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

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s and rays, in which the developing

embryo An embryo is an initial stage of development of 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 sperm ...

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 sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of g ...

reproduction encompassed by "

aplacental viviparity Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop insi ...

", 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 ( ) sometimes 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 ( ...

(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 and mucous cells. It ...

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

yolk sac The yolk sac is a membranous 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'' is fa ...

. This form of histotrophy is known to occur in the

dogfish shark Squalidae, more commonly known as dogfish, dog sharks, or spiny dogfish, are one of several families of sharks categorized under Squaliformes, making it the second largest order of sharks, numbering 119 species across 7 families. Having earned t ...

s (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 residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...

and

lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids incl ...

-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 An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic di ...

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