91 — Crab megalopa

Crab megalopa (live), Piha, 2/05//15.

Crab megalopa (live), Piha, 2/05//15.

Crab megalopa (dead), Piha, 2/05/15.

The lifecycle of a crab is really interesting and larval crabs are quite different from adults. In fact larval crabs were considered to be different species, until it was shown that they were just juvenile forms.

After a crab larvae hatches, it joins the plankton. Then, as they grow they go through several developmental stages, shedding their exoskeleton between each stage. Some of these stages bear very little resemblance to the adult crab. The megalopa stage is the last planktonic stage, during which the crab settles and then later moults and emerges as a small version of the adult form.

The cues controlling where and when a megalopa will settle are very important to the crab's survival, as various crab species are adapted to survive in particular habitats. Ambient sound is believed to be an important settling cue, with some crab species being more sensitive than others. For example, megalopae of the mud crab, Austrohelice crassa (Dana, 1851), are not very sensitive to sound cues and it is thought that this could be related their choice of habitat: soft sediments with little wave action (low ambient noise). Conversely the megalopae of the rocky-reef crab, Leptograpsus variegatus, (Fabricius, 1793) are quite sensitive to sound cues and their preferred habitats are rocky reefs with moderate to high wave exposure (high ambient noise).

These two megalopa were found in the cove at south Piha 2/05/15 and are probably (educated guess here) L. variegatus. They were about 1 cm across.




More info:

Stanley JA, Radford CA, Jeffs AG 2011. Behavioural response thresholds in New Zealand crab megalopae to ambient underwater sound. PLoS ONE 6(12): e28572. doi:10.1371/journal.pone.0028572