exists in the coastal waters of the Sea of Japan and around Okinawa; however, their catches are not reflected in FAO statistics (Arkhipkin et al., 2015). Recently, T. rhombus has represented a latent fishery resource and has been caught by jigging at a depth range of 145 m to 720 m in the Philippines where it is a source of food and livelihood for small-scale fishers (De Chavez et al., 2021). In addition, an emerging small-scale fishery is reported from the Canary Islands, the eastern Atlantic, and Jamaica in the Caribbean (Escánez Pérez et al., 2012).

Records of adult individuals are becoming more frequent around the world (e.g., Jereb & Roper, 2010; Arkhipkin et al., 2015; De Chavez et al., 2021). To date there are several records for egg masses (see review in Brown et al., 2022), paralarvae (e.g., De Silva-Dávila et al., 2019), and juveniles (e.g., Fernández-Álvarez et al., 2021). We report here the first record of juveniles of T. rhombus in the Gulf of Tehuantepec, Northeastern Tropical Pacific, along with Dosidicus gigas (d’Orbigny, 1835) juveniles and environmental data.

Sample collection

The Thysanoteuthis rhombus specimens were collected in April 2022, during a fishery and oceanographic cruise (JCFINP/2204) on the research vessel Dr. Jorge Carranza Fraser of the Instituto Nacional de Pesca y Acuacultura of the Mexican Government that systematically covered the Gulf of Tehuantepec (Fig. 1). Specimens of T. rhombus were identified based on the diagnostic characteristics described by Jereb & Roper (2010). Squid specimens were placed in bags to keep frozen at. For histological processing, fresh squid were fixed in Davidson solution. In the laboratory,

The squid samples were obtained from 50 midwater fishing hauls, four of which were positive for T. rhombus (Fig. 1). The hauls were made with a midwater net, which consists of four equal caps (top and bottom footrope length: 48.17 m) that was towed at an average speed of 6.5 km h^-1 during 45 min, at an average depth of 20 m.

The hauls were carried out following prior visualization of detections in echograms recorded 24 hours a day with a Simrad EK60 scientific echo sounder equipped with five split-beam transducers (18, 38, 70, 120 and 200 kHz).

The oceanographic sampling was carried out using a thermosalinograph Sea-Bird which uses a continuous flow pumping system of seawater approximately 3 m deep, directed towards a CT SBE-21 sensor that records temperature and salinity every 10 s. This system was equipped with a fluorometer for chlorophyll-a measurement at the same sampling rate.

In the laboratory, morphometric data for T. rhombus were taken according with the guidelines by Roper & Voss (1983). The sex was recorded by examination of gonads and annexes inside the mantle cavity. Stomach contents weight was measured to the nearest 0.1 mg. For Dosidicus gigas, the dorsal mantle length (ML, measured to nearest mm) and total weight (W, measured to the nearest 0.1 mg) were recorded. The maturity stage for each squid was based on Lipinski (1979).

The length-weight relationship was estimated for each gonadic stage based on the equation W = aML^b, where a is the intercept of the regression model and b is the coefficient of allometry. The estimated value of b was tested with Student’s t-test to determine whether growth was isometric or allometric.

The diamond squids used for this study are housed in the Collection of Cephalopods of Mexican Pacific in the Universidad del Mar (UMAR), Puerto Angel, Oaxaca, Mexico (reference number MHNUMAR-CEPHA 5102-505, and collected under permit PPF/DGOPA-004/22).

Thysanoteuthis rhombus and Dosidicus gigas

Four juvenile T. rhombus were found amongst hauls of Dosidicus gigas juveniles (Fig. 2). A total of 106 D. gigas juveniles were collected, with sizes ranging from 32.8 to 70 mm ML and from 1.1 to 5.8 g W. The length-weight relationship of the D. gigas juveniles, indicate isometric growth (b = 3, p<0.05). T. rhombus individuals were heavier than D. gigas, as seen in the size-weight curve (Fig. 3).

The four juveniles of T. rhombus were identified as three females and one male; data are reported in Table 1. The diamond squid juveniles showed one photophore on the ink sac (Fig. 4). The ingested items removed from the stomachs were highly macerated, belonging to bony fishes and crustaceans.

