DIVE DRY WITH DR. BILL #740: FARNSWORTH BANKS' LACE CORALS
A while back I received an e-mail from Clarissa Wei at KCET asking if I'd be willing to be interviewed for a story she was writing. The subject was the purple hydrocoral (Stylaster californicus) out on Farnsworth Banks off the windward side of Catalina. One question she was exploring is the impact of boat anchoring on these fragile colonies.
I checked my previous columns and I had not written about the hydrocoral in over 10 years, and all my images from my dives on Farnsworth are taken from lower resolution standard definition video. I set about doing a bit of new research on the species so I was prepared to answer her questions.
Unlike hermatypic (reef building) stony corals which are relatives of the sea anemones, hydrocorals are more closely related to the hydroids. Hydrocorals, often called lace corals, are also related to the fire corals which can give humans quite a sting. However Stylaster's nematocysts do not pose any threat to humans (unless they are very thin skinned!).
Reef building corals incorporate tiny symbiotic algae into their polyps which then give them their color. When they die, the calcium carbonate skeleton is usually a bright white. With the hydrocoral the purple, pink or blue color is actually deposited in the limestone skeleton and does not change upon death.
The polyps in lace corals are microscopic and embedded in two types of tiny pores on the colony's surface. Polyps in the larger dactylopores have long protruding hair-like structures with stinging cells. They function for defense although they may also capture food. The smaller gastropores contain the feeding polyps. The colony is connected by a network of minute canals which aid in food distribution.
Although this species is known from Cordell Bank in northern California to San Benito Island off central Baja, it is found at relatively few locations. It requires hard substrate to attach to, so Farnsworth Bank which is probably an igneous rock formation is perfect. In addition it needs strong currents to bring in food and very clear water with little sediment. Hydrocorals are considered deep water species. Based on surveys by Dr. Milton Love in 2008, it was most common from about 100 to 130 ft but found as deep as about 215 ft. Elsewhere in its range it has been observed down to a maximum depth of 2,700 ft.
Since I usually focus on munching and mating, I know you are all waiting to read about the sex life of hydrocorals. The sources I researched gave contradictory accounts. It appears a colony may be either entirely male or entirely female. Since the two genders are segregated, this requires expulsion of the gametes for fertilization to occur. One account reported that males and females release medusae (jellyfish-like forms) that cast sperm and eggs into the water column where they join to form a free swimming larva. It states that they may also reproduce asexually by fragmentation.
This appears to be contradicted by another source that states the eggs are retained by the female polyp and fertilized there, developing into a crawling larval stage that settles close to the parents. This would yield very limited dispersal and keep the new colony close to the optimal habitat of the parents. The new colony then grows through asexual reproduction, thus creating colonies of one gender or the other. I'll have to ask the hydrocorals which version is true on my next dive at Farnsworth.
A decade ago when I wrote my first column on these hydrocoral, I stated that the colonies grow very slowly taking decades to a century or more to reach the size we see on Farnsworth Banks. Recently I could find no source that mentions their growth rate.
The apparent slow growth rate, and delicate fan-like structure, make these corals very sensitive to disturbance. Years ago they were harvested to create jewelry and curios, but this practice was stopped back in the late 1970s. Current threats come from fishing and diving activity at the Farnsworth Banks site. Careless divers can break the coral by kicking it with their fins or landing on top of it due to poor buoyancy control. The anchors and chain from dive and fishing boats also cause significant breakage. I have observed fishing nets from purse seiners caught on the rock formations as well.
People have discussed placing fixed mooring buoys at the site to eliminate the need to anchor there. The issues that are usually raised involve who can use these buoys and what protocol is employed, who will maintain them and what the liability issues are for those involved. Additional concerns are that fixed buoys would make the site location much easier to find and therefore lead to an increase in diver visits to it.
© 2017 Dr. Bill Bushing. For the entire archived set of over 700 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Dense growth of purple hydrocoral and pink color variant; anchor chain resting right on top of colony and fragment broken off colony.
