By Paul Caiger.
The packhorse is a species of spiny lobster (aka crayfish) that only lives in this corner of the world, predominantly in northern New Zealand and eastern Australia. Packhorse lobsters are among the most primitive lobster species, with a unique olive green colouration and smooth abdomen, and have recently been moved into their own genus. Unrivalled for size, it is the largest of all the spiny lobsters, with unconfirmed reports of gargantuan 20kg animals almost 700mm in length – and along with the clawed lobster (Homarus americanus), are the largest decapod crustaceans in the world.
However, it is the remarkable early stages of their life history that is surely the most fascinating aspect of the largest spiny lobster in existence. They begin life as tiny larvae, starting out on an extraordinary journey which can take them thousands of kilometres, both within the water column and along the sea floor. Adult packhorse mate around October, then females head into slightly deeper water (100m or deeper) to hatch their 0.5–2 million eggs during spring.
Following a nine-month pelagic larval duration (red rock lobster is 18–24 months!) involving over a dozen different larval stages, the leaf-like phyllosoma metamorphose one last time into a still transparent peurulus and head for the reef. There is very little transport of larvae across the Tasman Sea, meaning New Zealand and Australian populations are largely genetically distinct. After settling onto the coast, the lobsters continue to undertake huge migrations. This was discovered in the late ’70s when scientists found that they were constantly migrating north, ultimately to join the breeding populations that accumulate within 25km of Cape Reinga, and so counteracting the current flow (and thus larval movement). In fact, one known tagged individual had walked 1070 kilometres! Supporting this discovery is that the proportion of undersized packhorse lobsters increases with distance south along the North Island’s east coast, with very few egg-bearing females caught south of Whangarei Heads.
Their long, variable and complex reproductive strategy leaves the packhorse lobster vulnerable to overfishing, especially when combined with natural disturbances such as strong El Niño. Anecdotally, there is an apparent decline in packhorse lobsters in diving depths. Furthermore, there are concerns about the lack of basic biological data and stock assessment on this species. However, overall the huge packhorse is a remarkably hardy beast, and remains one of the most enthralling animals in our coastal seas.
1 The largest spiny lobster in the World.
2 In Maori, known as pawharu.
3 The name Sagmarasius derives from the Greek sagmarion, meaning packhorse.
4 Undertake huge migrations, with one known individual known to have walked 1070 kilometres!
5 Most migrate to the top of the north island where they accumulate to breed.
6 New Zealand and Australian populations are genetically distinct.
7 Communicate with each other using urine.
8 Go through more than a dozen larval stages over the course of nine months before settling onto reefs as peurulus.