Getting into and out of mussels
The male pea crab leaves the safety of his green-lipped mussel host when it’s time to mate. He’s looking for a mussel that contains a female crab who’s also ready to mate – but how does he know which mussels contain females? And how does he avoid getting killed as he searches?
The risky life of a male pea crab
To mate, a male pea crab must leave the relative safety of his host mussel to seek out a female. In doing so, he is at risk of death by crushing (between the two shells of a mussel as it closes) and by predation (usually by fish). In this video, Oliver Trottier (Leigh Marine Laboratory) describes the risks taken by male pea crabs in search of a female.
Teaching point:
After watching this video, students could read the article Getting into and out of mussels.
The risky life of a male pea crab
To mate, a male pea crab must leave the relative safety of his host mussel to seek out a female. In doing so, he is at risk of death by crushing (between the two shells of a mussel as it closes) and by predation (usually by fish). In this video, Oliver Trottier (Leigh Marine Laboratory) describes the risks taken by male pea crabs in search of a female.
Teaching point:
After watching this video, students could read the article Getting into and out of mussels.
Oliver Trottier at the Leigh Marine Laboratory (University of Auckland) has been investigating these questions. His research so far suggests that male pea crabs can ‘smell’ which mussels contain females and that they use an unusual tickling technique to encourage those mussels to open and let them in.
Getting in and out: a dangerous time
The ocean is a dangerous place for a male pea crab. Each time he leaves his mussel host to find a mate, he risks getting crushed by his host mussel or the host mussel of a female crab. His crush risk is about 15% for every journey he makes into or out of a mussel. On top of that, he’s at risk of being eaten by fish such as snapper and spotties. These and other reef fish species are very common around mussel beds as they are hot spots for diverse marine life.
A long wait to get into the mussel shell
The long tickle
At mating time, male New Zealand pea crabs leave the safety of their host green-lipped mussel to search for a female. In this video, Oliver Trottier (Leigh Marine Laboratory) describes how he used infrared video cameras to document the behaviour of male crabs as they attempted to enter mussels containing female crabs. Oliver observed the crabs ‘tickling’ mussels, sometimes for hours on end. He thinks the tickling may stimulate a mussel to open wide enough to let the crab in.
Jargon alert:
The inhalant siphon is the structure through which water flows into the mussel.
The mantle edge is the frilly structure around the outer edge of the mussel’s flesh.
Focus questions:
What problem did Oliver face before he could begin observing pea crabs?
Oliver thinks that the male crab’s tickling might encourage the tickled mussel to open more widely. How could he test his hypothesis
The long tickle
At mating time, male New Zealand pea crabs leave the safety of their host green-lipped mussel to search for a female. In this video, Oliver Trottier (Leigh Marine Laboratory) describes how he used infrared video cameras to document the behaviour of male crabs as they attempted to enter mussels containing female crabs. Oliver observed the crabs ‘tickling’ mussels, sometimes for hours on end. He thinks the tickling may stimulate a mussel to open wide enough to let the crab in.
Jargon alert:
The inhalant siphon is the structure through which water flows into the mussel.
The mantle edge is the frilly structure around the outer edge of the mussel’s flesh.
Focus questions:
What problem did Oliver face before he could begin observing pea crabs?
Oliver thinks that the male crab’s tickling might encourage the tickled mussel to open more widely. How could he test his hypothesis
The male pea crab’s life is made even more dangerous by his method of gaining entry to the female’s host mussel. Using an infrared camera to film crabs at night, Oliver showed that the male crab can spend hours on the shell of a mussel containing a female, apparently ‘tickling’ it on and off to gently encourage it to open wide enough for him to climb in! The average ‘tickle time’ observed by Oliver was about 3 hours, and throughout that period, the male crab is at risk of being eaten by predators.
Finding a mate through smell?
Because leaving his mussel is so dangerous, it’s likely that the male pea crab has a mechanism for detecting which mussels contain a female crab. Without this, he’d enter too many shells that had no female inside and be crushed or eaten before finding a mate. Oliver’s observations from the ‘tickle’ experiment seemed to suggest this too. He noticed that the crabs spent far more time on mussel shells that had females inside.
