A catshark nursery in a deep gorgonian field in the Mississippi Canyon, Gulf of Mexico

Abstract

The habitat functions of Gulf of Mexico gorgonians are still poorly known and understood. Benthic transects were conducted at three deep-water (> 200 m) sites in the northwestern Gulf of Mexico in September 2003: Viosca Knoll (Lease Block 826–340 m), Green Canyon (Lease Block 354–538 m), and Mississippi Canyon (Lease Block 885–533 m) using the Sonsub Innovator ROV aboard the R/V Ronald H. Brown. Octocorals were present at all three sites and were the dominant benthic megafauna at one of the three dive sites. Colony heights were estimated to be 25–50 cm tall, with a mean height of ~35 cm. In total, 296 catshark egg cases were attached to 117 colonies (11% of the total). All egg cases observed appeared to be the same size and shape, suggesting that a single scyliorhinid species was responsible for depositing the eggs.

Full text

BULLETIN OF MARINE SCIENC E, 81(3): 553–559, 2007
553
Bulletin of Marine Science
© 2007 Rosenstiel School of Marine and Atmospheric Science
of the University of Miami
A CATSHARK NURSERY IN A DEEP GORGONIAN FIELD
IN THE MISSISSIPPI CANYON, GULF OF MEXICO
Peter Etnoyer and Jon Warrenchuk
Gorgonian corals (Anthozoa: Alcyonaria: Gorgonacea) are a conspicuous compo-
nent of the deep-water benthic megafauna in the Gulf of Mexico (Alcyonaria: Deich-
mann, 1936; Bayer, 1954; Giammona, 1978; MacDonald et al., 1996; Cairns and Bayer,
2002). Callogorgia spp. primnoid colonies in particular are widespread throughout the
Gulf, with one endemic species in the Mississippi delta region, Callogorgia americana
delta Cairns and Bayer, 2002. Primnoid octocorals in the North Pacific are considered
habitat-forming because they grow large enough to provide substrate and shelter for
associated species of fish and invertebrates (Krieger and Wing, 2002; Etnoyer and Mor-
gan, 2003, 2005). e habitat functions of Gulf of Mexico gorgonians are still poorly
known and understood.
e nature of the relationship between deep-water corals and fish in general is
unclear. Deep scleractinian reefs [Lophelia pertusa (Linnaeus, 1758) and Oculina
varicosa (Lesueur, 1821)] may provide nursery habitat for deep-water redfish Sebastes
viviparous (Kroyer, 1885) (Fossa et al., 2002), gag grouper [Mycteroperca microlepsis
(Goode and Bean, 1879)], and scamp Mycteroperca phenax Jordan and Swain, 1884
(Gilmore and Jones, 1992), but evidence of fish using gorgonian corals as nursery
habitat is limited. Krieger and Wing (2002) noted that most large rockfish occur near
boulders and corals on the continental shelf of the Gulf of Alaska. e question re-
mains whether gorgonian habitat for fishes is obligate or facultative (Auster, 2005).
An early woodcut print of a single egg case attached to a seafan from Lydekker’s
Royal Natural History (1896) shows a shallow gorgonian serving as nursery habitat
for a skate, ray, or oviparous shark, but physical specimens and in situ reports of this
habitat function are rare. One Paragorgia sp. colony with tendrils from a single egg
case attached to a branch resides at the Santa Barbara Museum of Natural History
(Id. no. 24471 CH, location unknown) (B. Horvath, Westmont College, pers. comm.).
Another Paragorgia sp. colony from the North Atlantic hosts a dozen catshark egg
cases at the Canadian Shark Research Laboratory (S. E. Campana, Bedford Institute
of Oceanography, pers. comm.). Submersible observations documented egg cases of
the chain catshark Scyliorhinus retifer (Garman , 1881) attached to soft corals, hy-
droids, and derelict fishing gear in the Mid-Atlantic Bight (Able and Flescher, 1991).
To date, catsharks using deep gorgonians as nursery habitat has not been document-
ed in the deep Gulf of Mexico.
Egg cases of an unknown species of scyliorhinid catshark were found in abundance
on a low-relief mound at 533 m depth in a large field of C. a. delta (family Primnoidae)
in the Mississippi Canyon, Gulf of Mexico during exploratory surveys in September
2003. A smaller field of Acanella sp. Gray, 1870 octocorals (Gorgonacea: Isididae)
without egg cases was also encountered at 340 m depth on Viosca Knoll (Fig. 1). Two
species of Acanella are known from the Gulf of Mexico. Acanella eburnea (Pourtalès,
1868) is common in the northern Gulf of Mexico between 283 and 1829 m, whereas
Acanella arbuscula (Johnson, 1862) was described from only one preserved speci-
NOTE

