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.

Newsletter: Pedal pedal pedal

Hi divers

Weekend dive plans

No diving This weekend we are hamstrung by the Cape Town Cycle Tour on Sunday – good luck if you are participating! I’m away in Port Elizabeth on Saturday, so I won’t be running dives even if it wasn’t going to be rather windy (it is). I’ll be diving with students during next week, so let me know if you want to tag along and I’ll keep you posted.
Sea star at Long Beach
Sea star at Long Beach

Ocean/Surf night at the Cape Town Adventure Film Festival

Film buff? Love the ocean? Think popcorn is a food group? This event on Friday, 12 April, may be right up your street. Get tickets here. 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!

Bookshelf: Eye of the Shoal

Eye of the Shoal: A Fishwatcher’s Guide to Life, the Ocean and Everything – Helen Scales

Eye of the Shoal
Eye of the Shoal

This is an absolutely wonderful book about fish. Everything about fish. Helen Scales is a marine biologist and the accomplished author of marine-themed books (I previously wrote about Poseidon’s Steedher book about seahorses).

Here, Scales delves into the world of an animal whose variety seems almost without limit. Her book overflows with wonders, and interweaves science, adventure and mythology to shed light on the under-appreciated inhabitants of the underwater realm.

Unsolicited (this is almost always the case), I read half of this book to Tony while I was busy with it, and it delighted both of us. We learned about bioluminescent fish, poisonous fish, the sounds fish make, and the colours of their skin. We learned about fish that use tools, fish cognition, and about the state of the science regarding whether fish experience pain. We even learned about moray eels and grouper hunting co-operatively.

As a scuba diver, Scales relates tales of dives on which she observed the behaviours and phenomena she describes, and I was inspired to pay more attention to the activities of the fish we see on dives around Cape Town. They may (almost) all be the same colour, but there are certainly things that they do, and fascinating ways of being, that I am failing to appreciate.

Scales provides a bibliography on her website with links to the open access scientific papers that she used to research the book.

Get the book here (South Africa), here (US) or here (UK).

Newsletter: From the heart

Hi divers

Weekend dive plans

Saturday: Boat dives from Hout Bay or shore dives at Long Beach

Weekend conditions don’t look all that great. Saturday will most likely be best for launching from Hout Bay , or there may be a slight chance of shore dive from Long Beach for students. I will make this decision late tomorrow depending on sea conditions. Let me know if you’re keen to dive.

A heart for you on Valentines day
A heart for you on Valentines day

Sunday is the Cape Peninsula marathon, starting in Green Point and finishing in Simon’s Town, so expect some road closures.

Film screening on the beach

There’s a free screening of the documentary Tidal at St James beach on Thursday 7 March. Read about the film here, and RSVP on Quicket. There’s no charge.

Tony Lindeque

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

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Bookshelf: Sea Change

Sea Change: Primal Joy and the Art of Underwater Tracking – Craig Foster & Ross Frylinck

Sea Change
Sea Change

The Sea Change project may be familiar to you from their large format photographic displays, one of which was for a time along the promenade in Sea Point, and is currently in Lamberts Bay. You may have read about the project, or seen its members – ocean-loving filmmakers, journalists, scientists – diving in the cold water of False Bay year round, without wetsuits. You may even have seen the BBC’s Blue Planet II series, in which the filmmakers, guided by Craig Foster and his fellows, captured an incredible octopus sequence filmed on our doorstep in False Bay.

The Sea-Change book has been a long time in the making, and is the product of hours upon hours upon hours spent in the water, observing the animals that call the kelp forests home. The book contains a story of loss and discovery – that of Ross Frylinck – interwoven with large-format photographs of scenes from the kelp forest, taken by filmmaker and naturalist Craig Foster, a co-founder of the Sea-Change project. Foster also provides the captions.

As a diver, it was immediately clear to me that a great deal of patience and close observation was required to gain the deep understanding that Foster has of the smallest creatures living among the kelp. There is no substitute for time in the water. There is no substitute for swimming slowly, deliberately, and for spending extended time looking at one place. The marks that animals leave on rocks, kelp stipes, the sand, and even on each other’s shells, can tell a story.

I learned a huge amount about animal behaviour from this book, and about the interconnectedness of all the elements of the watery, beautiful world around the Cape Peninsula. The photographs are beautiful and striking, capturing moments that one would be extremely lucky to see during the normal course of things. Diving for more than an hour a day, every day of the year, however, makes such things more commonplace.

