The jumbo or Humboldt squid, Dosidicus gigas, is an important fisheries resource and a significant participant in regional ecologies as both predator and prey. It is the largest species in the oceanic squid family Ommastrephidae and has the largest known potential fecundity of any cephalopod, yet little is understood about its reproductive biology. We report the first discovery of a naturally deposited egg mass of Dosidicus gigas, as well as the first spawning of eggs in captivity. The egg mass was found in warm water (25 -278C) at a depth of 16 m and was far larger than the egg masses of any squid species previously reported. Eggs were embedded in a watery, gelatinous matrix and were individually surrounded by a unique envelope external to the chorion. This envelope was present in both wild and captive-spawned egg masses, but it was not present in artificially fertilized eggs. The wild egg mass appeared to be resistant to microbial infection, unlike the incomplete and damaged egg masses spawned in captivity, suggesting that the intact egg mass protects the eggs within. Chorion expansion was also more extensive in the wild egg mass. Hatchling behaviours included proboscis extension, chromatophore activity, and a range of swimming speeds that may allow them to exercise some control over their distribution in the wild.

Changes in digestive enzyme activity and histology were studied in Atractosteus tropicus embryos, larvae and juvenile periods. Alkaline protease, chymotrypsin, carboxypeptidase A, lipase and α-amylase were detected in all periods and gradually increased until reaching the maximum peak in juveniles; meanwhile, acid protease was first detected at 5 days after hatching (dah) when first feeding started and trypsin and leucine aminopeptidase activities were detected from 19 dah, their values being increased gradually until reaching a maximum value at 31 dah. Acid and alkaline phosphatase activities increased from yolk-sac absorption (3 dah) until day 31 after hatching. Zymogram for acid protease showed two bands in active forms (0.4 and 0.5 Rfs) from day 5 after hatching and a third protease form (0.3 Rf) that appears at 31 dah. Two active forms (26.3 and 24.9 kDa) were detected using SDS-PAGE alkaline proteases zymogram at 5 dah, and an additional active form (44.1 kDa) was detected at 7 dah. Regarding the histological development of the digestive system, the exocrine pancreas containing zymogen granules was already visible at 3 dah, whereas at 5 dah first gastric glands were already detected in the stomach. Between 7 and 9 dah, the digestive tract of A. tropicus resembled that of a juvenile specimen with a well-developed and short oesophagus, stomach divided into a glandular and non-glandular (pyloric) stomach, folded intestine with pyloric caeca and a well-developed spiral valve (posterior intestine). Considering this, larvae of A. tropicus are capable of digesting several foods from yolk absorption (3 dah), maximizing its activities at 15 dah, age at which the organisms maximize its capability to absorb nutrients from diets provided.

The physiological transition to aerial breathing in larval air-breathing fishes is poorly understood. We investigated gill ventilation frequency (f), heart rate (f), and air breathing frequency (f) as a function of development, activity, hypoxia, and temperature in embryos/larvae from day (D) 2.5 to D30 posthatch of the tropical gar, Atractosteus tropicus, an obligate air breather. Gill ventilation at 28°C began at approximately D2, peaking at ∼75 beats/min on D5, before declining to ∼55 beats/min at D30. Heart beat began ∼36-48 h postfertilization and ∼1 day before hatching. f peaked between D3 and D10 at ∼140 beats/min, remaining at this level through D30. Air breathing started very early at D2.5 to D3.5 at 1-2 breaths/h, increasing to ∼30 breaths/h at D15 and D30. Forced activity at all stages resulted in a rapid but brief increase in both f and f, (but not f), indicating that even in these early larval stages, reflex control existed over both ventilation and circulation prior to its increasing importance in older fishes. Acute progressive hypoxia increased f in D2.5-D10 larvae, but decreased f in older larvae (≥D15), possibly to prevent branchial O loss into surrounding water. Temperature sensitivity of f and f measured at 20°C, 25°C, 28°C and 38°C was largely independent of development, with a Q between 20°C and 38°C of ∼2.4 and ∼1.5 for f and f, respectively. The rapid onset of air breathing, coupled with both respiratory and cardiovascular reflexes as early as D2.5, indicates that larval A. tropicus develops "in the fast lane."

The jumbo squid Dosidicus gigas is the only ommastrephid commercially caught in Mexico. Despite the economic and ecological importance of this species, little is known about its early life stages. The relationship between the presence of paralarvae and mesoscale oceanic features was investigated for the first time in the central Gulf of California, Mexico in February, April, June and September of 2008. A total of 86 paralarvae were found only in June and September (summer season), in the well-stratified column water where the thermocline was evident and warm sea surface waters (27.7° to 29.4°C) dominated. The greatest abundance of D. gigas paralarvae was observed within 2.23 to 3.48 km of the main front. The mantle length of the smallest paralarvae corresponded with the mantle length at hatching. The San Pedro Mártir Island–Santa Rosalia transect and Santa Rosalia–Guaymas transect were determined as the main hatching localities in June and September respectively. The number of paralarvae found in this study contrast with the potential fecundity of mature females which are found throughout the year.

