Horseshoe Crabs Past and Present: a NW Borneo Journey

Horseshoe crabs figured strongly in my week.

Last Saturday (April 1st 2017), I spoke with Kevin Laurie, a Hong Kong-based expert on horseshoe crab behaviour and conservation. He mentioned the existence of a wall decorated with horseshoe crabs in Kampung Ayer, the water village of Bandar, Brunei, in northwest Borneo. Armed with this tip, I visited Kampung Ayer on the evening of the following Thursday (April 6th), guided by two very knowledgeable locals, Wann and Mark Putera Delima.

Kampung Ayer is an interesting place in itself, with Antonio Pigafetta describing the place as he found it when the surviving Victoria and Trinidad of Ferdinand Magellan's fleet visited in July 1521 (shortly after Magellan's death in the Philippines). His description of "Burne" was as follows: "that city is entirely built in salt water, except the houses of the king and certain chiefs. It contains twenty-five thousand hearths. The houses are all constructed of wood and built up from the ground on tall pillars. When the tide is high women go in boats through the settlement selling articles necessary to maintain life". Today, the water village is said to house more than 30,000 people. I was reminded of the more rustic "palofitos" in southern Chile.

Kampung Ayer, April 6th 2017

In all my years, I had never seen a modern horseshoe crab in the wild, but I have long been interested in their evolution. I think this was initially triggered by awareness of the "Coal Age" horseshoe crabs such as Euproops rotundatus, a result of growing up on the Carboniferous of northern England. As a teenager, I had also collected primitive xiphosurans from the Silurian near the town of Leintwardine in Herefordshire. Later, in December 2007, I bought a specimen (labelled "aglaspid") from the Lower Ordovician of Morocco and excitedly recognised it as a horseshoe crab, the earliest known. I wrote to Peter van Roy (an expert in this type of animal) in January 2008, suggesting a collaboration (I had already started drafting a manuscript) and subsequently visited him with the specimen at University College Dublin in July 2008 (there was also an entertaining encounter here with some structural geologists from my past). Many specimens of this Moroccan form have been discovered since, but I still find this specimen to be the best individual example. I also own a fossil of a much younger (roughly a hundred million years old) horseshoe crab from the Upper Cretaceous of Lebanon, showing truly exceptional preservation of limbs, book gills and muscles (in calcium phosphate, apatite), and this specimen will reappear in the story shortly.

The original horseshoe crab, from the Lower Ordovician of Morocco (acquired 2007)

Before continuing, allow me to show some geographical context for the journey. We're on the northwestern margin of Borneo, between the Baram Delta and Brunei Bay.

The island of Borneo and the location of the horseshoe crab adventure

Zooming in on the Brunei Bay area, "horseshoe crabs" marks the location of Kampung Ayer water village.

Before Thursday came around, I had walked to the mouth of Sungai Belait (adjacent to the Baram, near the border with Sarawak) on the Monday evening and found there a couple of carapaces of large horseshoe crabs, my first encounter with tangible signs of the creatures in the modern. This served to whet my appetite for Thursday's river trip.

Carapace of a large horseshoe crab on the beach next to the mouth of Sungai Belait.

Over at Bandar on the Thursday evening, Mark and Wann were waiting at the pier with their small boat. Given the drizzling rain we were glad that the boat had a roof. Equipped with life jackets we crossed the water to the Kampung Ayer "water village" and the house of the distinguished Pengiran Haji Ahmad. The front wall of his house is adorned with a spectacular array of horseshoe crab specimens and further examples decorate an adjacent wall to the left. I counted 109 individuals. Some of these are huge and, briefed by Kevin Laurie, I could recognise them as Tachypleus tridentatus, the Chinese or tri-spine horseshoe crab. The males have a scalloped appearance to the front margin of their carapaces, with two marked indentations. A smaller species of Tachypleus (T. gigas) is also represented on the wall, as well as the smallest species in the area, the mangrove horseshoe crab (Carcinoscorpius rotundicauda), which, as its name suggests, has a rounded (in cross section) tail spine (telson).

Pengiran Haji Ahmad with part of his collection of more than a hundred horseshoe crabs.

The giant ones here are the Chinese or tri-spine horseshoe crab (Tachypleus tridentatus), males having scalloped frontal margins of their prosomas. The little ones are mangrove horseshoe crabs (Carcinoscorpius rotundicauda).

A giant female tri-spine horseshoe crab

A male tri-spine horseshoe crab

We said "Terima kasih" and "Selamat tinggal" to Pengiran Haji Ahmad and continued up the Brunei River to look for monkeys, reptiles and birds and this was where Wann and Mark surprised me with two live horseshoe crabs. After some close inspection and photo taking, back into the water they went. These were mangrove horseshoe crabs, both of them decorated with passengers in the form of barnacles and button corals.

Two live mangrove horseshoe crabs on the Brunei River. 

I picked this one up to take a look underneath. A girl.

Looking a bit closer, the second and third prosomatic appendages are not modified as claspers.

This one is a walking hotel for barnacles.

Further up the Brunei River, we encountered proboscis monkeys, monitor lizards, crocodiles and many birds, including three species of kingfisher.

A small gallery of other wildlife encountered along the Brunei River: proboscis monkeys, monitor lizards, and common, stork-billed and white-collared kingfishers.

Back in KL on the Saturday morning (over breakfast), I took another look at my Lebanese Cretaceous horseshoe crab. Having just learned about the distinguishing feature of the male Chinese or tri-spine horseshoe crab, I was delighted to see exactly the same feature in the 100 million year old fossil. A quick search revealed, however, that I am not the first to make this observation in the Lebanese Cretaceous material and I found a very interesting article on sexual dimorphism in Tachypleus syriacus by Lamsdell and McKenzie (2015). Lamsdell & McKenzie were also able to revise the divergence time for Tachypleinae, for which a Palaeogene or Neogene origin had been wrongly inferred from molecular clock analysis.

A complete male Tachypleus syriacus, 145 mm long with soft parts, including book gills, preserved in apatite. Cenomanian, Upper Cretaceous, Namoura, Lebanon.

Remembering my earlier interest in the deep time ancestors of today's horseshoe crabs, it's interesting to reflect that the lineage extends back around 480 million years, nearly half a billion years. I was delighted to encounter their modern representatives. This was a fascinating thread of experience during a single week and a wonderful antidote to the intensity of the day job.

- Lamsdell, J. C. & McKenzie, S. C. 2015. Tachypleus syriacus (Woodward) – a sexually dimorphic Cretaceous crown limulid reveals underestimated horseshoe crab divergence times. Organisms Diversity & Evolution.
-  Nicholl, Robert, 1980. Notes on Some Controversial Issues in Brunei History. Archipel 19, 25-42.


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