Having a Whale of a Time: A Barnacle’s Odyssey.
Whales might not stand out at first as an extreme marine habitat, but living in this environment requires unique adaptations. This article aims to briefly describe the challenges faced by cetacean barnacles and how there adaptations enable them to successfully colonise this unique and extreme environment.
Reproduction and Settlement
Whales are migratory and many move in great numbers to shared breeding grounds, the barnacle makes use of these events and times its reproduction to coincide allowing the greatest exposure to larger numbers of more densely aggregated individuals compared to their feeding grounds.
The barnacle’s life history consists of two halves, a free swimming larval stage and a sessile adult. The free swimming barnacle larva float in the open ocean feeding and growing until they reach a stage at which it can settle on a suitable substrate and grow to maturity. A study from japan found that barnacle larvae are only triggered to attach to a substrate in the presence of chemical cues from the cetacean skin.Even after successful settlement, the barnacle larva doesn’t stay in the place it first lands. The larva will crawl to a preferred location, one that receives a steady flow of water such as the nose, lower jaw or fin tips. After settlement and finding a suitable location, the larvae securely cement themselves with special adaptations not seen in other barnacles and begin to metamorphoses into their adult form.
But it’s not over yet, throughout the barnacles adult life it must remain attached to its host, this means withstanding traveling at speed and being dislodged whilst its host breaches the water in acrobatic displays. The cetacean barnacles remain attached in the same way as other barnacles by laying a hard substrate at their base in the form of basal plates. In cetacean barnacles however one might expect the barnacle to be shed in the regular cycle of epidermis renewal, this is not the case as the barnacle cement to the epidermis preventing it from being shed. As the barnacle grows it lays new basal material which acts to push surrounding epidermis outwards which sinks the barnacle into the skin of its host. Furthermore, the attachment of the barnacle to the host’s skin may invoke a defensive response to add stiff keratinous material to the surrounding area which, unintentionally further secures the barnacle.
Cetacean barnacles have evolved in two forms which help to prevent dislodgement, there is the typical barnacle shape consisting of a squat mound of calcium plates and the more unusual stalked form as can be seen in the image below. In the Squat form the barnacle has a wide basal disc and smooth plates that help to prevent being dislodged. In the stalked variation, the barnacle is flexible allowing it to absorb energy and remain attached.
A Constantly Changing Environment
Life atop a whale stands out from other habitats as the environmental conditions are changing constantly on multiple time scales. Whales are constantly moving, changing their position in the water column from shallow surface waters to depths of close to 3000m, in the case of the Curvier’s beaked whale as can be seen alongside other species in the diagram below. This means there is a constant change in pressure (up to 300 times the pressure of our atmosphere), temperature and salinity over scales of minutes.
Over hours and day’s whales constantly travel in search of food exposing their attached fauna to further variability in water temperature and food availability. Over an entire year, a whale may also migrate to warmer or cooler waters, in the case of the humpback whale an epic migration of nearly 12,000km from cold Antarctic waters to the tropical south pacific in search of warmer breeding grounds in which to breed and birth their calves (Image below).
Being constantly on the move the fauna attached to whales must also be able to withstand dislodgement. In the case of the acrobatic Humpback whale, the attached fauna must be able to withstand the large amounts of energy imparted upon impact after an impressive breach as can be seen in the video below.
The Benefits of Living Atop a Whale
What is the benefit to living on a whale? Well as hard as it might be to remain on a mobile substrate and tolerate the ever-changing physical conditions, the host does provide some attractive benefits to a filter feeding invertebrate.
Being constantly mobile means that the cetacean barnacle avoids being predated on by other animals such as fish that might predate on barnacles attached to a non-mobile substrate. Constantly travelling atop a whale means the barnacle doesn’t have to rely on currents to bring its food, instead the barnacle will travel with the whale to locations that have a higher density of food. This is especially beneficial in the baleen whales feeding on krill in nutritious phytoplankton rich waters. Furthermore the reproductive success is also likely to be improved with the high concentration of possible hosts during migratory behaviours.
As we can see, the life for a cetacean barnacle is not all plain sailing, they must beat the odds to find their host and adapt continually to rapidly changing physical conditions, however, the benefits of inhabiting this extreme environments are numerous.