Thorson's rule states that benthic marine invertebrates at low latitudes tend to produce large numbers of eggs developing to widely dispersing pelagic larvae, whereas at high latitudes such organisms tend to produce fewer and larger lecithotrophic ( yolk-feeding ) eggs and larger offspring.
12.
The length of the veliger stage in the natural environment is unknown and undoubtedly variable; however, in the lab, veligers of some species become competent to metamorphose in anywhere from a few days ( lecithotrophic larvae ) to a month or more after hatching ( planktotrophic larvae ).
13.
Many members of the family Poeciliidae are considered to be lecithotrophic ( the mother provisions the oocyte with all the resources it needs prior to fertilization, so the egg is independent of the mother ), but others are matrotrophic ( literally " mother feeding " : the mother provides the majority of resources to the developing offspring after fertilization ).
14.
In all but special cases of both ovuliparity and oviparity the overwhelming source of nourishment for the embryo is the yolk material deposited in the egg by the reproductive system of the mother; offspring that depend on yolk in this manner are said to be "'lecithotrophic "', which literally means " feeding on yolk ".
15.
Thorson's rule states that benthic marine invertebrates at low latitudes tend to produce large numbers of eggs developing to pelagic ( often planktotrophic [ plankton-feeding ] ) and widely dispersing larvae, whereas at high latitudes such organisms tend to produce fewer and larger lecithotrophic ( yolk-feeding ) eggs and larger offspring, often by viviparity or ovoviviparity, which are often brooded.
16.
However, their modes of maternal provisioning vary greatly . " P . monacha " can be considered to be lecithotrophic because it does not really provide any resources for its offspring after fertilization-the pregnant female is basically a swimming egg sac . " P . lucida " shows an intermediate level of matrotrophy, meaning that to a certain extent the offspring's metabolism can actually affect the mother's metabolism, allowing for increased nutrient exchange . " P . prolifica " is considered to be highly matrotrophic, and almost all of the nutrients and materials needed for fetal development are supplied to the oocyte after it has been fertilized.
