colour, camouflage & bioluminescence Flashcards
2 kinds of colour in fish
- Biochromes – from pigments
- Schematochromes (structural colours) – from reflection
what are Chromatophores
cells responsible for colour
- Characterised by irregular shapes with branched processes
- Found mainly in skin - Occasionally subdermally
what is Chromatosome
many chromatophores intertwined or overlaid
2 types of colour change
Physiological colour change
Morphological colour change
what is Physiological colour change
- Short-term by movement of pigment within chromatophore
- Response to background colour
- Response to social, behavioural or chemical stimuli
- Nervous or hormonal control (hormonal control = very slow, nervous = fast)
what is Pigment aggregated out through cell body
visible pigment
what is Pigment aggregated in centre of cell
underlying colour visible only, not pigment
what is Morphological colour change
- Long-term due to change in number or type of chromatophores
- Life-history stages e.g. juvinilles to adults, female to male
- Migration e.g. salmon are silver when living in sea, have bars (verticle stripes) when in fresh shallow water with more background – more camouflage
4 Functions of colouration
Protection of CNS from UV in larvae
Aid to thermoregulation
Optical filter
Main function: intra- and inter-specific signalling
Intra-specific signalling
(within species)
1. Advertising and a need to be seen
2. Social signals:
-Recognition of individuals in schools, or juvenile/adult recognition
-Threat or warning
-Sexual signalling
e.g. this cichlid is orange when brooding, young of this species show a preference for orange
what are sexual signals in Intra-specific signalling
- Identification of opposite sex in dimorphic spp.
- Attraction of one for another
- Good sexual signal for health – pigment = energetically costly – means you have enough energy to do so
what is Inter-specific signalling
(between species)
Concealment and disguise e.g:
- background matching
- countershading
- obliterative colouration
- disruptive colouration
what is background matching
- General colouration of fish resembles that of the environment
- Usually morphological
e.g. fish on substratum = brown, fish in weed/rocks = green/brown, pelagic fish = silver
explain background matching in Demersal fish
- Some fish can vary colour by physiological colour change to match background more fully
- Demersal colouration = ability for fish to blend into seabed, e.g. flatfish
- May be aided by skin flaps or irregular outline to break up recognisable shape
what is Vegetal colouration and some examples
Imitates vegetation in the water, often associated with shape
- hairy frogfish - same lifestyle as anglerfish, showing extensions and projections and colouration to blend in with seabed and surrounding soft corals
- Pygmy seahorse – good at physiological colour change to blend in
- Transparency = good method (but difficult) - common in planktonic larval fishes – organs tend to be near head
what is Countershading
- Distibution of light in sea = primarily from above
- Light from above will lighten dorsal surface of fish & gradually lessen as you go down sides of fish, casting the lower surface into shadow
- This provides 3D information to eyes
- Countershading darkens the dorsal surface and becomes progressively lighter as the intensity of the shadow increases
- This is coupled with colouration to match background, e.g. demersal fish are brown
- Fish away from the seabed = greyish e.g. hake
- Fish near the surface = blueish e.g. dolphin fish
- Silhouette can still be seen by anything directly beneath you, despite countershading
- So a round fish that is pelagic will be seen from below and is likely to be a large solitary predator e.g. sharks
- Small, pelagic, shoaling species (herring) tend to be laterally compressed – reduces their arc of visibility
what is obliterative colouration
- Refers to silvering in colour
- Many fish in open ocean will combine lateral compression with this
- Eyes assume light travels in a straight line and there is nothing there, even if it has reflected off something else first – works well in open oceans, not in more complex environments where there are more objects e.g. seaweed
what 2 layers has silvering araised from
- Stratum Argenteum : thick layer of guanine crystals subdermally along flanks, parallel to body surface (if random would be white)
- Iridophores in skin : non-motile guanine crystals act as iridescent plates - iridophores are oriented vertically and act like thousands of tiny mirrors
what is disruptive colouration
- Not so much to avoid being seen as to avoid being identified as potential prey
- Bold patterns, stripes and bars to break up outline of fish
- Often associated with non-fishlike body shape
- often also used to camouflage / draw attention away from important body parts e.g. eyespots placed on other end of body to draw animals towards the mouth
- Sometimes the patterns just used for total confusion…Is it a fish? Which bit do I attack?
- Advertising + mimicry often use bright colours and bold patterns to accompany unpalatability or venom e.g. lion fish
2 main types of mimicry
Müllerian mimicry
Batesian mimicry
what is Müllerian mimicry
Toxic/unpalatable animals that use similar colour and patterns to help teach potential predators to avoid them
what is Batesian mimicry
- Non-toxic animals use similar colour and pattern to imitate toxic animals to gain some protection from predators
- Important that these mimics are less abundant than their models otherwise warning becomes obsolete
3 sub-types of mimicry
facultative (or dynamic) mimicry
aggressive mimicry
protective mimicry
explain facultative (or dynamic) mimicry and example
ability to switch on mimic colouration at will, or change appearance to mimic a variety of species
e.g. blue striped fangblenny = The only animal other than the mimic octopus known to show this
