Baby (Smith’s swimming) crabs

Last week we met Smith’s swimming crab (Charybdis smithii), a rare visitor from the tropics that arrived in the southern reaches of False Bay during the third week of January 2019. We found some adult individuals at Long Beach, well inside False Bay, on 3 March.

Juvenile Smith's swimming crabs at Kommetjie
Juvenile Smith’s swimming crabs at Kommetjie

At the same time that fishermen were reporting adult Smith’s swimming crab off Cape Point, a huge number of tiny red crabs washed up on Long Beach, Kommetjie. There were great piles of them, but we only managed to visit a day or two later on Friday 18 January.

Juvenile Smith's swimming crab at Kommetjie
Juvenile Smith’s swimming crab at Kommetjie

We had to hunt a little, but after a while we found the crabs – now only a few remaining, but most of them very active and vital. Opinion from George Branch is that these are most likely the final larval stage of Smith’s swimming crab, and the same warm conditions wit anomalous westerlies that brought their adults to the peninsula, brought the juveniles.

Juvenile Smith's swimming crab at Kommetjie
Juvenile Smith’s swimming crab at Kommetjie

This conversation (facebook group, may be closed) suggests that the juvenile Smith’s swimming crabs were also seen on Milnerton Beach around the same time as they appeared on the beach at Kommetjie.

I took a couple of short videos of the tiny crabs, which show them in motion on the wet sand.

Apologies for the loud wave noise and less than perfectly steady camera work. Read more about these pelagic crabs, exceptionally infrequent visitors to our shores, in this post.

Hello, Smith’s swimming crab!

One of the things I love about False Bay, and about Long Beach in particular, is the tendency for creatures from all over South Africa’s coastline to end up here, often tropical marine life that got caught in the warm Agulhas current, and then within the circulation of False Bay, ending up just behind the Simons Town harbour wall.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

Thus it was, early in March, that we discovered several adult specimens of Smith’s swimming crab (Charybdis smithii) at Long Beach when we went for a dive. One or two were already dead, and the rest were struggling either on the sand, or in the shallows, looking unhealthy. The water temperature at the time was about 17 degrees.

The first hint that this unusual and rarely-seen visitor had arrived on our shores was a series of social media posts, from January, in one or two of the fishing groups I follow on facebook. (These are excellent places to keep tabs on what’s happening in parts of the ocean I might not routinely visit, and there’s a wealth of knowledge and experience among the members.) Here’s a conversation between local fishermen about seeing large numbers of adult Smith’s swimming crabs just off Cape Point (also facebook). You can also see some photos of one of the crabs from Sea-Change here (facebook), taken on 22 January in False Bay.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

At the same time as these social media posts, there was an influx of small, red crabs on the other side of the Cape Peninsula, at Long Beach in Kommetjie. (But more on that in another post.) Two Oceans says that Smith’s swimming crabs were first described in False Bay in 1838, and then again in 1978, 1983, and 1993. This facebook thread suggests that they may have been last seen off Muizenberg around 2005-2006.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

I read more about Smith’s swimming crab in two papers: this one (Romanov et al), from 2009, and this one (Van Couwelaar et al) from 1997. The more recent paper updates many of the findings of the earlier one. Both teams of scientists behind these papers used trawl data from pelagic cruises to learn about the distribution and life history of these crabs.

Smith’s swimming crab is a pelagic crab that spends the vast majority of its one year, monsoon-driven life cycle in the water column. They are endemic to the western Indian ocean, and are usually found in the area bounded by the Arabian sea (which is west of India) and the latitude of Dar Es Salaam in Tanzania (about 7 degrees south of the equator), and from the east African coast, across east to the Maldives. They may congregate in huge patches, larger than tens of square kilometres, and may reach a biomass of more than 130 kilograms per square kilometre. These swarms are densest between June and September. During July, their concentration can peak at more than 15,000 individuals per square kilometre.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

The crabs congregate on the seafloor of the continental shelf during the months of September to December, mating late in the year. No adult crabs are usually seen between April and June (Van Couwelaar et al speculate that the adults die after breeding), at which time, after metamorphosis, the swarms again become apparent in the western Indian ocean. The crabs grow to about 7.5 centimetres carapace width.

The crabs seem to perform a diel migration, moving deeper in the water column during the day (down to 350 metres’ depth), and returning to the surface at night. They swim continuously and are voracious predators in order to support the high metabolic demand created by this constant activity. They are able to regenerate all their limbs except for their swimming legs (Van Couwelaar et al deduced this in much the same way as Abraham Wald decided which parts of World War II bombers to reinforce – no crabs with partially grown swimming legs were caught in their trawls).

