Comparing comments on the ‘spiral death tracks’ of fossil juvenile horseshoe crabs found at Solnhofen with the normal spiral foraging trails of juvenile horseshoe crabs in their natural habitat on an intertidal mudflat in Hong Kong.

This post is one in a series of posts which compares the depositional environment of the late Jurassic lithographic limestones at Solnhofen in Germany based on extracts from Solnhofen: A Study in Mesozoic Palaeontology with observations made at a modern inter-tidal mudflat at Ha Pak Nai, Deep Bay, New Territories, Hong Kong and proposes an inter-tidal mudflat origin for the examples cited.

This post compares references to the presence of spiral death tracks made by horseshoe crabs at Solhofen - Eichstatt with photographs of spiral foraging tracks made by juvenile horseshoe crabs in their natural habitat on intertidal mudflats in Hong Kong.

Extract from: Solnhofen: A Study in Mesozoic Palaeontology

Species: Mesolimulus walchi

“A very few, relatively tolerant animals were able to survive the hypersaline milieu for a short time, perhaps a matter of hours, but all their traces end in the death of the beast. Washed into the stagnant, salty basin, they were merely able to crawl in a disoriented spiral before collapsing in their tracks. Most famous and comparatively common are the tracks of the horseshoe crab, Mesolimulus [See Figiures 5.5 and 5.7].” (Barthel et al - Pages 77 & 79).

“The other type of trace is the spiral death track with the body at the end.” (Barthel et al - Page 79).

“Limulids burrow in the mud to shallow depths in search of small invertebrates for food. They may spend short periods out of the water, and they are particularly tolerant to fluctuations in the salinity or temperature of the water. Consequently, it is not surprising they were one of the few animals still alive when they reached the poisonous lagoon floor, being responsible for some of the most famous spiral tracks or ‘death marches’ which terminate with the body at the centre.” (Barthel et al - Page 140).

“… the juvenile horseshoe crab, Mesolimulus and the crustacean Mecochirus were still alive for a short time on the lagoon floor. These creatures produced the famous spiraling trails with the body lying at the centre, interpreted as the last, disorientated crawl of a moribund animal before it collapsed in its tracks” (Barthel et al - Page 58).

Comments on juvenile horseshoe crab habitats and ecology in Hong Kong

There are four species of horseshoe crab living today, Limulus polyphemus,Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda. L. polyphemus  occurs along the east coast of the USA and Mexico. The other three species occur in South-East Asia. T. tridentatus and C. rotundicauda occur in Hong Kong.

When juvenile T. tridentatus and C. rotundicauda forage in Hong Kong, they leave spiral foraging trails. Such spiral trails represent normal feeding patterns and rather than being ‘disorientated’, they are suggestive of a systematic search for food, ensuring the same ground is not covered twice. In fact when searching for juvenile horseshoe crabs on intertidal mudflats, one of the first signs to look for is the presence of spiral foraging trails.

Spiral foraging and movement trails are therefore the norm, not the exception, so ‘spiral death tracks’ are likely the result of normal activity, albeit one that is fatal in its outcome.

Photographs of spiral foraging trails of juvenile T. tridentatus on an intertidal mudflat

 in Hong Kong

References

Barthel, K.W., Swinburne, N.H.M., and Conway Morris, S. (1994). Solnhofen: A Study in Mesozoic Palaeontology. Cambridge University Press, Cambridge.

Comparing comments on fossil juvenile horseshoe crabs found at Solnhofen spending “short times out of water” with sub-aerial activity of juvenile horseshoe crabs in their natural habitat on an intertidal mudflat in Hong Kong.

This post is one in a series of posts which compares the depositional environment of the late Jurassic lithographic limestones at Solnhofen in Germany based on extracts from Solnhofen: A Study in Mesozoic Palaeontology with observations made at a modern inter-tidal mudflat at Ha Pak Nai, Deep Bay, New Territories, Hong Kong and proposes an inter-tidal mudflat origin for the examples cited.

