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.

Sea squirts, lancelets and acorn worms

A sea squirt (Ciona intestinalis) - a member of Tunicata 
Wikimedia Commons uploaded by perezoso (GFDL)

Genomics has clarified our position in the tree of life. To explain this I need to define some taxonomic terms.

Phylum Chordata comprises three subphyla: Vertebrata (Craniata), Tunicata (Urochordata)and Cephalochordata. Tunicates include sea squirts  such as Ciona (pictured) above. A familiar cephalochordate is the lancelet Branchiostoma lanceolatum better known as Amphioxus (shown below).

Amphioxus or Branchiostoma lanceolatum
(c) Virginia Gewin here (CC-BY-SA 3.0)

Amphioxus has long been used to exemplify the general plan of chordate organization and lancelets used to be regarded as the closest relatives to vertebrates. The genomic evidence, however, has tunicates like the sea squirts as sister group to vertebrates with cephalochordates as a deeper branch. Additional support is given by conserved molecular signatures (here).

Chordates belong in the Superphylum Deuterostomia (brilliantly reviewed by Lowe et al. here) along with Phylum Hemichordata and Phylum Echinodermata. Echinoderms are richly represented in the fossil record and the five extant classes include sea urchins, sea cucumbers and starfish. Hemichordates include the acorn worms for which two genomes just became available (here and here).

Acorn Worms (Hemichordata: Enteropneusta)
from Spengel 1883 (public domain)

One of many findings was a cluster of six genes that are conserved across chordates and implicated in patterning of gill slits. This is significant because gill slits were an innovation in the deuterostome lineage (although secondarily lost in echinoderms).

Relationships between deuterostome phyla were largely worked out through their embryology, an example being the erection of Chordata by Haeckel. Understanding the genes involved in developmental processes remains a focus in working out our evolutionary history (see the review by Lowe et al. mentioned above).