DOI: 10.5061/dryad.15dv41nvt
title: A brain and a head for a different habitat: size variation in four morphs of Arctic charr (Salvelinus alpinus (L.)) in a deep oligotrophic lake

description: Adaptive radiation is the diversification of species to different
 ecological niches and has repeatedly occurred in different salmonid fish
 of postglacial lakes. In Lake Tinnsjøen, one of the largest and deepest
 lakes in Norway, the salmonid fish, Arctic charr (Salvelinus alpinus
 (L.)), has likely radiated within 9700 years after deglaciation into
 ecologically and genetically segregated Piscivore, Planktivore, Dwarf and
 Abyssal morphs in the pelagial, littoral, shallow-moderate profundal and
 deep-profundal habitats. We compared trait variation in the head shape,
 the eye and olfactory organs, as well as the volumes of five brain regions
 of these four Arctic charr morphs. We hypothesised that specific habitat
 characteristics have promoted divergent body, head and brain sizes related
 to utilized depth differing in environmental constraints (e.g. light,
 oxygen, pressure, temperature and food quality). The most important
 ecomorphological variables differentiating morphs were body length,
 habitat, optic tectum and eye area. The Abyssal morph living in the
 deepest areas of the lake had the smallest brain region volumes, head and
 eye size. Comparing the olfactory bulb with the optic tectum in size, it
 was larger in the Abyssal morph than in the Piscivore morph. The Piscivore
 and Planktivore morphs that use more illuminated habitats have the largest
 optic tectum volume, followed by the Dwarf. The observed differences in
 body size and sensory capacities in terms of vision and olfaction in
 shallow and deep-water morphs likely relates to foraging and mating
 habitats in Lake Tinnsjøen. Further seasonal and experimental studies of
 brain volume in polymorphic species are needed to test the role of
 plasticity and adaptive evolution behind the observed differences.
description: We sampled fish using
 gillnets, traps and baited anchored longlines in August-October 2013
 (Østbye et al., 2020). We sampled in four habitats; (i) the pelagial
 (setting gillnets positioned more than 50 m from shore and 20-30 m depth
 in midwater using a 12-panel multimesh Nordic series with mesh sizes  in
 this order of 43, 19.5, 10.0, 55.0, 12.5, 24, 15.5, 35.0, 29.0, 6.3, 5.0
 and 10.0 mm and Jensen floating series with mesh size of 13.5, 16.5, 19.5,
 22.5, 26.0, 29.0, 35.0, 39.0, 45.0 and 52.0 mm), (ii) the littoral
 (gillnets within 20 m from the shore using Nordic and Jensen littoral net
 series), (iii) the shallow-moderate profundal (Jensen littoral net series,
 traps and hook-line between 20-150 m depth), and (iv) the deep profundal
 (setting traps &gt; 150 m depth and &gt; 100 m from the shoreline
 using longlines of 220m long and 3 to 4 mm line with 180 hooks; see more
 detailed information in Østbye et al., (2020)). In the field, we assigned
 each individual to one of the four morphs (called field-assigned morphs:
 FA morphs) based on differences in body and head appearance and
 coloration. We also measured body length, and determined the sex and
 maturation stage visually (i.e., mature if the gonads covered more than
 half of the body cavity length; immature otherwise). We euthanized the
 fish with an overdose of benzocaine and we preserved the heads in formalin
 (10% unbuffered). We had 72 individuals with both
 genetic and morphological data for each individual. We digitized a set of
 30 common anatomical landmarks in tpsDIG2 v.2.22 (Rohlf, 2015) to capture
 head variation, which we included for landmark-based geometric
 morphometrics and statistical analyses. In addition, we measured the width
 (W) and height (H) of the eye in tpsDIG2 to calculate the eye area. We
 standardized for size with a Generalized Procrustes Analysis (Adams,
 Rohlf, & Slice, 2004; Zelditch, Swiderski, Sheets, & Fink,
 2004) and we calculated the centroid size (i.e. as a measure of size) for
 each individual to use in further analyses. We determined the age based on
 otoliths, which are more reliable than scales especially in Arctic charr
 (Christensen, 1964). Following Pollen et al., (2007), we measured five
 brain regions: olfactory bulb, telencephalon, optic tectum, cerebellum,
 and hypothalamus. We measured the width (W) of each brain structure from
 the dorsal and ventral image of the brain, as well as the length (L) and
 height (H) from lateral views of the left hemisphere. We used an ellipsoid
 model to estimate the volume (V = 1/6 π(LWH)) of each brain region (Huber
 et al., 1997). We dissected the olfactory rosettes and the nasal organ and
 stored them in 70% ethanol. We measured the width (W) and length (L) using
 a micrometer under the microscope in order to calculate the surface area
 of each olfactory rosette (A= 1/4 π(WL)). We also counted the number of
 olfactory lamellae in each rosette.
description: We counted (N1) and measured the olfactory organs (1) and
 measured five brain regions (olfactory bulb (2), telencephalon (3), optic
 tectum (4), cerebellum (5), and hypothalamus (6). We measured the width
 (W) of each brain structure from the dorsal and ventral image of the
 brain, as well as the length (L) and height (H) from lateral views of the
 left hemisphere. We also measured the the width (D1) and the height (D2)
 of the eye to calculate the eye area. For the head size, we calculated the
 centroid size.