This post compares references of juvenile horseshoe crabs found at Solhofen - Eichstatt spending short periods out of the water with photographs of sub-aerial foraging and movement by juvenile horseshoe crabs in their natural habitat on the intertidal mudflats at Ha Pak Nai.

Extract from: Solnhofen: A Study in Mesozoic Palaeontology

Species: Mesolimulus walchi

“Limulids burrow in the mud to shallow depths in search of small invertebrates for food. They may spend short periods out of the water, and they are particularly tolerant to fluctuations in the salinity or temperature of the water.” (Barthel et al - Page 140).

Comments on juvenile horseshoe crabs spending short periods out of the water in Hong Kong

There are four species of horseshoe crab living today, Limulus polyphemus,Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda. L. polyphemus  occurs along the east coast of the USA and Mexico. The other three species occur in South-East Asia. T. tridentatus and C. rotundicauda occur in Hong Kong.

In Hong Kong juvenile horseshoe crabs forage on intertidal mudflats and in areas in and around mangroves.

The foraging activity may take place in shallow water in stream beds or in the pools of water that form between seagrass bed mounds or sub-aerially when the tide has fully receded. Sub-aerial foraging and  movement is normal practice for juveniles of both T. tridentatus and C. rotundicauda and the juveniles can spend extended periods of time engaged in sub-aerial activity, as long as the surface of the sediment is moist or they are or they are not exposed for prolonged periods to high temperatures or direct sunlight. In addition, quite a lot of foraging activity is conducted on the surface of the sediment, not buried in it.

Photographic evidence from Ha Pak Nai. 

Juvenile T. tridentatus running across the surface of an intertidal mudflat.

Juvenile T. tridentatus foraging, then walking across the surface of an intertidal mudflat.

Sub-aerial juvenile T. tridentatus foraging trails, made on the surface of an intertidal mudflat.

References

Barthel, K.W., Swinburne, N.H.M., and Conway Morris, S. (1994). Solnhofen: A Study in Mesozoic Palaeontology. Cambridge University Press, Cambridge.

Comparing the fossil juvenile horseshoe crab moults found at Solnhofen with juvenile horseshoe crab moults and moulting in their natural habitat on an intertidal mudflat in Hong Kong.

This post is one in a series of posts which compares the depositional environment of the late Jurassic lithographic limestones at Solnhofen in Germany based on extracts from Solnhofen: A Study in Mesozoic Palaeontology with observations made at a modern inter-tidal mudflat at Ha Pak Nai, Deep Bay, New Territories, Hong Kong and proposes an inter-tidal mudflat origin for the examples cited.

This post compares references to the presence of horseshoe crab moulting and moulted exoskeletons found at Solhofen - Eichstatt with photographs of juvenile horseshoe crab moults and moulting in their natural habitat on intertidal mudflats in Hong Kong.

Extract from: Solnhofen: A Study in Mesozoic Palaeontology

Species: Mesolimulus walchi

“The other type of trace is the spiral death track with the body at the end. It has been suggested that these represent moulting activity but this cannot be the case as there are no marks made by the animal after it had crawled out of its skin” (Barthel et al - Page 79).

“Some of the fossils are the remains of moulted exoskeletons rather than body fossils. The moults are easily distinguished because they tend to be incomplete, distorted and lack the relief of the body fossils. The moulting activity must have taken place outside the lagoon and the moults transported to the lagoon independently.” (Barthel et al - Page 140).

Comments on juvenile horseshoe crab moulting activity in Hong Kong

There are four species of horseshoe crab living today, Limulus polyphemus,Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda. L. polyphemus  occurs along the east coast of the USA and Mexico. The other three species occur in South-East Asia. T. tridentatus and C. rotundicauda occur in Hong Kong.

Being arthropods, juvenile horseshoe crabs moult to grow and depending on the species, they may moult up to eighteen times before reaching maturity (Sekiguchi.1988). Juveniles horseshoe crabs develop in nursery areas in the intertidal zone, where they will spend up to sixteen years, according to species, feeding and growing, regularly moulting as they grow in size.

