SECRET SHARK NURSERIES

Catshark egg casings. Credit: OpenCage.info via Wikimedia Commons.

 

The fascinating deep-water cold-seep worlds of mud volcanoes and methane seeps are powered by the process of chemosynthesis not photosynthesis—a difference that gets them branded as 'extreme' environments.

But it turns out that cold seeps are also fantastically rich nursery grounds for deep-water sharks—specifically for catsharks, Galeus melastomus—and for skates—possibly of the genus Bathyraja.

Glass skatefish, Bathyraja transpicia. Credit: Sergio Gabriel Nahk via Wikimedia Commons.

 

The authors of a new paper in MEPS (Marine Ecology Progress Series) found living egg casings at two modern sites:

• the North Alex Mud Volcano in the eastern Mediterranean Sea, at water depths of about 500 meters/1,640feet

• the Concepción Methane Seep Area in the south-east Pacific Ocean, at depths of about 700 meters/2,300feet

 

Close-up photograph of the tubeworm Lamellibrachia luymesi from a cold seep at 550-meter/1,800-foot depth in the Gulf of Mexico. Credit: Charles Fisher via Wikimedia Commons.

  
They also found evidence in the fossil record at the Bear River Cold-Seep Deposit in the North Pacific off Washington that cold deep-water ecosystems have been important to elasmobranchs for at least 35 million years:

[W]e collected 30 fossilized shark egg capsules and fragments thereof. All of the capsules were found closely associated with abundant remains of bathymodiolin mussels, hexactinellid sponges, and tubeworms... The most similar extant egg capsules to those found at the BRSD are those of the deep-water catshark Apristurus spp.

Spongehead catshark, Apristurus spongiceps. Credit: NOAA Ocean Explorer.

As to why some sharks and skates lay their eggs in deep-water cold seeps—and have done so for so long—the authors suggest:

• that coral reeflike marine life in chemosynthetic ecosystems provide holdfasts for egg capsules—on tubeworms, gorgonians, and sponges, and between carbonate boulders 

• that enhanced currents around the reefs provide ventilation important for the development of the egg capsules 

What's not yet known is the role—if any—that seeps play in the ecology of the hatchling sharks and skates:

We do not know if the seep biostrome is still of importance for the neonate and juvenile sharks and skates after hatching because we could not confirm their presence in the vicinity of the egg capsules. Neonate catsharks and skates are reported to leave their  nurseries, probably to escape predation, and to migrate into deeper or shallower depths until they return for mating. However, the seep biostrome could provide ample and localized food sources in the form of small fishes, crustaceans, mollusks, and annelids.

Black catshark, Galeus melastomus. Via.

The authors conclude that 'extreme' seeps are hardly isolated from the 'non-extreme' waters around them:

By serving as nurseries for deep-water marine predators, cold seeps are important components of deep-sea ecosystems and should not be considered as only extreme and exceptional habitats; their presence or absence is likely to influence faunal diversity along continental margins.

The paper:

• ♥ Treude T, Kiel S, Linke P, Peckmann J, Goedert JL (2011) Elasmobranch egg capsules associated with modern and ancient cold seeps: a nursery for marine deep-water predators. Mar Ecol Prog Ser 437:175-181. DOI:10.3354/meps09305

♥ Open access.