Today more evolutionary gaps are filled
and gaffs are rectified as novel hypotheses of interrelationships between sturgeons and sharks are further cemented with more data. When all the parts keep falling into place like this, a hypothesis is more likely to be correct.
Yesterday I learned
that larval sturgeons (like larval paddlefish) have small sharp teeth. These are lost when hatchlings grow more than 3cm in length. At this point their diet changes from open water microscopic copepods to river bottom macroscopic arthropods. At the same time their mouth parts become extensible vacuum cleaner tubes, usually carried inside, sometimes everted (Fig. 2b).
Zarri and Palkovacs 2018 described
larval green sturgeon diets. “Fish smaller than 30 mm had teeth on the oral jaws and showed a strong reliance on zooplankton prey. The developmental loss of teeth in fish greater than 30 mm was associated with decreased zooplankton consumption and increased richness of benthic macroinvertebrates in diets.”
According to the online The Fish Report
“The study found that the most common larval sturgeon prey included copepods (a kind of tiny zooplankton), and macroinvertebrates such as mayflies, midges, and blackflies. The scientists also noted an interesting diet shift: larval sturgeon consumed zooplankton and macroinvertebrates in roughly equal amounts until they grew to 30 millimeters in total length, at which point their macroinvertebrate consumption increased. This shift coincided with the young sturgeon losing their teeth (fun fact: unlike humans, sturgeon start out life with teeth and lose them as they grow older).”
include copepods (a kind of tiny zooplankton) that floats freely in open waters.
such as larval mayflies, midges, and blackflies that live in river sands and muds.
Muir et al. 2000 report
a burrowing river amphipod about 1 cm long, Corophium spp., is the most important prey for bottom-feeding juvenile and sub-adult white sturgeon. In adult sturgeons,small bottom-dwelling fish, larvae, crayﬁsh, snails, clams and leeches are on their prey list.
So the loss of teeth and the change in diet
reflects a change from open water predation of microscopic forms that other fish would filter to visible worms and larvae living in river bottoms.
This somewhat mirrors more primitive behavior in lancelets
that feed in open waters as juveniles, then burrow tail first in river bottoms and become sessile feeders. One branch of lancelets kept evolving to become crinoids and later, starfish. The other branch, the one that kept active as adults, became vertebrates.
This supports the phylogeny
of the large reptile tree (LRT, 1780+ taxa) which recovers the toothless Chondrosteus + Rhinchodon + Manta clade as the proximal descendants of sturgeons. These increasingly larger taxa continue to feed like larval sturgeons on plankton filtered from open water with larger, more anteriorly directed jaws and branchial cavities.
The second largest and second most basal shark in the LRT,
the basking shark, Cetorhinus, is likewise toothless and feeds on open water zooplankton.
it was not until larval teeth were retained in adults, like Isurus and similar sharks, that made the capture of larger and larger prey in open water conditions possible. Contra tradition, filter-feeding, like a whale shark, is a a primitive trait, as documented in the LRT.
After marginal teeth appeared on shark jaws, and stayed there in adults,
evolution took several courses, including a return to benthic feeding in guitarfish, sawfish, rays, ratfish all with pavement-like teeth. Sharks with sharp teeth kept their open water feeding habits. Some of these gradually lost the long rostrum and evolved into several forms, including 2m Hybodus close to the base of bony fish represented by 4cm Prohalecites (Fig. 5).
Given the above gathered data points,
now I’m looking for a juvenile osteostracan. Wonder what it looks like? If less bony, as in sturgeons, they might be hard to find.
Muir WD, McCabe, Parsley and Hinton 2000. Diet of first-feeding larval and young-of-the-year white sturgeon in the Lower Columbia River. Northwest Science 74(1):25–33.
Sewertzoff AN 1928. The head skeleton and muscles of Acipenser ruthensus. Acta Zoologica 13:193–320.
Zarri LJ and Palkovacs EP 2018. Temperature, discharge and development shape the larval diets of threatened green sturgeon in a highly managed section of the Sacramento River. Ecology of freshwater fish 28(2): https://doi.org/10.1111/eff.12450