Lancelets and clams compared

Molluscs and chordates are getting closer and closer lately.
Earlier we looked at nautilus and lancelet similarities. Also earlier we looked at garden slug and hagfish similarities.

Today
we look at lancelet (Fig. 1) and clam (Fig. 2) similarities.

Figure 1. Extant lancelet (genus: Amphioxus) in cross section and lateral view. The gill basket nearly fills an atrium, which intakes water + food, sends the food into the intestine and expels the rest of the water. Compare to the clam in figure 2. Pretty much the same.
Figure 2. Clam diagrams modified from Markus Ruchter. As in lophorates, the rectum bends dorsally over the buccal cilia and mouth in clams compared to the straight intestine in lancelets (Fig. 1). The clam foot is homologoous with the lancelet tail. Both are used for digging backwards into the substrate. Both have an atrium for filtering plankton from sea water. Both have a stomach opening posterior to the atrium to collect plankton captured on mucous strands traveling along the atrial walls.

At first clams seem odd and inscrutable,
but when you simplify their structures (Fig. 2), many previously overlooked similarities to lancelets begin to appear. Lancelets (Fig. 1) have a straight intestine terminating below a terminal tail. Clams also have a terminal tail, but it is traditionally called a foot. In clams the stomach and intestine arch dorsally and terminate dorsal to the ciliated mouth (as in lophophorates), expanding to produce a funnel, as in Nautilus (which has a ventral funnel, likely due to a close, but separate ancestry from the clam).

The phylogenetic origin of the bilateral clam shell
remains a mystery at present. Ontogeny (Fig. 3) provides clues. Clam shells develop during the clam’s planktonic (= free-swimming) embryo stage, shortly after feeding commences and prior to settling on or burrowing tail first into the sea floor (like a lancelet, Figs. 1, 2).

Figure 3. Clam embryo development from fao.org. Though overall similar to the protostomate trochophore, the clam mouth appears at the oral pole surrounded by buccal cilia, as in lancelets, not at the equator, as in other protostomates. Not sure what the single strand arising from the oral pole is yet, but appears to be a swimming organ that is absorbed as the buccal cirri take over that job.

All molluscs are traditionally considered protostomates,
but note a subtle difference: the traditional protostomate trochophore (= early embryo) has a mouth that appears at the so-called equator (Fig. 3). By contrast the clam mouth appears in the middle of buccal cilia, at the oral pole, as in the lancelet. In clams, as in the nautilus, octopus and starfish, the buccal cilia double as organs of locomotion. This is distinct from lancelets that depend on their tail, not their mouth, to swim and dig. By this evidence, this early stage (Fig. 3) is where the switch from one to another took place, if ontogeny recapitulates phylogeny.

Both clams and lancelets have an atrium
for filtering plankton from sea water. Both have a stomach opening posterior to the atrium to collect plankton captured on mucous strands traveling along the atrial walls.

The benthic, burrowing, plankton-feeding lifestyle of a clam
(Fig. 2) remains very much like that of its unarmored ancestor, the lancelet (Fig. 1). The armored body looks extremely different, from the outside, but take away the armor and the similarities become more noticeable.

Lancelets came first.
They are closer in morphology to their elongate nematode ancestors (Fig. 4) and only develop buccal cilia as they near adulthood. In clams the buccal cilia appear early in embryology and take over as swimming organs. Timing is everything. And it looks more and more like the traditional phylum Mollusca is polyphyletic, like traditional diapsids, protorosaurs, pterodactyloids, turtles and whales.

Figure 4. From Mansfield et al. 2015. Lancelets do not go through a trochophore embryo stage, but rather quickly become elongate, like their nematode ancestors, during their planktonic, free-swimming stage. Note the temporary appearance of eyes and a tail better for swimming than digging along with a late appearance of buccal cilia, all key factors in the present hypothesis of interrelationships with molluscs. Lancelets are Ediacaran in origin.

This, too, appears to be a novel hypothesis of interrelationships.
If not please provide an earlier citation so I can promote it here.

References
Mansfield JH, Halaler E, Holland ND and Brent AE 2015. Development of somites and their derivatives in amphioxus, and implications for the evolution of vertebrate somites. EvoDevo 6(21): DOI 10.1186/s13227-015-0007-5

Clam hatchery: http://www.fao.org/3/y5720e/y5720e0a.htm

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