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men (Giammona, 1978). Areal coverage and density for both species are unknown
(Gulf of Mexico and South Atlantic Fishery Management Council, 1982).
M
Video transects were conducted at three deep-water (> 200 m) sites in the northwestern
Gulf of Mexico in September 2003: Viosca Knoll (Lease Block 826–340 m), Green Canyon
(Lease Block 354–538 m), and Mississippi Canyon (Lease Block 885–533 m) using the Son-
sub Innovator ROV aboard the R/V R H. B. A 2.5-km round trip linear video
transect was conducted on September 31, 2003 in the southwest saddle of the Mississippi
Canyon (28°3.9´N, 89°43.1´W) on a low-relief knoll at 533 m depth. e linear transect mea-
sured 1.25 km, but the ROV deviated from this path several times. e ROV was positioned
approximately 1 m off the bottom and stopped intermittently to collect samples and conduct
close-up video taping with a Sony TRV900 3-chip camera recording to Mini-DV tape. e
total survey time was 7 hrs 3 min. e dominant macrofauna along the entire transect was
the primnoid octocoral C. a. delta. Colony heights were approximated by comparison with
the known measures of the ROV and its tools, e.g., the manipulator claws and arms. Colony
density was estimated based upon the number of colonies in the field of view. Nearly 51% of
the dive was spent motionless, making observations and collections, and waiting for visibility
to improve.
e dive site on Viosca Knoll (29°06´N, 88°23´W) was surveyed on September 23, 2003. is
site is also a low-relief knoll, 200 km to the northeast of Mississippi Canyon, slightly shallower
than the previous site. e total elevation of the feature was ~70 m height. e ROV survey was
conducted at depths of 300–350 m in a series of eight east-west parallel belt video transects
Figure 1. Map of Callogorgia spp. occurrences in the Gulf of Mexico, illustrating the range and
distribution of primnoid habitat. Callogorgia americana delta is endemic to the Mississippi
Delta region, but Callogorgia spp. primnoid corals are widespread. Octocoral occurrence data
from Smithsonian NMNH, Texas Cooperative Wildlife Collection (TCWC), and Harbor Branch
Oceanographic Institution. Bathymetry data from Smith and Sandwell, 1997.

NOTES 555
~200 m in length across a northeast trending spur of the knoll with two rises. e area surveyed
at Viosca Knoll was 50,000 m2 in a 1.85-km linear distance over an 8 hr period.
Octocoral samples and attached epibionts were collected. Branched segments of octocoral
colonies were preserved in 90% ethanol; duplicate collections were deposited at the California
Academy of Sciences and the Smithsonian Institution National Museum of Natural History
(USNM 1085704 and 1085705). Acanella sp. was not collected. Four egg case specimens were
collected with the ROV’s suction device and preserved in 70% ethanol for identification. ese
could not be identified because eggs had hatched, and no embryos were present. e video
transect was analyzed by quantifying the numbers of octocoral colonies with and without egg
cases and the number of egg cases per colony.
R
Octocorals were present at all three sites and were the dominant benthic megafau-
na at one of the three dive sites. Callogorgia americana delta was the most conspicu-
ous benthic megafauna in Mississippi Canyon, while C. a. delta, Acanella sp. bamboo
corals, and Leiopathes spp. black corals were all observed in surveys of Viosca Knoll.
e scleractinian L. pertusa dominated some crests and mounds at Viosca Knoll
(Schroeder, 2006). Only two species of zoantharian black corals, Stichopathes sp.
Brook, 1889, and Leiopathes sp. were collected from third deep site at Green Canyon.
ese occurred as isolated or scattered individuals.
e epibenthos at the Mississippi Canyon site was heavily silted. No hard bottom
substrate was evident, though the presence of C. a. delta implied buried hard attach-
ment surfaces. A total of 1010 individual colonies of C. a. delta were counted along
the 2.5 km round trip transect. Colonies were sparsely spaced, between 1 and 2 m
apart on average, with several colonies visible at any given time. e greatest number
of visible colonies at any one time along the transect was 18. Colony heights were
estimated to be 25–50 cm tall, with a mean height of ~35 cm. In total, 296 egg cases
were attached to 117 colonies (11% of the total). A maximum of 17 cases were ob-
served on a single colony and the mode was 1 egg case per colony (Figs. 2, 3). Octoc-
orals with egg cases appeared to be somewhat clustered, that is, corals with egg cases
tended to be located near other corals with egg cases. No egg cases were observed in
the field of Acanella sp. at Viosca Knoll.
All egg cases observed appeared to be the same size and shape, suggesting that a
single scyliorhinid species was responsible for depositing the eggs. All four egg cases
collected were hatched and did not contain embryos. e cases ranged from 48–57
mm in length and 18–22 mm in width, consistent with the description of the chain
catshark S. retifer (Castro et al., 1988). However, egg cases of many other scyliorhinid
species have yet to be described, no sharks were definitively identified in situ, and no
living embryos were obtained, so the species identification remains inconclusive.
D
Scyliorhinid catsharks are a specious group of small benthic sharks that range
throughout the deep-sea (Compagno, 1988). eir egg cases are distinctive, with
long tendrils, and a bulging fusiform shape. Scyliorhinids reported from the Gulf
of Mexico (Kiraly et al., 2003; Froese and Pauly, 2006) include: smallfin catshark
Apristurus parvipinnis Springer and Heemstra, 1979; broadgill catshark Apristurus
riveri Bigelow and Schroeder, 1944; the marbled (aka roughtail) catshark Galeus arae

