Cyclograpsus lavauxi

Cyclograpsus lavauxi found in a tidal rock pool

Cyclograpsus lavauxi found in a tidal rock pool

The smooth shore crab, Cyclograpsus lavauxi, is a marine large-eyed crab of the family Grapsidae, found in New Zealand and the Juan Fernández Islands of Chile.^[1]

Description

Cyclograpsus lavauxi is a small, temperate crab in the Grapsidae family.^[2] The crab is known to be the size of a coin,^[3] with the adult males averaging 28 mm and the females averaging 26 mm.^[1] The shell, carapace, is polished ^[3] with slightly more width than length.^[4] On average, the length is 9 mm and the breadth is 11 mm.^[5] The brim of Cyclograpsus lavauxi carapace is smooth, a lot like other crab species.^[3] The anterior of the carapace is linear edged with two eye orbits, which are curved and situated in the two frontal corners. The antennas are well-developed and are used for sensing texture, food, sound etc.^[6] C. lavauxi has eight legs and two large pincers at the front of the crab. Its legs are compressed slender and long, which includes six lines of hair running the full 4mm length of the legs which are fairly robust.^[5] Between the first, second and third legs are also small visible tuffs.^[4] Its long-legs enable it to run at rapids speeds. In addition to this the Cyclograpsus spp. are agile and alert resulting in a very mobile and versatile crab.^[7]

Cyclograpsus lavauxi has been seen in a number of different colours. Their carapace can be the colour of slate blue, bluish grey, fawn, yellowish-brown.^[1] C. lavauxi can also have several shades of grey, green and brown with dark brown and red speckles.^[5] The underside of the crab is a lighter plain colour than the carapace.^[1] The legs carry the same colouring as the carapace with brighter speckled colouring.^[4]

Distribution

Natural global range

The smooth shore crabs natural global range is New Zealand and Chile in the Juan Fernandez Islands.^[1][8]

New Zealand range

C. lavauxi has been found to reside in a number of New Zealand sites from the Hohoura Harbour to Westland ^[1] with significant populations being found present in the North Island.^[5]

Habitat preferences

C. lavauxi occurs in a multitude of habitats as it is such a versatile creature that can withstand extreme weather conditions.^[9] It has been found on boulders under stones and beaches ranging from open and exposed coasts to sheltered bays.^[1] It is a fairly common crab on New Zealand shores and will typically outnumber the larger common rock crab species.^[1] It can be found near the littoral sea fringe where it is wetted only slightly or not at all.^[10] It has also been found in intertidal levels, mudflats, estuaries and rocky shores.^[11] The larger individuals from the Cyclograpsus lavauxi species are often found higher on the foreshore or mudflats than the younger Cyclograpsus lavauxi generations. The younger generation tend to live in the lower lines of the foreshore and mudflats. During the summer months, the Cyclograpsus lavauxi population move down into the lower foreshore line.^[2]

Life cycle/Phenology

Reproduction

The mating season of C. lavauxi is during the summer months from the middle of October to late December.^[12] This mating season is highly synchronous within the population. These crabs are not monogamous and both females and males will mate multiple times during the breeding season.^[13] During this reproduction season, there is a lot of male-male competition where bigger males will often attack other males who are in the process of mating with a female, frequently giving the female an opportunity to escape.^[13] Likely due to this competition, mating pairs can be found under rocks to hide from other males who might interfere.^[13]

Brockerhoff and McLay were able to find that males would approach the females and hold on to her carapace for an hour, after which there was a two hour long copulation.^[13] After the two hours were up, it would abruptly end.^[13] Oftentimes, the female will resist the males and try to escape his clutches.^[13]

It was found that females only carried eggs between November and February.^[14] They also had a very short reception time of less than 24 hours, or almost a week in captivity.^[13] They would oviposit their eggs in synchronization with others in the population within a four-week period.^[13] There are thousands of eggs in each batch laid.^[14] Females who were unable to mate during the breeding season were able to lay fertile eggs by storing sperm from the previous breeding season about a year before.^[13] This extra sperm was stored in the spermatheca after mating.^[13] Females only lay eggs once per mating season.^[12]

