Degree of herbivory and intestinal morphology in nine notothenioid fishes from the western Antarctic Peninsula

Abstract

Although many notothenioid fishes are primarily carnivorous, some species consistently feed on macroalgae and are therefore omnivorous. Among fish, the degree of herbivory is usually reflected in the morphology of the gastrointestinal system especially intestine length. We examined a large number of juvenile and adult specimens of nine sympatric notothenioid species collected sequentially over eight summer seasons at Potter Cove, South Shetland Islands. We provide relative intestine lengths (RIL), distinct proportions of algae and animal prey in the diets (W%), and numbers of pyloric caeca for all species. The sister species Notothenia coriiceps (NOC) and N. rossii (NOR) evidenced significantly different intestinal growth over ontogeny and ate distinctly different proportions of algae and animal prey. We establish a ranking of the degree of herbivory for the fish species in the local ecosystem, and this was found to be related to their distinct feeding types and strategies. There is a correspondence between intestine length/RILs and degree of herbivory in six of the nine species analysed but no clear association between the number of pyloric caeca and degree of omnivory or carnivory. Compared to other teleosts, our results, and those in the literature, indicate modest divergence in notothenioids that include a phylogenetic decrease in the number of pyloric caeca, from 6 to 7 in most nototheniids to 2-3 in channichthyids, and a 1.8-fold difference in average relative intestine lengths which are most frequently 50-70% of body length and never exceed body length. This is consistent with the unspecialized gastrointestinal morphology that reflects the dietary and ecological plasticity of many notothenioids, exemplified by the high degree of omnivory in species such as NOC and NOR.

Histology and ultrastructure of the gastrointestinal tract in four temperate marine herbivorous fishes

“…There were small differences between the epithelial cells in the pyloric caeca and anterior GIT of the present study species, but their generally similar structures imply that these two regions have a similar function. The large variation in number of pyloric caeca among study species suggests that the number does not relate to function or specific diet type, consistent with studies on other groups of fishes (e.g., Moreira et al, 2020).…”

Comparative Morphology of the Digestive Tract of African Bush Fish (Ctenopoma acutirostre) and Paradise Fish (Macropodus opercularis) Inhabiting Asian and African Freshwaters

“…Their number is a species-specific trait, varying from a few appendages in fish such as spotted snakehead ( Channa punctata ) to several hundred in Atlantic cod ( Gadus morhua ) [ 26 ]. They are absent in stomachless fish [ 43 , 47 , 48 ]. Previous studies have not shown a correlation between the number of pyloric caeca and the type of food [ 26 ]; however, their numbers are lower in omnivorous and herbivorous species than in carnivorous species [ 49 ] due to the need to digest chemically and structurally more demanding foods.…”

The Axes of Divergence for the Evolutionary Radiation of Notothenioid Fishes in Antarctica

“…The heads of some «cryonotothenioids» display specialized morphology for the acquisition of prey, but the alimentary canal from the esophagus to the rectum is generally similar in its gross morphology and histology in the species studied to date. The interspecific differences include: (1) a phylogenetic decrease in mean number of pyloric caeca from 6-7 in most nototheniids to 2 in channichthyids, although Dacodraco has 4; (2) a maximum ≈1.5-fold difference in average relative stomach lengths; and (3) a maximum ≈2-fold difference in average relative intestine lengths (Ojeda, 1986; Eastman and DeVries, 1997; Eastman 1999; Moreira et al, 2020) [195][196][197][198]. The absence of specialization may be attributable to the opportunistic feeding and dietary plasticity of many species.…”

“…For example, depending on the time of year and location, the alimentary canal of Notothenia coriiceps processes food on a scale ranging from carnivory to omnivory to herbivory (Barrera-Oro and Casaux et al, 1990) [69,153], although perhaps less efficiently that if it was more specialized for only carnivory or herbivory. Hence there is no morphology that reflects intermittent herbivory on kelp, and attempts to correlate diet with stomach length, intestine length, or number of pyloric caeca are inconclusive (Moreira et al, 2020) [198]. There may also be non-dietary influences on gut morphology.…”

“…During benthic browsing, for example, the ingested inorganic detritus occupies space in the intestinal lumen and may reduce the efficiency of digestion and absorption. This could necessitate a longer intestine, with Gobionotothen gibberifrons the exemplar among «cryonotothenioids» (Moreira et al, 2020) [198]. The absence of specialization in the alimentary canal is neither a new observation nor unique to the «cryonotothenioids»-"the ecological situation of fish may often favor a generalized rather than a highly specialized digestive system" (Barrington, 1957, p. 112) [199].…”