BULLETIN OF MARINE SCIENC E, VOL. 81, NO. 3, 2007
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(Nichols, 1927); chain catshark S. retifer (Garman , 1881); the Iceland catshark Apris-
turus laurussonii (Saemundsson, 1922); and the Campeche catshark Parmaturus
campechiensis Springer, 1979. Of these, the egg cases of only S. retifer and A. riveri
have been described (Castro et al., 1988; Compagno, 1988).
Elasmobranch egg cases have been found attached to several types of substrates,
including octocorals, but to our knowledge, this is the first in situ documentation of
a shark nursery in a deep gorgonian field. Unfortunately, embryos were not collected
but the characteristics of the egg case fit the known description of S. retifer. Scyliorhi-
nus retifer is a small, slender oviparous benthic catshark (Nichols, 1931) with a strik-
ing charismatic pattern and fluorescent coloration (Matz, 2005) found commonly
along North Atlantic continental shelf and slope at depths of 50–750 m (Sminkey
and Tabit, 1992). In captivity, mature S. retifer females lay eggs every 14–17 d. e
observed egg-laying behavior shows that egg case tendrils extending from the cloaca
must snag on a structure as the shark passes close, then the shark circles the struc-
ture until the egg case is pulled from the cloaca and the egg case is attached (Castro
et al., 1988).
Callogorgia americana delta colonies on average are elevated above the sediment
by nearly 35 cm, with likely benefits of increased survivorship for the shark embryos
due to increased water circulation and perhaps reduced predation. e branches of
C. a. delta are strong and flexible. Colonies are plumose, with broadly spaced pinnate
branches. ey have a uniform proteinaceous axis with heavily-armored retractile
polyps arranged in whorls. e polyps are protected by on all sides by large roughly
textured calcitic scales (Cairns and Bayer, 2002). ese factors may contribute to the
shark’s choice of C. a. delta nursery substrate.
Callogorgia spp. primnoid corals differ greatly from Acanella spp. bamboo coral
colonies in form and character. Acanella spp. colonies have a bushy appearance, with
an alternating axis of calcitic internodes and gorgonaceous nodes. Colonies branch
from the nodes. In contrast to the primnoids, isidid polyps are fleshy and non-re-
Figure 2. Callogorgia americana delta colonies with catshark egg cases attached. Depth 533 m.
ROV suction hose on the left is 15.25 cm (6 in) in diameter.

NOTES 557
tractile, protected only by eight small spines projecting from the mesenteries (Ver-
rill, 1883; Kukenthal, 1924; Bayer, 1990). erefore, the texture of Callogorgia spp.
branches is rough, while the texture of bamboo coral branches is fleshy and smooth.
e rough texture of Callogorgia spp. primnoid corals may enhance egg case adhe-
sion.
is occurrence demonstrates that large (> 1 km) monotypic fields of gorgonian
octocorals on low relief mounds can provide nursery habitat to fish species in the
Gulf of Mexico. Yet, this one occurrence is insufficient to determine whether cat-
shark nursery habitat is substrate-specific, taxon-specific, or site-specific. Callogor-
gia spp. primnoids are widespread throughout the Gulf of Mexico (Cairns and Bayer,
2002) (see Fig. 1). Catshark nursery habitat may or may not be widespread. Egg cases
have not been reported on other commonly occurring benthic megafauna in the Gulf
of Mexico such as antipatharian black corals. Deep scleractinian thickets are rare in
the Gulf, therefore, gorgonians may provide essential fish habitat for catsharks sim-
ply due to a lack of alternatives. Future research and management of catshark popu-
lations should consider the status and distribution of deep-water gorgonian fields, as
they may have an important role in the life history of sharks. Widespread and local
anthropogenic impacts to gorgonian communities, such as bottom trawling, longlin-
ing, and ocean dumping may inadvertently impact deep-water nursery habitat for
catshark populations.
A
e authors thank the scientists and crew of the 2003 Gulf of Mexico Deep Habitat Expe-
dition. We thank M. Wicksten, J. MacEachran, and R. A. Martin for sharing their expertise
on scyliorhinid sharks. We also thank F. M. Bayer, S. D. Cairns, and T. Coffer of Smithso-
nian NMNH for identifying and curating octocoral specimens and D. Opresko for identifying
black corals specimens. Special thanks to J. Reed of Harbor Branch Oceanographic Institu-
tion and M. Wicksten of the Texas Cooperative Wildlife Collection at Texas A&M University
for octocoral data. Finally, we express our gratitude to the editors and three anonymous re-
Figure 3. Frequency of Callogorgia americana delta colonies in Mississippi Canyon with cat-
shark egg cases attached.

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A: (P.E.) Harte Research Institute, Texas A&M University-Corpus Christi, 6300
Ocean Dr., Unit 5869, Corpus Christi, Texas 78421. (J.W.) Oceana, 175 South Franklin Street,
Suite 418, Juneau, Alaska 99801. C A: (P.E.) E-mail: <peter.etnoyer@
tamucc.edu>.