Abstract. The rhynchoteuthion stage of the jumbo squid, Dosidicus gigas, has morphological characteristics similar to the paralarvae of the purpleback squid, Sthenoteuthis oualaniensis, making it difficult to determine the locations, seasons, and conditions where spawning of jumbo squid occurs. In this study, 180 paralarvae of D. gigas were collected off the west coast of the Baja California peninsula and identified by sequencing a 369‐bp fragment of the cytochrome oxidase I gene. Of these, 77 specimens, 0.8–6.0 mm in mantle length, were described. Indices based on morphometric ratios were used to determine whether the shapes of different body structures were reliable for identifying the paralarvae. For two ratios, the results appear to discriminate members of D. gigas from those of S. oualaniensis. Additionally, eye or intestinal photophores were not observed in any paralarvae of D. gigas. Morphological and morphometric information provided a valuable basis for the discrimination and identification of these two species.

Tropical gar, Atractosteus tropicus, is a carnivorous fish species from Southern México with high value and acceptance in local markets. Therefore, the present study aims to spare proteins in diets for larviculture of this species. An experiment was performed using three different experimental diets with increasing carbohydrate levels based on potato starch (S) and decreasing protein content (P) and total energy: 16% S (16% S-44% P), 22% S (22% S-40% P) and 28% S (28% S-36% P). Tropical gar larvae (five days post-hatching, 2.1 mm average notochordal length) were assessed for growth, survival, degree of cannibalism, and digestive enzyme activities for 30 days. Highest growth and survival (24%), as well as the lowest cannibalism (33%), was seen in larvae fed the 28% S diet, and these larvae also had the highest lipase, amylase and glucosidase activities (0.28, 0.56 and 0.11 units, respectively). Protease activity (alkaline protease, trypsin, chymotrypsin, and leucine aminopeptidase) was higher in the larvae fed the 22% S diet than in those given the 16% S and 28% S diets. In conclusion, dietary inclusion of potato starch at 28% S enhanced growth, survival and some digestive enzyme activities, and decreased cannibalism in the larval gar. Potato starch could replace dietary protein as a major source of energy for A. tropicus larvae, thereby reducing the cost of diets.

Common snook (Centropomus undecimalis) is a marine species with high aquaculture potential; although its digestive physiology is still unknown and knowledge of that could allow the development of a balanced feed for commercial culture of this fish. The objective of this study was to partially characterize the digestive proteases in C. undecimalis using electrophoretic and biochemical techniques. A total of 50 wild snook juveniles were used to determine the optimal values of pH stability and temperature as well as the effect of inhibitors on digestive, gastric and intestinal proteases. The optimal pH for gastric proteases was obtained to be 2 with stability obtained between 2 and 8; the optimal temperature was detected at 75ºC for in vitro test, and the thermal stability was between 25 and 45ºC. Intestinal proteases showed two peaks of activity at a pH of 7 and 11; meanwhile, the greatest stability was found between a pH of 4 and 10; the optimal temperature was at 65ºC, and the greatest stability was detected between 35 and 45ºC. Up to 86% of the gastric protease activity was inhibited by pepstatin A; meanwhile, the intestinal proteases TPCK, TLCK, 1-10 Phenanthroline, SBT1, EDTA, PMSF and ovalbumin reduced the activity by 17%, 68%, 85%, 41%, 40.5%, 60% and 59%, respectively.

El diseño de dietas con base en la fisiología digestiva, así como un adecuado esquema de alimentación, per- miten la disminución del canibalismo y el incremento de la supervivencia en la etapa larvaria de peces. Por lo anterior, el objetivo fue evaluar el crecimiento, supervivencia y canibalismo en la larvicultura del pejelagarto (Atractosteus tropicus Gill) bajo cuatro esquemas de alimentación: CT (co-alimentación con nauplio de Artemia por 10 días y dieta comercial de trucha), T1 (alimentación con la dieta comercial de trucha desde la apertura de la boca), T2 (co-alimentación con nau- plio de Artemiay dieta diseñada para larvas de A. tropicus por 10 días) y T3 (alimentación con la dieta diseñada para A. tropicus desde la apertura de la boca), así como determinar la correlación entre el canibalismo y la supervivencia mediante un modelo de regresión lineal. Los mejores crecimientos y supervivencias se obtuvieron en los tratamientos T2 y CT. En todos los tratamientos se presentó canibalismo, siendo más alto cuando no se co-alimentó con nauplio de Artemia (T1 y T3). Se encontró una fuerte correlación (r = -0.88, p < 0.05) entre la supervivencia y el canibalismo, lo que demuestra que el canibalismo tiene efecto sobre la supervivencia. El tratamiento T2 representa la mejor alternativa para la larvicultura de A. tropicus, con mayor crecimiento y menor canibalismo.