These crabs are important prey for yellowtail, as for other pelagic fish species such as blue sharks, yellowfin tuna and bigeye tuna. The fishermen of False Bay observed that they made excellent bait.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

So what brings Smith’s swimming crab this far south? False Bay is way out of their range. This paper (Chapman, 1988) suggests that prior arrivals of these crabs on our shores have co-incided with weaker than usual summer south easterly winds (which has definitely been a feature of late 2018 and the start of 2019 – we had a gloriously wind-still summer for the most part) and the westward movement of warm water containing the crabs. We did have a spell of unusual westerly wind just prior to these crabs’ arrival.

A fascinating 1984 paper by George Branch describes a temperature anomaly during the summer of 1982-83. This particular Cape summer was characterised by very little of the typical south easterly winds, leading to reduced upwelling, and relatively high sea temperatures (Duffy et al, Effects of the 1982-3 Warm Water Event on the Breeding of South African Seabirds, 1984). The exceptionally warm water on the south and west coasts of South Africa caused mass strandings of some tropical animals (such as portuguese man ‘o war), mortalities of others (such as black mussels), changes in abundance of some species, and extensions of some species’ geographic range. For example, an exceptional number of juvenile turtles washed up on the beaches of False Bay, several months before the usual start of the usual turtle stranding season (which is, very loosely speaking, March-July). Prof Branch records that large numbers of healthy, adult Smith’s swimming crab washed up at Cape Hangklip, and smaller numbers at Boulders Beach, Strand, Milnerton and Blouberg. About 62% of the crabs were female, and many of them survived in aquaria for some time after stranding.

In short, it seems that we had our own little temperature anomlay, however brief, in early 2019, and the pulse of warm water brought with it these rarely seen (in Cape Town) crabs. What luck to spot this unusual visitor!

There are some lovely pictures of Smith’s swimming crab, healthy and in mid water, taken off Tanzania, here.

Cape Columbine lighthouse

The lighthouse at Columbine Nature Reserve
The lighthouse at Columbine Nature Reserve

The Cape Columbine Nature Reserve is just outside the small fishing town of Paternoster on the West Coast. It’s the reserve that contains Tietiesbaai campsite, and is a popular camping location during crayfishing season. Tony and I camped there several years ago, and enjoyed the space and the ability to set up anywhere we wanted to.

Cape Columbine lighthouse
Cape Columbine lighthouse

Inside the reserve is the Cape Columbine lighthouse, which has an art deco feel to it. Built on top of a rocky outcrop called Castle Rock, it was commissioned in 1936. The lighthouse is a 15 metre high masonry tower topped by a 5,040,000 candela light with a range of 30 nautical miles. It covers a particularly treacherous coast, prone to fog and gales, and with many hidden reefs.

Cape Columbine lighthouse
Cape Columbine lighthouse

Cape Columbine lighthouse is manned, and can be visited by the public on weekdays between 10.00 and 15.00.Cape Columbine was the last manned lighthouse to be constructed in South Africa. We haven’t passed by on a weekday yet, so I haven’t been inside.

As of late 2018, the Cape Columbine lighthouse needs a coat of paint!
As of late 2018, the Cape Columbine lighthouse needs a coat of paint!

When we camped at Columbine Nature Reserve in 2009, the lighthouse was in much better shape. I took the picture below on that trip. If you drive around the lighthouse, you may see a small green tower inside a fenced off area that houses a fog detector, and a fog signal that sounds when fog is detected. This apparatus used to be housed at the lighthouse, but in 1995 the opportunity was taken to move both sets of devices (detector and signaller) closer to the sea.

Cape Columbine lighthouse
Cape Columbine lighthouse

Learn more about South Africa’s lighthouses from Lighthouses of South Africa.

Shrimp news from False Bay

The University of Cape Town has announced that a further three new species of shrimp, all spotted close to shore near Millers Point in False Bay, have been described and named. All three belong to the same genus (Heteromysis), and look similar, with pale bodies marked by red spots and stripes. One of these new (to science) species lives inside octopus dens, and another lives inside the shell of certain types of hermit crab. These three shrimps join the stargazer shrimp that was discovered by and named for Guido Zsilavecz, citizen scientist and author of several books on False Bay’s marine wildlife.

Two of the new species were discovered by local film maker Craig Foster, founder of the Sea-Change project about which we read last week. These types of discoveries are very exciting and should be a great inspiration and encouragement to divers and other water users. Time in the water is rewarded. If you can’t identify something, send an email with its photo to SURG. It is possible to make significant contributions to science while holding down an entirely non-scientific day job!

