DIVE DRY WITH DR. BILL #787: HERMAN'S HERMITS?
Ah, "Mrs Brown You've Got A Lovely Daughter..." "I'm Henry the VIII, I Am..." "Can't You Hear My Heartbeat..." and what a "Wonderful World" this could be. Oops, sorry. I got distracted by nostalgia for the original British invasion group Herman's Hermits. My columns aren't about musical history... my focus is on the critters of the sea. Therefore I'll redirect to this week's subject, hermit crabs.
Now many a landlubber has observed hermit crabs crawling around in tide pools so you don't have to be an experienced SCUBA diver to witness them (but it helps!). For that matter, many SCUBA divers miss them completely, thinking the shells they see on the ocean floor are snails. Well, they were but once deceased, hermit crabs may select them for their home. How do you tell the difference? Snails are slow and smoothly crawl away while hermits are faster and more erratic as they try to escape our prying eyes.
Hermit crabs are a group of over 1,000 species crustaceans classified in the super family Paguroidea. My family is pretty super too. They (the crabs, not my family) are separate from the true crabs in the classification Brachyura. The differences can be summarized by the nature of the abdomen. True crabs have hard, calcified abdomens and tails. Hermit crabs are softies with no calcification on their long "bellies."
Because of the soft abdomen, hermits require added protection from predators. Most choose the discarded shells of snails to avoid the embarrassment of having a hungry fish chow down on them. Back in the daze when I was teaching marine biology at the Catalina Island School, I devised an interesting lab exercise. We collected hermit crabs from tidepools, then identified the snail shells they were using. The "porpoise" was to compare which shells they chose vs the abundance of that kind of snail in the habitat. Hermits can vigorously compete with their brothers and sisters for empty shells. This was an example of resource utilization. Some hermits like the ones I filmed in the Philippines also carry stinging sea anemones on their shells to help discourage predators!
Hermit crabs are generally scavengers. It is said they are not fussy eaters and are willing to try most anything. Algae and dead meat from other marine critters are consumed with pleasure. However, in my night dives on our Pacific coast I have observed hermit crabs acting as predators! Under cover of darkness with shell-crushing predators like sheephead sound asleep, I often see a number of hermit crabs up in colonies of gorgonians nipping at the tasty zooids.
Like most critters, hermit crabs grow if food is available. Since the anterior portion of their body has a hard, calcified exoskeleton, there is a limit to how big they can get. When they get too big for their britches, hermit crabs (and other hard bodies) split their exoskeletons and grow a new, larger one. Divers can occasionally find the discarded exoskeletons or molts.
Their snail shell homes also make for an interesting sex life! I have observed pairs of hermits appearing to be mating while each is still inside their shell. Of course it may have just been foreplay. Males may battle one another for the right lady (humans exhibit similar behavior on occasion). Some have said the male may try to coax the female out of her shell by tapping or rocking it (rock my world!). The females genital openings are on the first segment of the rear legs while the male's reproductive opening is on its fifth pair of appendages.
Sadly for the hermits, copulation may last but a few seconds and few have the staying power to continue as long as a minute. The eggs are fertilized and usually cast into the sea. The first two stages of development occur within the egg itself. Most larvae hatch out of the egg in the third stage known as the zoea. This stage can often be seen in plankton tows. The zoea has long spines and large fuzzy antennae which help it remain in the water column without sinking. This larval stage undergoes several molts before reaching the final stage known as the megalopa.
While most live in the marine environment, there is at least one species that lives in freshwater. There is also a group of hermits that live on land! The coconut crab Birgus latro is one example, and it is shell-less. I guess that could be because land snails aren't as common and often have thin shells. The land hermits need to be reasonably close to water for reproduction. I need to be close to water, too... but not necessarily for that "porpoise."
© 2018 Dr. Bill Bushing. For the entire archived set of over 750 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Hairy hermit crab and molt from one; hermit crab from the Philippines with sea anemones and hermits feeding on gorgonian.
