Brain and sense organ anatomy and histology of the Falkland Islands mullet, Eleginops maclovinus (Eleginopidae), the sister group of the Antarctic notothenioid fishes (Perciformes: Notothenioidei)

Abstract

The perciform notothenioid fish Eleginops maclovinus, representing the monotypic family Eleginopidae, has a non-Antarctic distribution in the Falkland Islands and southern South America. It is the sister group of the five families and 103 species of Antarctic notothenioids that dominate the cold shelf waters of Antarctica. Eleginops is the ideal subject for documenting the ancestral morphology of nervous and sensory systems that have not had historical exposure to the unusual Antarctic thermal and light regimes, and for comparing these systems with those of the phyletically derived Antarctic species. We present a detailed description of the brain and cranial nerves of Eleginops and ask how does the neural and sensory morphology of this non-Antarctic notothenioid differ from that seen in the phyletically derived Antarctic notothenioids? The brain of Eleginops is similar to those of visually oriented temperate and tropical perciforms. The tectum is smaller but it has well-developed olfactory and mechanoreceptive lateral line areas and a large, caudally projecting corpus cerebellum. Eye diameter is about twofold smaller in Eleginops than in many Antarctic species. Eleginops has a duplex (rod and cone) retina with single and occasional twin cones conspicuous centrally. Ocular vascular structures include a large choroid rete mirabile and a small lentiform body; a falciform process and hyaloid arteries are absent. The olfactory rosette is oval with 50-55 lamellae, a large number for notothenioids. The inconspicuous bony canals of the cephalic lateral line system are simple with membranous secondary branches that lack neuromasts. In Antarctic species, the corpus cerebellum is the most variable brain region, ranging in size from large and caudally projecting to small and round. "Stalked" brains showing reduction in the size of the telencephalon, tectum, and corpus cerebellum are present in the deep-living artedidraconid Dolloidraco longedorsalis and in most of the deep-living members of the Bathydraconini. Eye diameter is generally larger in Antarctic species but there is a phylogenetic loss of cellularity in the retina, including cone photoreceptors. Some deep-living Antarctic species have lost most of their cones. Mechanosensation is expanded in some species, most notably the nototheniid Pleuragramma antarcticum, the artedidraconid genera Dolloidraco and Pogonophryne, and the deep living members of the bathydraconid tribe Bathydraconini. Reduction in retinal cellularity, expansion of mechanoreception, and stalking are the most noteworthy departures from the morphology seen in Eleginops. These features reflect a modest depth or deep-sea effect, and they are not uniquely "Antarctic" attributes. Thus, at the level of organ system morphology, perciform brain and sensory systems are suitable for conditions on the Antarctic shelf, with only minor alterations in structure in directions exhibited by other fish groups inhabiting deep water. Notothenioids retain a relative balance among their array of senses that reflects their ...

Divergence of brain and retinal anatomy and histology in pelagic antarctic notothenioid fishes of the sister taxa Dissostichus and Pleuragramma

“…Although notothenioid eyes are devoid of the extreme specializations seen in primary deep‐sea species, the retina of Dissostichu s bears some resemblance to scotopic retinae of these species in having a high degree of summation and good sensitivity (Munk,1984). The retina of Pleuragramma , on the other hand, is remarkably similar to the photopic retina of the non‐Antarctic Eleginops (Eastman and Lannoo,2008) with a large number of total cells, including many rods (Table 2). Furthermore, the single cones are the same size, although Pleuragramma lacks the twin cones seen in Eleginops .…”

“…With the exception of the weights, information for the brain morphology of Eleginops maclovinus (Fig. 3), the sister group, is from Eastman and Lannoo (2008).…”

“… ( A ) Lateral and ( B ) dorsal views of the brain of Eleginops maclovinus (SL = 34.5 cm). From Eastman and Lannoo (2008). To illustrate features of the rhombencephalon in dorsal view, the drawing was made from a slightly oblique dorsocaudal angle.…”

“…As a means of apportioning the influence of phylogentic and ecological factors shaping brain morphology, Kotrschal et al (1998) call for investigations of closely related perciform species that differ in body size, habitat utilization and behavior. Having established the large‐scale pattern of neural and sensory morphology for families and genera of Antarctic perciform notothenioids (Eastman and Lannoo,1995,2003a,b,2004,2007,2008; Lannoo and Eastman,2000), we are in a position to address this topic. Using the ecologically vital but neuroanatomically unstudied genera Dissostichus and Pleuragramma , we will provide such a comparison and analysis of the brains and retinae of sympatric sister species of notothenioids that, while sharing the pelagic lifestyle, have diverged in body size, diet, habitat depth, activity levels and tendency to migrate.…”

The brain of Brycon orbignyanus (Valenciennes, 1850) (Teleostei: Characiformes: Bryconidae): gross morphology and phylogenetic considerations

“…When the described procedure is complete, the brain should be completely free from the neurocranium. Meek & Nieuwenhuys (1998) and Striedter (2005), with a single modification: fish brains descriptions usually follow the posteroanterior direction (e.g., Meek & Nieuwenhuys, 1998), but herein we chose to follow Striedter (2005) in adopting the anteroposterior direction, the most commonly used for vertebrates as a whole (see Bauchot et al, 1989;Striedter, 2005;Eastman & Lannoo, 2007, 2008 for fishes; ten Donkelaar, 1998a for amphibians; ten Donkelaar, 1998b for reptiles and Walsh & Milner, 2011 for avians). For illustration and description purposes, the brains were divided into telencephalon, mesencephalon, diencephalon, rhombencephalon and medulla spinalis (Fig.…”