Oceanographic conditions

Thysanoteuthis rhombus juveniles were collected in the oceanic zone with upwelling and thermohaline front, with a maximum level of chlorophyll-a (5.5 µg/L), unlike the neritic zone that is saltier, with a higher relative level of oxygen (Fig. 5A–B) and lower level of chlorophyll-a (Fig. 5D).

In the oceanic and neritic zone (only the temperature and chlorophyll profiles are shown, Fig. 5D), surface water intrusion of lower relative salinity (34.3 psu) was observed. In the oceanic zone, the saline intrusion was interrupted by an anticyclonic eddy with a haloclinic uplift between oceanographic stations (OS) 12 and 21. This process created the maximum chlorophyll-a observed in OS 24, but not in OS 27. The stations of the neritic zone contrast with the oceanic zone with a cyclonic eddy of lower sinking intensity, but the effects were similar. The distribution of chlorophyll-a detected at different levels of this parameter in the neighboring stations was affected with a level relatively high (3 µg/L) presence in OS 59 but 50% lower in OS 62 (Fig. 5D). The gyres detected in both sampling zones did not affect the relatively high oxygen levels (Fig. 5B). The temperature in the area of interest was only affected by the oceanic eddy with a rise of up to 5°C, but not by the neritic. These processes are consistent with a depth of the mixed layer between 17 and 15 m, for the oceanic and neritic zones, respectively (Fig. 5E).

Due to its mid ocean habitat T. rhombus is difficult to detect (Fernández-Álvarez et al., 2021). In Mexico, records of T. rhombus are limited to an egg mass fragment and one paralarva found in the Northeastern Tropical Pacific and Gulf of California (De Silva-Dávila et al., 2019), and records of two adult females in the Gulf of Tehuantepec (Alejo-Plata & Urbano-Alonso, 2018). To these records, the findings reported here have to be added.

This is the first work that reports juveniles in the Eastern Tropical Pacific, consisting of three females and one male with mantle length less than 70 mm. Notably, the distributions of T. rhombus and D. gigas overlap to some extent (Jereb & Roper, 2010), which explains the joint capture of juveniles of both species in the Gulf of Tehuantepec. Similarly, in the Peruvian Sea, there are isolated data on the by-catch of diamond squid in fisheries targeting D. gigas (Roque-Sánchez & Donayre-Salazar, 2021).

The FAO global production database is relied upon for collecting sightings of ocean species. While all wild catches are recorded as “target species” (Blasco et al., 2020), catches from fishing areas that are reported but not identified to species level are identified only as “not elsewhere included” (NEI). Their relatively rare collection suggests that T. rhombus squids are included among the various squids in “NEI category” (Boyle & Rodhouse, 2005).

Thysanoteuthis rhombus is an epipelagic to mesopelagic species that inhabits open ocean water, rarely approaching the coast (Jereb & Roper, 2010). Previous studies have reported that the passive migration of this species can be regulated by the oceanographic conditions (Miyahara et al., 2008; Sajikumar et al., 2020). In the Gulf of Tehuantepec, the high seasonal productivity and low sea-surface water temperature are due to strong vertical mixing and entrainment associated with strong winds called ‘Tehuanos’ that originate in the Gulf of Mexico and blow across the Isthmus of Tehuantepec (Trasviña & Barton, 2008), mainly from November to April. The occurrence of the T. rhombus juveniles (as well as those of D. gigas) reported in this work may in fact be related with this upwelling.

The diamond squid has the potential for a fishery resource in the different parts of its distributional range as bycatch whenever squid and finfish pelagic trawl fisheries develop in slope and oceanic areas, especially in highly productive zones (Jereb & Roper, 2010). The egg masses of T. rhombus have not been observed in the Gulf of Tehuantepec, probably due to lack of targeted study. Nonetheless, the previous record of a mature female (Alejo-Plata & Urbano-Alonso, 2018) in the Gulf of Tehuantepec as well as the occurrence of paralarvae (De Silva-Dávila et al., 2019 ) in close by areas, and newly reported juveniles here, suggest that T. rhombus is most probably is a common resident in this gulf.

MCAP and RIRG thank the Sistema Nacional de Investigadores (SNI-CONAHCyT). We also thank the National Institute of Fisheries and Aquaculture for the support with the R/V Dr. Jorge Carranza Fraser. We thank the anonymous reviewers for their helpful comments that improve this manuscript. English translation by Karen Englander.