Oliver hypothesised that female crabs were releasing a pheromone (a chemical ‘scent’) into the surrounding seawater when they were ready to mate. This would let males know which mussel shells contained receptive females.
How male pea crabs might find females
Male pea crabs need to work out which mussels contain females so they can avoid getting crushed by a closing mussel shell while searching for a mate. It is likely that males detect a pheromone that is released by females who are ready to mate. This chemical signal would guide the male crab to a mussel that contains a female.
Testing the pheromone idea
Oliver set up an experiment to look at whether male pea crabs could detect a chemical released by female pea crabs. He used a plastic tube with seawater flowing through it. Within the tube, he placed two mussels – one containing a male crab and the other a female. The male crab’s mussel was placed downstream to make sure the male crab could detect any scent being released by the female.
Crucially, Oliver included a trap in the tube between the two mussels – so if the male crab detected a pheromone from the female and left his mussel to find her, he would fall into the trap and be stuck there. Male crabs are more likely to leave their shells under cover of darkness, so Oliver set up his first experiment one afternoon and left it overnight. He says he was “pretty stoked” in the morning when he opened the trap to find a male crab inside.
Oliver’s pheromone hypothesis
Pheromones are chemical ‘smells’ that are released by individuals of a species and detected by other members of the same species. Oliver Trottier has proposed that female New Zealand pea crabs release a pheromone when they are ready to mate and that male pea crabs follow the ‘scent’ of the pheromone to locate the receptive female within her host mussel. In this video, Oliver discusses what pheromones are and describes his ideas about how pea crabs use them.
Teaching point:
After watching this video, students could read the article, Getting into and out of mussels.
Oliver’s pheromone hypothesis
Pheromones are chemical ‘smells’ that are released by individuals of a species and detected by other members of the same species. Oliver Trottier has proposed that female New Zealand pea crabs release a pheromone when they are ready to mate and that male pea crabs follow the ‘scent’ of the pheromone to locate the receptive female within her host mussel. In this video, Oliver discusses what pheromones are and describes his ideas about how pea crabs use them.
Teaching point:
After watching this video, students could read the article, Getting into and out of mussels.
Testing the pheromone hypothesis
Oliver Trottier (Leigh Marine Laboratory) set out to test whether pea crabs use pheromones at mating time. In this video, Oliver talks us through his experimental set-up. He demonstrates how male and female pea crabs (each in their host mussels) were placed close to each other in flowing seawater and shows how he used a trap to test whether male crabs were leaving their hosts in response to a signal from female crabs.
Teaching point:
Try showing this clip without sound. You could ask students to write about, draw a sketch of or explain Oliver’s experimental design. What are the strengths of Oliver’s experimental set-up?
Point of interest:
Leigh Marine Laboratory is situated right beside the sea, within the Cape Rodney-Ōkakari (Goat Island) Marine Reserve. This site means that the laboratory has access to piped seawater from the ocean. It is this seawater that is flowing through the pheromone tubes in Oliver’s experiment.
Testing the pheromone hypothesis
Oliver Trottier (Leigh Marine Laboratory) set out to test whether pea crabs use pheromones at mating time. In this video, Oliver talks us through his experimental set-up. He demonstrates how male and female pea crabs (each in their host mussels) were placed close to each other in flowing seawater and shows how he used a trap to test whether male crabs were leaving their hosts in response to a signal from female crabs.
Teaching point:
Try showing this clip without sound. You could ask students to write about, draw a sketch of or explain Oliver’s experimental design. What are the strengths of Oliver’s experimental set-up?
Point of interest:
Leigh Marine Laboratory is situated right beside the sea, within the Cape Rodney-Ōkakari (Goat Island) Marine Reserve. This site means that the laboratory has access to piped seawater from the ocean. It is this seawater that is flowing through the pheromone tubes in Oliver’s experiment.
Next steps
Oliver’s next task is to nail down exactly what pheromone the female pea crab is releasing. If the identity of the pheromone can be determined, the pheromone might be used to lower the infection level of pea crabs on mussel farms.
Find out more in the article, Biocontrol of the New Zealand pea crab.
