Faunal mortality associated with massive beaching and decomposition of pelagic Sargassum.

In 2018, the Mexican Caribbean coast received a massive influx of pelagic Sargassum spp. that accumulated and decayed on beaches producing organic decomposition products that made the water turbid and brown. Between May and September of the same year there were several reports of mass mortality of marine biota in this area. From these reports we estimate that organisms belonging to 78 faunal species died as result of this event, with demersal neritic fish and Crustacea being the most affected groups. The cause of mortality appears to be the combined effect of high ammonium and hydrogen sulfide concentrations, together with hypoxic conditions. If massive arrival of pelagic Sargassum spp. continues and algae is left to decay on the beach in large volumes then deterioration in water quality could affect coral reefs close to shore. Furthermore, barriers placed in lagoons to intercept the Sargassum spp. before it reaches the beach could impact reef fauna if the algae is left to die and sink on site.

Identifying pelagic fish eggs in the southeast Yucatan Peninsula using DNA barcodes.

In the waters surrounding Banco Chinchorro in the Mexican Caribbean are spawning and nursery areas for many types of fish. In this natural environment, as opposed to under controlled laboratory conditions, it is almost impossible to link an individual egg to the adult that laid it. This makes identifying the species of the eggs difficult. However, DNA barcodes have made this easier. In the present study, 300 eggs were processed for molecular analysis, from which 139 sequences were obtained. We identified 42 taxa (33 species with their binomial names), 35 genera, and 24 families. The identified eggs included those from Ariomma melanum, which is the first recording of this species in the Mexican Caribbean. Eggs from economically important fish species were also identified, including frigate tuna (Auxis thazard), crevalle jack (Caranx hippos), common dolphinfish (Coryphaena hippurus), sailfish (Istiophorus platypterus), white marlin (Kajikia albida), skipjack tuna (Katsuwonus pelamis), blackfin tuna (Thunnus atlanticus), and swordfish (Xiphias gladius). We have also described new morphological characteristics and captured photographs for 21 species, as well as obtained new information about spawning locality and time for 16 species. This valuable information will provide the basis to develop more effective conservation measures for sustainable fisheries and protection of the Mesoamerican Barrier Reef System.

Asociaciones de larvas de peces en una bahía del Caribe mexicano / Fish larvae association in a Mexican Caribbean bay

Se analizaron las variaciones interanuales del ictioplancton, en la Bahía de la Ascensión, Quintana Roo durante diciembre de 1994 a 1997. Fueron identificadas 32 familias, 35 géneros y 21 especies de peces. Las familias dominantes fueron la Gobiidae, Callionymidae, Clupeidae y Tetraodontidae. La diversidad larval fue baja comparada con la registrada en otras épocas del año (secas y lluvias). En diciembre de 1994 y 1995 se observaron tres asociaciones espaciales (interna, media y externa). En diciembre de 1996 y 1997 las asociaciones se conformaron por la mezcla de estaciones de la parte interna y externa de la bahía. Las familias dominantes caracterizaron a la mayoría de las asociaciones faunisticas. La densidad de huevos de peces fue más alta en la zona externa de la bahía, mientras que las larvas fueron más abundantes en la zona interna. Los principales factores que afectan a las asociaciones de larvas de peces durante diciembre (1994-1997) en la bahía de la Ascensión parecen estar relacionados a la localización de áreas de crianza, zonas de desove y al periodo y estrategia reproductiva de peces estuarinos y arrecifales.

[Fish larvae association in a Mexican Caribbean bay]. / Asociaciones de larvas de peces en una bahía del Caribe mexicano.

Interannual ichthyoplankton variation, was analyzed in Bahía de la Ascensión, Mexico, during December of four consecutive years (1994-1997). A total of 32 families, 35 genera and 21 species of fish larvae were identified. The most abundant fish larvae were the Gobiidae followed by the Callionymidae, Clupeidae and Tetraodontidae. Larval diversity was low when compared with other periods ("dry" and "rainy"). Three spatial associations (internal, medium and external) were found in December 1994 and 1995. In 1996-1997, stations of the inner and outer parts of the bay were mixed. The dominant families characterized most of the faunal associations. Egg density was highest in the external zone of the bay, whereas larvae were most abundant in the inner area. Major factors affecting the fish larval assemblages during December (1994-1997) in Bahía de la Ascensión seem to be related to the nursery areas location and to the tropical fishes reproductive period.

Electrostatic forces as a possible mechanism underlying skeletal muscle contraction.

A possible mechanism is put forward to explain the sliding of thin filaments during muscle contraction. In our model, repulsion due to electrostatic forces is the mechanism which triggers crossbridges to cause the thin filaments to slide. The mechanism proposed could operate regardless of whether the myosin heads rotate or bend, although recent experimental evidence seems to confirm the latter action. In spite of its simplicity, the model prediction of the velocity of sliding of the thin filaments agrees well with experimental values from in vitro motility assays.