Ah, "Mrs Brown You've Got A Lovely Daughter..." "I'm Henry the VIII, I Am..." "Can't You Hear My Heartbeat..." and what a "Wonderful World" this could be. Oops, sorry. I got distracted by nostalgia for the original British invasion group Herman's Hermits. My columns aren't about musical history... my focus is on the critters of the sea. Therefore I'll redirect to this week's subject, hermit crabs.
Now many a landlubber has observed hermit crabs crawling around in tide pools so you don't have to be an experienced SCUBA diver to witness them (but it helps!). For that matter, many SCUBA divers miss them completely, thinking the shells they see on the ocean floor are snails. Well, they were but once deceased, hermit crabs may select them for their home. How do you tell the difference? Snails are slow and smoothly crawl away while hermits are faster and more erratic as they try to escape our prying eyes.
Hermit crabs are a group of over 1,000 species crustaceans classified in the super family Paguroidea. My family is pretty super too. They (the crabs, not my family) are separate from the true crabs in the classification Brachyura. The differences can be summarized by the nature of the abdomen. True crabs have hard, calcified abdomens and tails. Hermit crabs are softies with no calcification on their long "bellies."
Because of the soft abdomen, hermits require added protection from predators. Most choose the discarded shells of snails to avoid the embarrassment of having a hungry fish chow down on them. Back in the daze when I was teaching marine biology at the Catalina Island School, I devised an interesting lab exercise. We collected hermit crabs from tidepools, then identified the snail shells they were using. The "porpoise" was to compare which shells they chose vs the abundance of that kind of snail in the habitat. Hermits can vigorously compete with their brothers and sisters for empty shells. This was an example of resource utilization. Some hermits like the ones I filmed in the Philippines also carry stinging sea anemones on their shells to help discourage predators!
Hermit crabs are generally scavengers. It is said they are not fussy eaters and are willing to try most anything. Algae and dead meat from other marine critters are consumed with pleasure. However, in my night dives on our Pacific coast I have observed hermit crabs acting as predators! Under cover of darkness with shell-crushing predators like sheephead sound asleep, I often see a number of hermit crabs up in colonies of gorgonians nipping at the tasty zooids.
Like most critters, hermit crabs grow if food is available. Since the anterior portion of their body has a hard, calcified exoskeleton, there is a limit to how big they can get. When they get too big for their britches, hermit crabs (and other hard bodies) split their exoskeletons and grow a new, larger one. Divers can occasionally find the discarded exoskeletons or molts.
Their snail shell homes also make for an interesting sex life! I have observed pairs of hermits appearing to be mating while each is still inside their shell. Of course it may have just been foreplay. Males may battle one another for the right lady (humans exhibit similar behavior on occasion). Some have said the male may try to coax the female out of her shell by tapping or rocking it (rock my world!). The females genital openings are on the first segment of the rear legs while the male's reproductive opening is on its fifth pair of appendages.
Sadly for the hermits, copulation may last but a few seconds and few have the staying power to continue as long as a minute. The eggs are fertilized and usually cast into the sea. The first two stages of development occur within the egg itself. Most larvae hatch out of the egg in the third stage known as the zoea. This stage can often be seen in plankton tows. The zoea has long spines and large fuzzy antennae which help it remain in the water column without sinking. This larval stage undergoes several molts before reaching the final stage known as the megalopa.
While most live in the marine environment, there is at least one species that lives in freshwater. There is also a group of hermits that live on land! The coconut crab Birgus latro is one example, and it is shell-less. I guess that could be because land snails aren't as common and often have thin shells. The land hermits need to be reasonably close to water for reproduction. I need to be close to water, too... but not necessarily for that "porpoise."
© 2018 Dr. Bill Bushing. For the entire archived set of over 750 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Hairy hermit crab and molt from one; hermit crab from the Philippines with sea anemones and hermits feeding on gorgonian.
