General descriptions of the dermis structure of a juvenile whale shark Rhincodon typus from the Gulf of California.

The aim of this study is to provide preliminary observations on the microanatomy of Rhincodon typus skin using histology and electron microscopy analyses. Skin biopsies were obtained from a deceased juvenile male shark (548 cm total length) stranded in La Paz, Mexico, during February 2018. The results of this study evidenced the basic structure of the dermal denticles in the epidermis of the trunk of the shark, as well as the composition of the connective tissue in the hypodermis. Histological images of the hypodermis showed a high concentration of collagen fibres, formed by a large number of fine and wavy fibres of compact shape and little intercellular substance.

First record of the nematode Echinocephalus pseudouncinatus (Gnathostomatidae, Spirurida) in an edible, commercial host, the pen shell Atrina maura (Bivalvia: Pinnidae).

We describe a nematode infestation of the Atrina maura, the most valuable pen shell fished in Mexico, for the first time. Observations by Mexican authorities indicated that parasites were found encysted in pen shell gonads. The aims of this study included: (i) to identify the parasite morphologically; (ii) to quantify the infestation in different tissues of the pen shell; and (iii) to establish the seasonal variation in gonadal development. During 2015, 10 to 40 pen shells were randomly collected each month from the same commercial bed, and gonads were preserved and processed histologically to establish the seasonal and monthly frequency of infested gonads. A subsample of 35 pen shells was used to identify the parasite and to characterize the infestation of the muscle, mantle, gonad, and digestive gland tissue components. All tissues were dissected, and parasites were collected, photographed, quantified, and adequately preserved for species identification using scanning electron microscopy. Prevalence, intensity of infestation, abundance and crowding data were analyzed by Quantitative Parasitology software and compared monthly and seasonally using a chi test and analysis of means. Results indicated that all parasites were juvenile (second stage) nematodes, Echinocephalus pseudouncinatus. Parasites were found only in muscle and gonad tissues in both male and female pen shells. Hyperpigmentation was observed in the muscle and atretic oocytes with phagocytosis in the gonads. The frequency of infested gonads was significantly higher during winter but was not related to shell size or sex (p > 0.05). A greater number of pen shells were observed to be infested during December and August and during pen shell spawning and development stages. The highest frequency of infested gonads (40%) occurred in December when the lowest temperature and chlorophyll a concentration were recorded. The effects of the biotic and abiotic parameters on this host-pathogen relationship need further research. This nematode species is related to species that are causal agents of gnathostomiasis, which may constitute a health hazard for raw muscle consumers and could affect the reproductive activity of pen shell.

Are Prorocentrum hoffmannianum and Prorocentrum belizeanum (DINOPHYCEAE, PROROCENTRALES), the same species? An integration of morphological and molecular data.

The taxonomic assignment of Prorocentrum species is based on morphological characteristics; however, morphological variability has been found for several taxa isolated from different geographical regions. In this study, we evaluated species boundaries of Prorocentrum hoffmannianum and Prorocentrum belizeanum based on morphological and molecular data. A detailed morphological analysis was done, concentrating on the periflagellar architecture. Molecular analyses were performed on partial Small Sub-Unit (SSU) rDNA, partial Large Sub-Unit (LSU) rDNA, complete Internal Transcribed Spacer Regions (ITS1-5.8S-ITS2), and partial cytochrome b (cob) sequences. We concatenated the SSU-ITS-LSU fragments and constructed a phylogenetic tree using Bayesian Inference (BI) and maximum likelihood (ML) methods. Morphological analyses indicated that the main characters, such as cell size and number of depressions per valve, normally used to distinguish P. hoffmannianum from P. belizeanum, overlapped. No clear differences were found in the periflagellar area architecture. Prorocentrum hoffmannianum and P. belizeanum were a highly supported monophyletic clade separated into three subclades, which broadly corresponded to the sample collection regions. Subtle morphological overlaps found in cell shape, size, and ornamentation lead us to conclude that P. hoffmanianum and P. belizeanum might be considered conspecific. The molecular data analyses did not separate P. hoffmannianum and P. belizeanum into two morphospecies, and thus, we considered them to be the P. hoffmannianum species complex because their clades are separated by their geographic origin. These geographic and genetically distinct clades could be referred to as ribotypes: (A) Belize, (B) Florida-Cuba, (C1) India, and (C2) Australia.