Where juvenile moulting (ecdysis) has been observed in the wild in Hong Kong it takes place once the tide retreats. As the tide retreats, the juvenile will emerge from the substrate and remain still while the entire exoskeleton is moulted. Once separated from its’ old exoskeleton (exuviae), the newly emerged animal will expand in size, then take up to one hour for the chitin in its’ new exoskeleton to harden before it moves off.

The discarded moult (exuviae), being lighter than water is usually picked up on the next incoming tide and deposited on the high tide line.

Video of juvenile T. tridentatus moulting (in a pool)

https://www.youtube.com/watch?v=HZjuCc22NoI

Juvenile T. tridentatus moulting - sub-aerial on moist sediment

T. tridentatus moult floating in a pool of water on an incoming tide

T. tridentatus moults found along the high tide line at Ha Pak Nai

Size range of juvenile T. tridentatus moults found along the high tide line at Ha Pak Nai in 2009

References

Barthel, K.W., Swinburne, N.H.M., and Conway Morris, S. (1994). Solnhofen: A Study in Mesozoic Palaeontology. Cambridge University Press, Cambridge.

Comparing the fossil juvenile horseshoe crabs found at Solnhofen with juvenile horseshoe crabs in their natural habitat on intertidal mudflats in Hong Kong.

This post is one in a series of posts which compares the depositional environment of the late Jurassic lithographic limestones at Solnhofen in Germany based on extracts from Solnhofen: A Study in Mesozoic Palaeontology with observations made at modern intertidal mudflats at Ha Pak Nai, Deep Bay, New Territories; Yi O on Lantau and Luk Geng in the New Territories, Hong Kong and proposes an intertidal mudflat origin for the examples cited.

This post compares references to the presence of juvenile horseshoe crabs found at Solhofen - Eichstatt with photographs of juvenile horseshoe crabs in their natural habitat on three intertidal mudflats in Hong Kong.

Extract from: Solnhofen: A Study in Mesozoic Palaeontology

Species: Mesolimulus walchi

“… the juvenile horseshoe crab, Mesolimulus and the crustacean Mecochirus were still alive for a short time on the lagoon floor.” (Barthel et al - Page 58).

“The individuals which crawled and died on the plattenkalk sediment in the Solnhofen- Eichstatt area are all juveniles” (Barthel et al - Pages 77 & 79).

“Almost all fossil limulids from the western area of Solnhofen and Eichstatt are thought to be juveniles” (Barthel et al - Page 140).

Comments on juvenile horseshoe crab habitats and ecology in Hong Kong

There are four species of horseshoe crab living today, Limulus polyphemus,Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda. L. polyphemus  occurs along the east coast of the USA and Mexico. The other three species occur in South-East Asia. T. tridentatus and C. rotundicauda occur in Hong Kong.

Each of the living species of horseshoe crabs has different habitat requirements during their lives, related to spawning, juvenile development and adulthood.

In all living horseshoe crab species, spawning takes place on the high tide mark. After spawning, eggs take up to 45 days to develop. Once hatched, small trilobite larvae are released and according to the species, these develop in the spawning area, or very close to it.

About one year after hatching, small juveniles move to a nursery area in the intertidal zone, where they will spend up to sixteen years, according to species, feeding and growing, regularly moulting as they grow in size. Depending on the species, they may moult up to eighteen times before reaching maturity (Sekiguchi.1988).

In Hong Kong those nursery areas are on open intertidal mudflats, often associated with seagrass beds in the case of T. tridentatus and in and around mangroves in the case of C. rotundicauda. The juveniles of these two species in Hong Kong also appear to have preferences for substrate type in their intertidal areas where they forage.

