The flatfish Citharichthys spilopterus Günther 1862 is the most common bothid in the coastal lagoons of the Gulf of Mexico. The objective of the present study was to describe the trophic biology of this species in the tropical coastal lagoon of Tampamachoco, Mexico. For the diet analysis, we used multivariate discriminant analysis and trophic niche breadth. The morphological analysis showed that the features of the bucco‐pharyngeal cavity, the large stomach and the short intestine of this flatfish relate to a carnivorous habit. The overall diet showed that this species is a third‐order consumer, feeding mainly on fish (52%, mainly gobies) and crustaceans (36%, mostly decapods). Diet and trophic niche breadth showed no significant differences between sexes (P > 0.1). By contrast, there were significant ontogenetic differences in the diet and trophic niche breadth, where larger C. spilopterus were almost entirely piscivorous and showed the narrowest trophic niche. As the flatfish grew in size there was a trend toward the consumption of larger prey. The importance of copepods and peracarids correlated inversely with flatfish size (P < 0.001), but fish prey correlated directly to flatfish size (P < 0.001). Likewise, there was an inverse significant correlation between niche trophic breadth and flatfish size (P < 0.005). There were also significant seasonal differences in the diet (P < 0.02), related to the availability and vulnerability of prey in the lagoon during the dry and rainy seasons. However, the trophic niche showed no significant differences between seasons. Finally, we discuss the advantages of discriminant analysis applied to evaluate differences among diets of fish groups when compared with other bivariate and multivariate techniques.

The menhadens Brevoortia gunteri and B. patronus are sympatric and morphologically similar. The two species are planktophagous and exhibited significant (P<0·0001) food resource partitioning, with phytoplankton predominating in B. patronus stomachs and zooplankton in B. gunteri. The branchial apparatus of both species is of the typical alosine form. Brevoortia patronus has significantly more branchiospinules per mm (x =19·47) and significantly longer gill rakers (x =13·35 mm) than B. gunteri (x =14·11, x =12·01 mm respectively). B. patronus is characterized by a gill raker system forming a fine-meshed filter capable of retaining smaller food items. The results support the hypothesis that food resource partitioning is related to different morphological features of the branchial apparatus. 1996 The Fisheries Society of the British Isles

The anatomical organization of the reproductive system of the flounder, Bothus robinsi, is similar to other fish in the order Pleuronectiformes. Accentuated, external secondary sexual characters are observed in males such as increased body length, larger left pectoral fin, and greater interorbital distance. As a qualitative character, the rostral spine, which is part of the maxillary bone, is pronounced. The interorbital distance implies noticeable internal sexual differences, such as the broadening of the left frontal and lateral ethmoid bones, and a greater length of the oblique muscles of both eyes, the optic nerves, and the left olfactory nerve. Five male specimens examined had female external secondary sexual characters.

In order to know the morphological organization of the digestive system of the smooth butterfly ray Gymnura micrura, specimens were collected with a trawl net in coastal lagoons and littoral of the Gulf of Mexico. Samples were fixed in formaldehyde and preserved in ethylic alcohol. In the laboratory, the anatomic analysis of the digestive tract components and its glands, together with the comparison with representative species of the Rhinobatidae, Rajidae, Urotrygonidae, and Dasyatidae families, allowed for the recognition that the general organization of the digestive system of G. micrura fits to the general morphological pattern of Batoidea. However, some significant differences were recognized between this species and the remaining rays, such as the slender nature of its jaws, the reduced volume of the mandible musculature, and the welldefined development of two anterior hepatic lobes separated from each other. It is outstanding the number of two posterior hepatic lobes as a shared morphological character with other Myliobatoidei. This species shows sexual dental dimorphism, consisting in a greater size of the dental cuspid in each tooth of males

Anatomía comparada del sistema digestivo de las rayas ABSTRACTThe general organization of the digestive system of the rays Urotrygon chilensis and Dasyatis sabina fits with the general morphological pattern of the Myliobatiformes. Dentition of both species shows sexual dimorphism, having the females flattened teeth whereas pointed teeth the males. Among the mandibular muscles, the adductor mandibulae is the most complex. Both the buccopharyngeal and the visceral cavities are dorsoventrally flattened. The esophagus is long, the stomach is U shaped, and the intestine is formed by the duodene, valvular intestine and rectum. The dorsal mesentery is restricted to the rectum and rectal gland. The main morphological characters differentiating both species are: the presence in U. chilensis of the cardiac sphincter and the S shape of the rectum, and the presence in D. sabina of mandibular notches and a higher number of coils of the valvular fold of the intestine.