Sea Change presents a beautiful opportunity for the wider community of ocean lovers to learn from the unique approach taken by the Sea-Change team (this article gives a good sense of it), and to learn how to understand and observe the animals that surround us when we look beneath the surface of the inshore kelp forests. The project also has something to say about how science happens, and the vital connection between science and storytelling. Identifying animals is fun, but – as any veteran twitcher will tell you – the next level is understanding behaviour. This is a challenge I’m happy to take up.

Get a copy here, or directly from the Sea Change team.

Newsletter: Seal the deal

Hi divers

Weekend dive plans

No diving This weekend’s wind is forecast to be between 30 and 50 km/h, which, in my book, renders the weekend not suitable for diving. Sorry, kids.

Elephant seal on Fish Hoek beach
Elephant seal on Fish Hoek beach

See a seal instead

If you feel like a natural exfoliation, the visiting elephant seal is still on Fish Hoek beach, just after the little yellow and white mini lighthouse half way down the beach. He’ll be leaving soonish, so don’t delay. Read more about elephant seals here.

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

Diving is addictive!

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Newsletter: Lend a hand

Hi divers

Weekend dive plans

Sunday: Shore or boat dives

Most forecasts predict a southerly swell tomorrow, which does not do good things for False Bay. If anything, Sunday will be the day to dive and we will make a decision on whether to launch or shore dive during the early afternoon on Saturday. Let me know if you need a dose of thalassotherapy and I’ll put you on the list.

Greater flamingos at Strandfontein
Greater flamingos at Strandfontein

Help a flamingo

Thousands of lesser flamingo chicks have been rescued from the parched Kamfers Dam near Kimberly, and are being hand-reared at facilities around South Africa. If you are in Cape Town and have some free time, consider volunteering at SANCCOB. More information here.

Meet a seal

If you haven’t already visited the southern elephant seal that is moulting on Fish Hoek beach, consider doing so this weekend. He’s a magnificent beast and it’s a real privilege – though perhaps not as rare as you might think – to see this type of seal on our shores. Read more about southern elephant seals here. regards

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

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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.

Newsletter: The joy of kelp

Hi divers

Weekend dive plans

No diving The forecasts vary wildly this weekend. Windy says howling south easter everywhere, whilst other sites say mild wind suitable for some Atlantic diving. The mountain will break some of the wind so I am sure Table Bay sites will be good, and Hout Bay much the same. I have students for the pool this weekend so there are no launches planned.

Kelp forest near Pyramid Rock
Kelp forest near Pyramid Rock

Kelp night

Learn all about kelp at the Two Oceans Aquarium next Thursday evening, 31 January. There will be talks on kelp science, kelp as a habitat, and kelp as a snack. Read more here, and book tickets at Quicket.

Camera housing

Clare is selling her Sony underwater housing that fits the Sony RX100 range of cameras. If you’re interested, drop me a mail and I’ll put you in touch with her.

regards

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

Diving is addictive!

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Stompneuspunt beacon

View of Stompneuspunt beacon from inside Shelly Point
View of Stompneuspunt beacon from inside Shelly Point

During the course of a West Coast road trip late last year, we stopped at the unmanned Stompneuspunt beacon. This striking, squat structure sits at the southern end of St Helena Bay. To get there, we had to drive through the eerie, deserted, badly laid out Shelley Point golf estate development (tell the guard at the gate that you want to visit the lighthouse). Persistence through the maze of narrow roads turning in upon each other is well rewarded.

Stompneuspunt beacon
Stompneuspunt beacon

The green-painted lantern house atop the structure looks like a minaret, and the whole building looks like an exotic transplant from the Middle East. The beacon is situated on a beach of coarse sand covered with thousands of empty mussel shells and inhabited by flocks of cormorants. The mussel shells wash up after winter storms and red tides, and because of predation by rock lobsters and other shellfish.

Stompneuspunt beacon
Stompneuspunt beacon

The beacon was commissioned in 1934, at which time it was a pyramid-shaped wooden structure. The present building was completed in 2001. The tower is 8 metres high, and the focal plane of the light is 12 metres above sea level. The intensity of the light is a modest 1,403 candelas, but this beacon doesn’t have to compete with much in the way of onshore light pollution. It’s visible from 10 nautical miles away.

Stompneuspunt beacon
Stompneuspunt beacon

Hit up Lighthouses of South Africa for more information on this charming light.