Histotrophy is a form of matrotrophy exhibited by some live-bearing

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s and rays, in which the developing

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

receives additional nutrition from its mother in the form of uterine secretions, known as histotroph (or "uterine milk"). It is one of the three major modes of

elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks ( division Selachii), and batomorphs (division Batomorphi, including rays, skates, and sawfish). Members of this subclass are characterised by h ...

reproduction encompassed by " aplacental viviparity", and can be contrasted with yolk-sac viviparity (in which the embryo is solely sustained by

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...

) and

oophagy Oophagy ( ) or ovophagy, literally "egg eating", is the practice of embryos feeding on eggs produced by the ovary while still inside the mother's uterus. The word oophagy is formed from the classical Greek (, "egg") and classical Greek (, "to ...

(in which the embryo feeds on ova). There are two categories of histotrophy: *In mucoid or limited histotrophy, the developing embryo ingests uterine

mucus Mucus (, ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both Serous fluid, serous and muc ...

or histotroph as a supplement to the energy supplies provided by its

yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...

. This form of histotrophy is known to occur in the dogfish sharks (Squaliformes) and the

electric ray The electric rays are a group of rays, flattened cartilaginous fish with enlarged pectoral fins, composing the order Torpediniformes . They are known for being capable of producing an electric discharge, ranging from 8 to 220 volts, depending ...

s (Torpediniformes), and may be more widespread. *In lipid histotrophy, the developing embryo is supplied with

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

and

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

-enriched histotroph through specialized finger-like structures known as trophonemata. The additional nutrition provided by the enriched histotroph allows the embryo to increase in mass from the egg by several

orders of magnitude In a ratio scale based on powers of ten, the order of magnitude is a measure of the nearness of two figures. Two numbers are "within an order of magnitude" of each other if their ratio is between 1/10 and 10. In other words, the two numbers are wi ...

by the time it is born, much greater than is possible in mucoid histotrophy. This form of histotrophy is found in stingrays and their relatives (Myliobatiformes).

References

* * {{cite book , title=Reproductive Biology and Phylogeny of Chondrichthyes: Sharks, Batoids and Chimaeras , editor=Hamlett, W.C. , publisher=Science Publishers , year=2005, isbn=1-57808-314-1 , pages=46–47 Embryology Ichthyology