The eggs initially are a dark purple color after laying, but as they mature they grow paler and have obvious black eyespots.^[12] The eggs are 0.25-0.3mm in diameter and hatch in about two months.^[14] Once hatched, the larvae are just over 1 mm in length.^[12]

Moulting

C. lavauxi tend to moult soon after the breeding season has ended which means late summer to early autumn.^[13]

Diet and foraging

Cyclograpsis lavauxi is a scavenger crab, which means it eats deceased animals and plants,^[3] although in most cases it is seen as a herbivore.^[5] Its general diet includes algae, drift and seaweed.^[1] The crabs feed only during high tides, and they have adapted the ability to synchronize or modify their feeding pattern to ensure that they feed under optimal conditions, maximizing their feeding hours and nutrition.^[9]

Predators, Parasites, and Diseases

There are a number of different species that instigate predation on C.lavauxi. Because they inhabit the area where the tide does not often reach, their predators are not generally fish but birds.^[15] Their main predators are generalist birds such as Seagulls or Kingfisher.^[9] The major form of predation from fish includes the Smooth Hound Fish, Spined Dogfish, Terakihi, Sea Perch, Moki and Red Gurnard.^[16] One defense mechanism the crab has is being able to live on the beaches where fish are unable to eat them.^[16] They are also at times able to evade the predators by using their quick reactions and speed.^[15]

The nematode Ascarophsis sp. and the parasitic isopod Portunion sp. are found in Cyclograpsus lavauxi.^[17] Brockerhoff ^[18] found that the majority of C. lavauxi are infected by the internal parasite Portunion sp. Brockerhoff’s ^[18] study showed that the reproduction of female C. lavauxi was remarkably reduced by the parasite Portunion sp., this was believed to be because all parasitized females were castrated. She also noted in her study that there were no visible signs of this parasite affecting mating behaviour in the males. The male verse male competition was also not seen to affect the crab that was infected with the parasite Portunion sp. Brockerhoff ^[18] study also suggests that females did not avoid either of the unparasitized and parasitized males because there was no visible disadvantages to either.

Other interesting features of Cyclograpsus lavauxi

Some other intriguing attributes and behaviour of the C.lavauxi is the ability to live successfully under large amounts of sediment. Bacon ^[5] found that Cyclograpsus insularum was only able to tolerate 18% of sediment underneath boulders. When comparing this to C.lavauxi there is a stark discrepancy as C.lavauxi can withstand up to 63% of sediment which is sand and silt. There are a number of other unique behavioural traits associated with C.lavauxi including its aggressiveness when cornered which is rather unusual for a crab, with its pincers raised to accentuate their size along with showing the paleness of their pincers.^[1] Their major body postures include being flat with their pincers folded and laying on the ground with their walking legs close and side-by-side. When in a raised position they place their body at a 45 degree angle with their back part still laying on the ground with their pincers resting still either flat or poised at the midway mark. Their final position is the tiptoe position with their body angled and lifted off the ground and in the air, pincers still folded but held off the ground with their walking legs fully extended.^[1]

Moreover, another fascinating thing that has been found about the C.lavauxi is that they are usually found under stones, with their hind legs grasping stones which pushes the back half of the crab up onto the stones clasping it with its talons with the crab using its other legs to support its weight. This is in order to most likely to increase its mobility and stabilize the crab against the effects of tidal action.^[1] They also use the pincers and talons to dig burrows and push stones away. These holes have been found to be between 20 and 40 mm deep under the cover of rock.^[1] However one of the most perplexing behaviours C.lavauxi is its ability to make clicking noises with the sound acting as a sort of distant artillery.^[1] It is one of the only crabs in New Zealand who can perform such an action.

C.lavauxi has evolved to become a versatile and robust species that is able to cope in extreme conditions that many species of crab in New Zealand cannot. This has resulted in an overall greater abundance and success in their population leading them to be one of New Zealand’s most common crabs. More in-depth studies are needed to fully comprehend the capabilities, genetics and reasons why this crab has become so successful.

References