Read all about the new shrimps here.

Newsletter: Keeping current

Hi divers

Weekend dive plans

No diving

We dived Long Beach today in surge, strong current and lousy visibility. It is full moon and we have had some swell but I was hoping it would be ok – it wasn’t.

Weekend conditions do not look ideal for recreational (fun!) diving. Better luck next week I hope.

Rock lobster munching a sea jelly
Rock lobster munching a sea jelly

Date to diarise

There’s a talk on South Africa’s inshore marine resources, with special reference to West Coast rock lobster stocks, happening on Wednesday 18 October under the auspices of Kommetjie Environmental Awareness Group. It starts at 7.30pm at Kommetjie Christian Church. There’s a blurb with more info here. It should be an extremely interesting event, and I’d particularly encourage you to attend if you are a keen crayfisher in season…

regards

Tony Lindeque
076 817 1099
www.learntodivetoday.co.za
www.learntodivetoday.co.za/blog/

Diving is addictive!

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Article(s): The Longform guide to sea creatures (some holiday reading!)

Here are some holiday reading recommendations – not too taxing, not entirely insubstantial – to enjoy while lounging under an umbrella by the pool or waiting for a flight to board. You will probably enjoy them because they’re about marine life, and I assume that if you didn’t have a passing interest in the ocean, you wouldn’t be reading this blog.

Octopus at Pyramid Rock
Octopus at Pyramid Rock

Longform is a website that provides reading recommendations – usually (as the name suggests) long form stories, not restricted to a particular range of topics. I am a subscriber to the Longform newsletter, and lately a user of their iPad app.

The Longform guide to sea creatures is a short list of juicy long articles whose common thread is that they focus on marine animals. I’ve shared some of them with you already – Killer in the Pool and Moby Duck being the most notable. Others are about giant squid, octopus, tuna, whales, and the Loch Ness monster. It’s a page worth bookmarking, should you anticipate requiring a couple of hours of thoughtful, fact-checked, well researched reading on the subject of marine life.

You can find the list of Longform sea creature articles here, and a mostly overlapping but slightly different version on Slate.com, here. (The advantage of the Slate list is that you can send the articles to your Kindle, to read later.)

 

Newsletter: Try it out

Hi divers

Weekend dive plans

Saturday: 6.00 am on Simon’s Town jetty for double tank dives in the Roman Rock vicinity

Saturday: Dives from Simon’s Town jetty at 9.00 and 11.30 am, sites dependent on conditions

Dive conditions

We had good conditions last week; they held for the weekend and then kept going at the start of this week. Yesterday and today, however, the viz took a bit of a nosedive and is possibly going to settle into the summer visibility groove of a warm 4- 6 metres, depending on your eyesight. There is very little swell or wind in the forecast which will help.

I doubt there will be too much difference between Saturday and Sunday so the plan is as follows: a screechingly early double tank launch on Saturday (6.00 am on the Simon’s Town jetty). On Sunday we will meet for 9.00 am and 11.30 am. The sites will depend on what we find on Saturday.

A Cape long-legged spider crab
A Cape long-legged spider crab

This Cape long-legged spider crab hitched a ride to the surface on one of the divers’ booties this week. Isn’t he a handsome chap? He is back where he belongs!

Try diving in the pool

In the month of December until Christmas, we are offering Discover Scuba sessions (try dives) in our pool, free of charge, every Wednesday and Thursday after 3.00 pm. If you have a friend that needs a little persuasion to qualify as your future dive buddy, then bring them along. Booking is essential. Get in touch if you want to reserve a slot.

regards

Tony Lindeque
076 817 1099
www.learntodivetoday.co.za
www.learntodivetoday.co.za/blog/

Diving is addictive!

To subscribe to receive this newsletter by email, use the form on this page!

Dive sites: Vulcan Rock

Esther exploring
Esther exploring

Vulcan Rock is a dive site just outside Hout Bay. We dived it on a day when the south easter had been blowing for a while, so the visibility was quite good. The dive site is essentially a huge stack of granite boulders, with Vulcan Rock as the highest point. The rocks are covered with sea urchins, rock lobster, corals, sea cucumbers, brittle stars, and a bit of red seaweed.

Urchins, brittle stars, corals at Vulcan Rock
Urchins, brittle stars, corals at Vulcan Rock

Esther and I stayed within the range of Open Water divers, not going deeper than 18 metres, but it is possible to go as deep as 30 metres at this site if you go for a bit of a swim. This is definitely a site to visit when the surface conditions are good and the swell is low (not that Hout Bay diving is ever great – or particularly safe – in a big swell).