A while back I received an e-mail from Clarissa Wei at KCET asking if I'd be willing to be interviewed for a story she was writing. The subject was the purple hydrocoral (Stylaster californicus) out on Farnsworth Banks off the windward side of Catalina. One question she was exploring is the impact of boat anchoring on these fragile colonies.
I checked my previous columns and I had not written about the hydrocoral in over 10 years, and all my images from my dives on Farnsworth are taken from lower resolution standard definition video. I set about doing a bit of new research on the species so I was prepared to answer her questions.
Unlike hermatypic (reef building) stony corals which are relatives of the sea anemones, hydrocorals are more closely related to the hydroids. Hydrocorals, often called lace corals, are also related to the fire corals which can give humans quite a sting. However Stylaster's nematocysts do not pose any threat to humans (unless they are very thin skinned!).
Reef building corals incorporate tiny symbiotic algae into their polyps which then give them their color. When they die, the calcium carbonate skeleton is usually a bright white. With the hydrocoral the purple, pink or blue color is actually deposited in the limestone skeleton and does not change upon death.
The polyps in lace corals are microscopic and embedded in two types of tiny pores on the colony's surface. Polyps in the larger dactylopores have long protruding hair-like structures with stinging cells. They function for defense although they may also capture food. The smaller gastropores contain the feeding polyps. The colony is connected by a network of minute canals which aid in food distribution.
Although this species is known from Cordell Bank in northern California to San Benito Island off central Baja, it is found at relatively few locations. It requires hard substrate to attach to, so Farnsworth Bank which is probably an igneous rock formation is perfect. In addition it needs strong currents to bring in food and very clear water with little sediment. Hydrocorals are considered deep water species. Based on surveys by Dr. Milton Love in 2008, it was most common from about 100 to 130 ft but found as deep as about 215 ft. Elsewhere in its range it has been observed down to a maximum depth of 2,700 ft.
Since I usually focus on munching and mating, I know you are all waiting to read about the sex life of hydrocorals. The sources I researched gave contradictory accounts. It appears a colony may be either entirely male or entirely female. Since the two genders are segregated, this requires expulsion of the gametes for fertilization to occur. One account reported that males and females release medusae (jellyfish-like forms) that cast sperm and eggs into the water column where they join to form a free swimming larva. It states that they may also reproduce asexually by fragmentation.
This appears to be contradicted by another source that states the eggs are retained by the female polyp and fertilized there, developing into a crawling larval stage that settles close to the parents. This would yield very limited dispersal and keep the new colony close to the optimal habitat of the parents. The new colony then grows through asexual reproduction, thus creating colonies of one gender or the other. I'll have to ask the hydrocorals which version is true on my next dive at Farnsworth.
A decade ago when I wrote my first column on these hydrocoral, I stated that the colonies grow very slowly taking decades to a century or more to reach the size we see on Farnsworth Banks. Recently I could find no source that mentions their growth rate.
The apparent slow growth rate, and delicate fan-like structure, make these corals very sensitive to disturbance. Years ago they were harvested to create jewelry and curios, but this practice was stopped back in the late 1970s. Current threats come from fishing and diving activity at the Farnsworth Banks site. Careless divers can break the coral by kicking it with their fins or landing on top of it due to poor buoyancy control. The anchors and chain from dive and fishing boats also cause significant breakage. I have observed fishing nets from purse seiners caught on the rock formations as well.
People have discussed placing fixed mooring buoys at the site to eliminate the need to anchor there. The issues that are usually raised involve who can use these buoys and what protocol is employed, who will maintain them and what the liability issues are for those involved. Additional concerns are that fixed buoys would make the site location much easier to find and therefore lead to an increase in diver visits to it.
© 2017 Dr. Bill Bushing. For the entire archived set of over 700 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Dense growth of purple hydrocoral and pink color variant; anchor chain resting right on top of colony and fragment broken off colony.
