Nekton

contrast - nekton can make headway against the currents have developed powers of locomotion for swimming. - however they can be very closely related

many nektonic organisms have planktonic eggs and larval stages - its apart of their life style

pelagic eggs - they tend to be smaller eggs -transparent and all practically spherical -contains yolk which maybe heterogeneous or segmented - positively bouyant due to content

1. the big bit is the yolk and the small bit is the embyo2.the next two show division - every couple of minutes3. produces a glastular = undifferentiated cells4. starts to bend inwards - gastrulation = early digestive system5. favouring development around the head and eyes6. muscles block start to develop and it starts to look like a little fish

a fish egg is a perfect meal = a lot of predators - a lot of protein and lipids - cannibalism within the sea

spawn as plankton or laid on the benthic the benthic are more devloped due to a bigger egg = more yolk

development is punctuated by series of events and transitions either - gradual and remarkable - abrupt and dynamicthere is no clear distinction between stages its a slow morph = but there are key characteristics that dictic the stage such as guts and yolk gone

LEPTOCEPHALUS - larvae form of eel like organisms -largest one of = emphisis on eye development there is very little happening with the body but it starts out flat and the coils in to produce that identifying shape = helps with swimming transparences

flatfish -wide range of sizes and shapes which is due to the fin raystay in the plankton for a very long time

these have developed exaggerations to make themselfs look bigger for defense against predators and to keep them selves up in the water column as it increase drag.

carapid fish has fleshy tabs on the dorsal fin - very good for sensory function elaborted and elongated fish this is used to increase surface area = increased dragg also it is good for predator defense as elongated bits are more detectable then the head so the individual is more likely to survive

spines they are mainly for defense however they are lost during metamorphism which indicates that they are a storage to help skeleton ossification as they are made out of calcium carbonate.

Melanophores each species will have a specific pigment they are used to-hide from predators -conceal the gut and swim bladder- protection from ultraviolet rays- also facilitates intraspecific recognition the gut can never be transparent

Gut adaptations can be slender long instines can be exturnal - allowing large prey to be eaten

extra adaptations such as extending eyes can move around so able to see all around them also used to make predators go for the eyes instead of the head

Argentinoid smelts - anal and dorsal fins develop within a fin fold some distance away from tbody - fins connect at metamorphism this could be to enhance + locomotion and stalked eyes also for predator defense

scale development this generally commences at the end of the larval stage squamation - being scaly can happen early but mostly individuals without scales like swordfishes

Clupeiform larvae elongated bodies, long striaght intestines and relative uniformity of morphology pigment along the gut - camouflage strategy also intraspecific for schooling fin migration - dorsal and anal migrate anterior- wise during during development. function unknown

Patchiness -schooling of adults -diurnal periodicity in spawning -local sites of high mortality - ocean events - larval aggregation behaviour

survival depends on maternal input egg size - size of larvea at first feed -avaliable food at time of first feed larger eggs have more time to find food small - 1 to 2 days large - 6 days as embryo ages it become more restintant to starvation

visual feeders = confined to daylight hours to be seen prey must be relatively narrow mainly to do with luck stratergy

prey -nauplier through copepod adult stages - euryphagous = will eat anything this is typical for early stages - tintinids, phytoplankton, mollusc larvae and copepods larvae eat smaller larvae many fish are cannibalistic tendencies

At start of pelagic period they are morphologically & behaviourally poorly developed; they are small, lack fins other than fin folds; have incompletely developed sensory organs & they swim poorly, probably in a viscous environment. In this period they approach the true planktonic condition. At end of pelagic period they are morphologically & behaviourally well developed; they are 1 or more cm in length; they have all or nearly all their fins; they have functional sensory organs that can detect predators & settlement habitats (where applicable), and they swim well, certainly in an inertial environment. In this period the larvae graduate to the nekton. Real challenge is accurately describing the phase of gradualtransition between the plankton & nekton.

cephalopods little is known about thier biology and ecology develop is unique - true larval stages are absent they are paralarvae - newly hatched cephalopds found in near shore surface waters during day and night - distinct distribution from adult conspecifics that commonly indergo vertical diel migrations

cephalopod eggs vary greatly in size among species - pelagic or benthic hatchlings from pelagic eggs are always planktonic benthic could be planktonic or benthic they have internal yolk reserve - no exturnal sac organs -hatching glands -posterior spines - tufts of spines -banded cilia these help in leaving the egg - many lost then

Cranchiidae young squids often have eyes with long stalks. these and lobes extend lateral to the head - connect to brain by long optic nerve - can be seen

Crustaceans krill swarm to confuse predators - 1000 krill per m thier visions has evolved so that they can see 360 and so never crash into the each other

Resource summary

	Created by chloe allen over 7 years ago