Intertidal mudflats in Hong Kong can be broadly categorized as intertidal mudflats with a predominant sand component and intertidal mudflats with a predominant mud component.This is important in understanding juvenile horseshoe crab ecology because in Hong Kong juvenile T. tridentatus favour intertidal mudflats with a high sand component, whilst juvenile C. rotundicauda favour intertidal mudflats with a high mud component

In addition, because of these preferences, whilst juveniles of these two species occur sympiatrically at a number of locations, their differing spawning and juvenile habitat requirements suggest their substrate foraging preferences mean they do not directly compete with each other.

Photographs of juvenile horseshoe crabs and their habitats in Hong Kong

Intertidal mudflat at Ha Pak Nai, with Halophila beccari seagrass beds

Juvenile T. tridentatus foraging trail on intertidal mudflat at Ha Pak Nai

Series of T. tridentatus juvenile moults collected at Ha Pak Nai in 2009

Photograph showing the size range of the juvenile moults collected at Ha Pak Nai in 2009

Intertidal mudflats with juvenile C. rotundicauda

Yi O, Lantau

Luk Geng, New Territories

References

Barthel, K.W., Swinburne, N.H.M., and Conway Morris, S. (1994). Solnhofen: A Study in Mesozoic Palaeontology. Cambridge University Press, Cambridge.

Sekiguchi. K. (1988). Biology of Horseshoe Crabs. Science House Co., Ltd, Tokyo

Juvenile horseshoe crab behaviour – self righting

This post documents the behaviour of a juvenile horseshoe crab (Carcinoscorpius rotundicauda) at an inter-tidal mudflat at Luk Geng, New Territories, Hong Kong.

This photograph was taken just as the tide was receding and shows a juvenile C. rotundicauda on its back trying to self-right itself using its tail. The tail marks left by the animal’s actions can clearly be seen in the muddy substrate (if horseshoe crabs end up on their backs, they use their tail as a lever to self-right themselves).

Juvenile horseshoe crab behaviour - burrowing

This post documents the behaviour of juvenile horseshoe crabs (Tachypleus tridentatus) based on observations made at an inter-tidal mudflat at Ha Pak Nai, Deep Bay, New Territories, Hong Kong.

All observable juvenile horseshoe crab activity on the mudflat appears to be synchronized with the tides. When the tide comes in, the juveniles bury themselves in the substrate. When the tide recedes, they emerge to forage.

The following photographs show two instances of juvenile T. tridentatus burrowing on the exposed mudflat:

Burrowing 1

Juvenile T. tridentatus sprints across the mudflat, then burrows into the sandy substrate.

Burrowing 2

Juvenile T. tridentatus burrowing into sandy substrate

Comment

Understanding the way that tidal cycles dictate the foraging and burrowing activities of juvenile horseshoe crabs may help in the interpretation of the palaeo-environment of other localities with horseshoe crab fossils, such as Mazon Creek.

Juvenile horseshoe crab behaviour – walking, wandering and sprinting

This post documents the behaviour of juvenile horseshoe crabs (Tachypleus tridentatus) based on observations made at an inter-tidal mudflat at Ha Pak Nai, Deep Bay, New Territories, Hong Kong.

All observable juvenile horseshoe crab activity on the mudflat appears to be synchronized with the tides. Before the tide comes in, the juveniles bury themselves in the substrate. When the tide recedes, they emerge to forage.

The following are examples of walking, wandering and sprinting behaviour, either after the tide has receded, or after foraging activity has taken place.

Wandering as the tide is receding

This video is of a juvenile T. tridentatus wandering from pool to pool in a well developed seagrass bed, just as the tide is receding, perhaps searching for a new foraging area:

http://www.youtube.com/watch?v=CKdfHu1Wjas

Olympic sprint

These photographs show a juvenile T. tridentatus running at high speed across an exposed area of mudflat and quickly burying itself:

Transition from foraging to walking

The following series of photographs shows the sequence of trails left by a juvenile T. tridentatus as it transitions from foraging to walking:

Pool on sandy area of beach where the photographs below were taken.

Foraging horseshoe crab(s)

Transition from foraging to walking trail (walking starts with tail marks)

Walking horseshoe crab