Granite and kelp
Granite and kelp

I had the little Sony camera with me and took some happy snaps. Underneath all the granite is an enormous cave, with several entrances. Peet, who joined us on this dive, made a video of the cave that I will share with you later this week.

Mark bringing the boat
Mark bringing the boat

Dive date: 19 April 2015

Air temperature: 23 degrees

Water temperature:  10 degrees

Maximum depth: 15.6  metres

Visibility: 12 metres

Dive duration: 39 minutes

Bookshelf: Shorelines, Strandlopers and Shell Middens

Shorelines, Strandlopers and Shell Middens – John Parkington

Shorelines, strandlopers and shell middens
Shorelines, strandlopers and shell middens

I’m currently delving into some subjects I know nothing about, stone age archaeology being one. I harboured a childhood dream of excavating ancient Egyptian artefacts, but that came to nought and I ended up a mathematician – manipulating things I can’t even see or touch. Ah well.

Strandlopers are the people who lived as hunter gatherers along the Southern African coastline, related to the San. A shell midden is basically an ancient dustbin, and the strandlopers would have generated these when they stopped at a location for any length of time. They are typically densely packed collections of shells that are all of a certain size (large enough to be worth harvesting and eating) and from a limited set of edible species. In this way they are distinguishable from random heaps of shells. A midden of considerable size would have been built up in a relatively short time owing to the sheer number of molluscs it is necessary to consume in order to have something approaching a decent diet! Shell middens exist around South Africa’s coast, including a couple in False Bay which I plan to hunt down. It is believed that the consumption of shellfish – which are extremely nutritious – by early humans contributed to increased brain size and the development of more complex civilisations.

John Parkington, the author of this book, is a lecturer in archaeology at the University of Cape Town. His writing style is clear and expository, and enabled me to follow along with little to no background knowledge on the subject. The book is extensively illustrated with photographs, including of Parkington’s collaborators at the university. I appreciated the fact that each person’s full name was provided.

If you’re interested in knowing more about the early history of humans along South Africa’s coast, I’d recommend starting with the series Shoreline, which provided an excellent introduction to the subject. The Sea-Change project also explores our ancestors’ interactions with the marine world.

You can find this book here, or perhaps at Kalk Bay Books.

Bookshelf: Shrimp

Shrimp: The Endless Quest for Pink Gold – Jack & Anne Rudloe

Shrimp
Shrimp

Before reading this book it was important to determine the difference between shrimp (which we almost never talk about or eat in South Africa, except for little tins that go into paella when there’s not enough of anything else to make a full meal), and prawns (which are large, and frequently enjoyed). My meanderings around the internet revealed that “shrimp” and “prawn” are common names, not related to any scientific classification, and that by convention shrimp are often small and prawns are large… But it’s not clear cut and no one should be dogmatic about anything in this debate. What Americans call shrimp encompasses our prawns, as well as our tiny shrimps… Hence the term “jumbo shrimp”, which sounds nonsensical to me!

Shrimp capitalises on the popularity of nature books about a single species, often with one word titles, a boom initiated (I think) by Marc Kurlansky’s bestseller CodThe Rudloes are marine biologists and founded the Gulf Specimen Marine Laboratories together (Anne passed away in 2012). Jack Rudloe spent a lot of time on commercial fishing boats, and this book – like Trevor Corson’s book The Secret Life of Lobster – reveals how difficult it is for a writer to remain objective when confronted with the hard working, salt of the earth fisherman archetype.

Despite this, which bothered me slightly, this is a fascinating insight into everything you ever wanted to know about shrimp (slash prawns). Shrimp fisheries and farms worldwide are examined (with a focus on the American Gulf of Mexico fishery), and the dizzying variety of shrimps and prawns is elucidated in some detail. There are diagrams of their bodies, descriptions of their habitats, and details of how they are harvested. The book’s structure is difficult to discern, even chaotic, which throws off a disorganised reader like me (six books on the go, and often fragmentary reading times in elevators and in queues).

I did not find much information in this book about the impact of trawl fisheries, which include horrific bycatch and damage to the ocean floor that is so serious it can be seen from space. It was here that I felt Rudloe’s affinity with the fishermen who do this damage got in the way of a frank assessment of how bad it is. Prawn farming is also not environmentally neutral. The means by which we get prawns onto our plate are actually so bad that the decision whether to eat shrimp and prawns at all should be weighed very carefully.

You can get the book here if you’re in South Africa, and here if you’re not. If you want to read it on your Kindle, go here.