Placentation in salps

Placenta of a salp (Salpa fusiformis)
Reproduced from Bone, Pulsford and Amoroso (here)
(C) 1985 with permission from Elsevier

Sea squirts? asked a faithful reader of this blog. OK, sea squirts do not have placentas, but salps do! They, too, are tunicates, and thus included in the sister group to vertebrates (previous post). Whilst sea squirts are mainly sessile, salps are pelagic, living near the surface of oceans and at times more numerous than krill. They are tubular animals and swim by jet propulsion.

The life cycle involves alternation between asexual and sexual generations. The asexual phase (oozooid) develops within the jet chamber of the sexual phase (blastozooid). The image above is from Amoroso's last paper, published almost three years after his death (see previous post for Amo). The placenta consists of two layers: an outer cortex (co) and an inner central layer (c). These separate the embryonic (E) and maternal (M). circulations. Both layers are syncytial and both are maternal in origin. However, embryonic leucocytes pass into and add to the cortical layer.

Blastozooids are hermaphrodite, but the egg develops before the testis matures so is fertilized by sperm from a different blastozooid

HMS Rattlesnake on which Thomas Henry Huxley served as
Assistant Surgeon during the voyage to Australia and New Guinea 1846-50
National Maritime Museum (public domain)

The first English language description of placentation in the Family Salpida was given by T. H. Huxley R.N., "late of HMS Rattlesnake," in 1851 (here) although he cites even earlier work by Cuvier, Chamisso and Meyen.

We tend to think of placentation in terms of mammals, reptiles and fish, but a current paper in Biological Reviews (here) shows that maternal provision of nutrients (matrotrophy) and even placentation is not infrequent in invertebrate phyla.

Amo's chapter on placentation

Amo Cambridge 1972

Emmanuel Ciprian Amoroso FRS (Amo) is best remembered for his monumental chapter on Placentation written for the 1952 edition of Marshall's Physiology of Reproduction edited by A. S. Parkes (Volume II pages 127-309).

Amo was born in Trinidad and graduated in medicine from the National University of Ireland. He then spent 1930-32 in Germany and his knowledge of the German literature (almost unread in the English speaking world) was one reason his chapter was so important.

Amo then came to University College London where he completed his Ph.D. under the auspices of J. P. Hill. From there he moved to the Royal Veterinary College where he was Professor of Veterinary Physiology from 1950 to his retirement in 1968. Amo was a mentor for many scientists with distinguished careers. He continued in this role on retirement when he found a new home at the Institute of Animal Physiology at Babraham near Cambridge. He died in 1982.

An engaging account of Amo's life was written by R. V. Short (here). The true monument remains his chapter which has a place of honour on my bookshelf.