#parophrys vetulus

[unofficial-sean]

An english sole (Parophrys vetulus)

[vulturvolanss]

You've disected a Parophrys vetulus?! Tell me about it!

I have! it was extremely fresh from the fish market (not preserved with anything) and I was extremely inexperienced, so I ended up making a bad incision and dissolving a lot of its organs with its own digestive enzymes. the gill rakers had a lot of sediment in them, which was pretty cool. I was the only one strong enough to break them enough to look inside—they have surprising sturdy rakers. nothing else of extreme note. just a lot of blood, LOL.

[unofficial-sean]

I got my ROV back together and took her out for an expedition! It's been almost 5 whole months since her last foray! Many sea cucumbers, flatfish, sculpins, and crabs to be seen!

[unofficial-sean]

Parophrys vetulus, Lepidopsetta bilineata, Citharichthys stigmaeus, Metacarcinus magister, Metacarcinus gracilis, penpoint gunnel, crescent gunnels, unidentified sculpin, jellyfish, and more can be found beneath the surface of Les Davis Marine Park!

The park is most notable for its concrete reef, located about 18m down near a marker buoy. I did not visit the reef, unfortunately, due to low visibility conditions that made me uncomfortable going deeper than 9m. Perhaps next time I will record a definitive set of wildlife seen on the reef! Among them are rockfish, lingcod, and Metridium farcimen in the hundreds!

[unofficial-sean]

First off, I love the title of this study.

This study examines depth data from 32 Hexanchus griseus in Puget Sound's main basin using acoustic tags from 2005-2007. They were monitored by an impressive array of acoustic receivers--some 270--throughout the Sound. The study aims to answer for what reason H. griseus migrates vertically over 24-hour periods. Three hypothesis are proposed: 1) For foraging 2) Predator or competitor avoidance 3) Bioenergetic benefits

H. griseus is the largest resident fish in Puget Sound, so their only competitors are other sharks. Smaller H. griseus may also be prey to larger sharks.

Diel vertical migrations observed in the spotted catshark (Scyliorhinus canicula) show a bioenergetic benefit of being able to hunt in prey-rich warm waters at night, while resting in cooler waters during the day.

Through passive and active monitoring of the acoustic tags over this period, the researchers were able to monitor depth and water temperature following the sharks. Interestingly enough, on three occasions, pairs of sharks were observed making very similar movements, as shown in a, b, and c in the figure below. It was also noted that the sharks follow the contours of the sea floor when migrating, as opposed to direct vertical movement in the water column.

So, in the relatively shallower and level environment of Elliot Bay, sharks occupied relatively shallower depths and the changes in depth were more gradual (as in a and b). Sharks observed at Bainbridge Island display more extreme depth changes in comparison due to the more extreme slopes of the sea floor, there. The mean depth of the sharks depended on the maximum depth of their site.

Consistent patterns of vertical movement and levels of activity across size and sex across multiple spatial scales best supports the first hypothesis; that H. griseus' vertical migrations are related to foraging behavior. Competition and predation between sharks is unlikely due to the synchronicity of paired sharks observed, and they are responding to similar stimuli.

In addition, temperature changes in Elliot Bay were found only to have change less that 1 degree C, and generally H. griseus did not cross the thermocline. Plainly, there is little to suggest that bioenergetic efficiency is the cause of these vertical movements.

As we can see in the plots above, sharks in Elliot Bay (a, b) make one large vertical movement at dusk and dawn. Whereas at Bainbridge Island (c, d), sharks made multiple vertical movements in shallower waters during the night.

 This is likely due to the difference in distribution and abundance of prey in the shallow embayment vs. the steeper main channel, thus necessitating differing foraging behavior. Fish that display similar vertical migrations are the pacific hake (Merluccius productus), english sole (Parophyrs vetulus), and dungeness crab (Cancer magister). Though, as we've observed before, H. griseus seems to struggle with crab consumption, so I'm doubtful Cancer magister is on the menu. Attempts to examine stomach contents of the sharks came up empty, so nothing could be confirmed.

While changes in light intensity are hypothesized to be responsible for initiating foraging behaviors in pelagic some fish, such changes are unlikely to be of import to H. griseus, and may only initiate a transition of behaviors, instead.

There is spatial segregation between older, larger and younger, smaller sharks (I wonder if that has to do with distribution of prey items. If smaller prey for smaller sharks are present in deeper depths) with large sharks sticking to the shallows.

The fastest rates of ascent and descent were consistently observed on the night ebb tide, and highest rate of ascent/descent observed was 15m/minute and 13m/minute, respectively.

In conclusion, the evidence supports the hypothesis that Hexancus griseus' diel vertical migrations are related to foraging above all else. As active predators and passive scavengers, H. griseus is able to forage across the entire food web, making them a good candidate for overall ecosystem health indicators.

[unofficial-sean]

INCREDIBLE new footage from John Sanders at Redondo!!!

Here's why:

The question is up in the air as to whether the bluntnose sixgill shark (Hexanchus griseus) is a primarily a predator or scavenger. The answer is likely both when looking at other predators in its niche, but this footage is another key to confirming the hypothesis that they are primarily scavengers.

In the past, H. griseus has been recorded ingesting crabs and spitting them out at a bait station specifically for the crabs. This was interesting to me because crustaceans supposedly make up a decent portion of their diet. Why wouldn't the observed shark eat the crabs even if it couldn't get the fish meat inside the trap?

A more recent study, which I also wrote about, found tissues from a young dolphin in the stomach of an H. griseus from the western Mediterranean Sea. These are not fast sharks. There were not brittle stars or other scavenging organisms in the stomach, suggesting that the shark got to the dolphin before the others, if it was scavenged and not predated. Among other little details that I go into further detail in that post.

To the point: In this video, we see three H. griseus at a bait station crowded with pacific spiny dogfish (Squalus suckleyi) and red rock crabs (Cancer productus). There was one Dungeness crab (Metacarcinus magister) and one flatfish that may be a rock sole (Lepidopsetta bilineata) or an English sole (Parophrys vetulus). All gathered to feed on the salmon meat placed by the divers. Repeatedly in the footage, we see the H. griseus ingest crabs and spit them out alive.

Why? Well, it appears that they are only interested in the salmon flesh and may have been trying to steal the morsels that the crabs were pinching away at. In one instance, an H. griseus goes for a whole gathering of crabs. Doesn't eat a single one. Now, to draw any conclusion from this would require a lot of assumptions. I assume that these crabs had salmon meat in their claws and I assume that the sharks got the flesh before spitting the crabs out. That need not be the case to still support my hypothesis that these sharks are primarily scavengers.

These sharks had multiple opportunities to eat any number of the other visitors to the feast. In one moment, an S. suckleyi was right at the rostrum of one of the H. griseus and the latter didn't even try to eat the former. This could be because S. suckleyi has venomous spines; perhaps it is learned or instinctual not to try and eat them. But then there's the C. productus. Other that having a slippery carapace, there is little the crab could do that would prove fatal if eaten live. And then there's the flatfish, which has no natural defenses or hazards.

All of this points to scavenger behavior.

I am blown away by this footage (and extremely envious of this diver). Even though my ROV is OOC, I still get observe these sharks in one way or another.

[unofficial-sean]

Here it is! Another expedition completed.

This time, I came across an English sole (Parophrys vetulus)! Who jumpscared me! There were also many shiner perch (Cymatogaste aggregata), plumose anemone (Metridium farcimen), a dungeness crab (Metacarcinus magister), three red rock crabs (Cancer productis), and miscellaneous gunnel fish and an unknown sea star.